928 Tech Tips

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Oxygen Sensor Information

Written by Rick Kirchoff (rick@posms.cactus.org). Edited to html by Kyle
Hamar

>From Terrill_Yuhas@smtpsc1.sc.pima.gov Fri Nov 4 12:42:39 1994

In response to several requests for more information about Oxygen (O2) sensors, perhaps the following information will help.

Comment:

These procedures are only for self powered conventional sensors. Some very new cars are using a different style sensor that is powered. *Many* Oxygen sensors are replaced that are good to excellent. *Many* people don't know how to test them. They routinely last 50,000 or more miles, and if the engine is in good shape, can last the life of the car.

What does the O2 sensor do?

It is the primary measurement device for the fuel control computer in your car to know if the engine is too rich or too lean. The O2 sensor is active anytime it is hot enough, but the computer only uses this information in the closed loop mode. Closed loop is the operating mode where all engine control sensors including the Oxygen sensor are used to get best fuel economy, lowest emissions, and good power.

Should the O2 sensor be replaced when the sensor light comes on in your car?

Probably not, but you should test it to make sure it is alive and well. This assumes that the light you see is simply an emissions service reminder light and not a failure light. A reminder light is triggered by a mileage event (20-40,000 miles usually) or something like 2000 key start cycles. EGR dash lights usually fall into the reminder category. Consult your owners manual, auto repair manual, dealer, or repair shop for help on what your light means.

How do I know if my O2 sensor may be bad?

If your car has lost several miles per gallon of fuel economy and the usual tune up steps do not improve it. This *is not* a pointer to O2 failure, it just brings up the possibility. Vacuum leaks and ignition problems are common fuel economy destroyers. As mentioned by others, the on board computer may also set one of several failure "codes". If the computer has issued a code pertaining to the O2 sensor, the sensor and it's wiring should be tested. Usually when the sensor is bad, the engine will show some loss of power, and will not seem to respond quickly.

What will damage my O2 sensor?

Home or professional auto repairs that have used silicone gasket sealer that is not specifically labeled "Oxygen sensor safe", "Sensor safe", or something similar, if used in an area that is connected to the crankcase. This includes valve covers, oil pan, or nearly any other gasket or seal that controls engine oil. Leaded fuel will ruin the O2 sensor in a short time. If a car is running rich over a long period, the sensor may become plugged up or even destroyed. Just shorting out the sensor output wire will not usually hurt the sensor. This simply grounds the output voltage to zero. Once the wiring is repaired, the circuit operates normally. Undercoating, antifreeze or oil on the *outside* surface of the sensor can kill it. See how does an Oxygen sensor work.

Will testing the O2 sensor hurt it?

Almost always, the answer is no. You must be careful to not *apply* voltage to the sensor, but measuring it's output voltage is not harmful. As noted by other posters, a cheap voltmeter will not be accurate, but will cause no damage. This is *not* true if you try to measure the resistance of the sensor. Resistance measurements send voltage into a circuit and check the amount returning.

How does an O2 sensor work?

An Oxygen sensor is a chemical generator. It is constantly making a comparison between the Oxygen inside the exhaust manifold and air outside the engine. If this comparison shows little or no Oxygen in the exhaust manifold, a voltage is generated. The output of the sensor is usually between 0 and 1.1 volts. All spark combustion engines need the proper air fuel ratio to operate correctly. For gasoline this is 14.7 parts of air to one part of fuel. When the engine has more fuel than needed, all available Oxygen is consumed in the cylinder and gasses leaving through the exhaust contain almost no Oxygen. This sends out a voltage greater than 0.45 volts. If the engine is running lean, all fuel is burned, and the extra Oxygen leaves the cylinder and flows into the exhaust. In this case, the sensor voltage goes lower than 0.45 volts. Usually the output range seen seen is 0.2 to 0.7 volts.

The sensor does not begin to generate it's full output until it reaches about 600 degrees F. Prior to this time the sensor is not conductive. It is as if the circuit between the sensor and computer is not complete. The mid point is about 0.45 volts.
This is neither rich nor lean. A fully warm O2 sensor *will not spend any time at 0.45 volts*. In many cars, the computer sends out a bias voltage of 0.45 through the O2 sensor wire. If the sensor is not warm, or if the circuit is not complete, the computer picks up a steady 0.45 volts. Since the computer knows this is an "illegal" value, it judges the sensor to not be ready. It remains in open loop operation, and uses all sensors except the O2 to determine fuel delivery. Any time an engine is operated in open loop, it runs somewhat rich and makes more exhaust emissions. This translates into lost power, poor fuel economy and air pollution.

The O2 sensor is constantly in a state of transition between high and low voltage. Manufacturers call this crossing of the 0.45 volt mark O2 cross counts. The higher the number of O2 cross counts, the better the sensor and other parts of the computer control system are working. It is important to remember that the O2 sensor is comparing the amount of Oxygen inside and outside the engine. If the outside of the sensor should become blocked, or coated with oil, sound insulation, undercoating or antifreeze, (among other things), this comparison is not possible.

How can I test my O2 sensor?

They can be tested both in the car and out. If you have a high impedance volt meter, the procedure is fairly simple. It will help you to have some background on the way the sensor does it's job. Read how does an O2 sensor work first.

Testing O2 sensors that are installed

The engine must first be fully warm. If you have a defective thermostat, this test may not be possible due to a minimum temperature required for closed loop operation. Attach the positive lead of a high impedance DC voltmeter to the Oxygen sensor output wire. This wire should remain attached to the computer. You will have to back probe the connection or use a jumper wire to get access. The negative lead should be attached to a good clean ground on the engine block or accessory bracket. Cheap voltmeters will not give accurate results because they load down the circuit and absorb the voltage that they are attempting to measure. A acceptable value is 1,000,000 ohms/volt or more on the DC voltage. Most (if not all) digital voltmeters meet this need. Few (if any) non-powered analog (needle style) voltmeters do.
Check the specs for your meter to find out. Set your meter to look for 1 volt DC. Many late model cars use a heated O2 sensor. These have either two or three wires instead of one. Heated sensors will have 12 volts on one lead, ground on the other, and the sensor signal on the third. If you have two or three wires, use a 15 or higher volt scale on the meter until you know which is the sensor output wire.

When you turn the key on, do not start the engine. You should see a change in voltage on the meter in most late model cars. If not, check your connections. Next, check your leads to make sure you won't wrap up any wires in the belts, etc. then start the engine. You should run the engine above 2000 rpm for two minutes to warm the O2 sensor and try to get into closed loop. Closed loop operation is indicated by the sensor showing several cross counts per second. It may help to rev the engine between idle and about 3000 rpm several times. The computer recognizes the sensor as hot and active once there are several cross counts.

You are looking for voltage to go above and below 0.45 volts. If you see less than 0.2 and more than 0.7 volts and the value changes rapidly, you are through, your sensor is good. If not, is it steady high (> 0.45) near 0.45 or steady low (< 0.45). If the voltage is near the middle, you may not be hot yet. Run the engine above 2000 rpm again. If the reading is steady low, add richness by partially closing the choke or adding some propane through the air intake. Be very careful if you work with any extra gasoline, you can easily be burned or have an explosion.
If the voltage now rises above 0.7 to 0.9, and you can change it at will by changing the extra fuel, the O2 sensor is usually good.

If the voltage is steady high, create a vacuum leak. Try pulling the PCV valve out of it's hose and letting air enter. You can also use the power brake vacuum supply hose. If this drives the voltage to 0.2 to 0.3 or less and you can control it at will by opening and closing the vacuum leak, the sensor is usually good.

If you are not able to make a change either way, stop the engine, unhook the sensor wire from the computer harness, and reattach your voltmeter to the sensor output wire. Repeat the rich and lean steps. If you can't get the sensor voltage to change, and you have a good sensor and ground connection, try heating it once more. Repeat the rich and lean steps. If still no voltage or fixed voltage, you have a bad sensor.
If you are not getting a voltage and the car has been running rich lately, the sensor may be carbon fouled. It is sometimes possible to clean a sensor in the car. Do this by unplugging the sensor harness, warming up the engine, and creating a lean condition at about 2000 rpm for 1 or 2 minutes. Create a big enough vacuum leak so that the engine begins to slow down.

The extra heat will clean it off if possible. If not, it was dead anyway, no loss. In either case, fix the cause of the rich mixture and retest. If you don't, the new sensor will fail.

Testing O2 sensors on the workbench.

Use a high impedance DC voltmeter as above. Clamp the sensor in a vice, or use pliers or vice-grip to hold it. Clamp your negative voltmeter lead to the case, and the positive to the output wire. Use a propane torch set to high and the inner blue flame tip to heat the fluted or perforated area of the sensor. You should see a DC voltage of at least 0.6 within 20 seconds. If not, most likely cause is open circuit internally or lead fouling. If OK so far, remove from flame. You should see a drop to under 0.1 volt within 4 seconds. If not likely silicone fouled. If still OK, heat for two full minutes and watch for drops in voltage. Sometimes, the internal connections will open up under heat. This is the same a loose wire and is a failure. If the sensor is OK at this point, and will switch from high to low quickly as you move the flame, the sensor is good. Bear in mind that good or bad is relative, with port fuel injection needing faster information than carbureted systems.

ANY O2 sensor that will generate 0.9 volts or more when heated, show 0.1 volts or less within one second of flame removal, AND pass the two minute heat test is good regardless of age. When replacing a sensor, don't miss the opportunity to use the test above on the replacement. This will calibrate your evaluation skills and save you money in the future. There is almost always *no* benefit in replacing an oxygen sensor that will pass the test in the first line of this paragraph.
--
Rick Kirchhof Austin, Texas

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Subject: O2 Sensor

From: "McMullen, Russ" russ@so.co.lee.fl.us

A while back someone posted on the list they had installed an O2 sensor for a mustang in their 944. I was wondering how it worked as I am going to replace my sensor soon. I was shopping around for prices and found the following: Porsche direct replacement ‑ $179.00, Universal 3 wire ‑ $69.00, Mustang 3 wire ‑ $31.00. So the questions are, will the mustang O2 sensor work or should I shell out the extra $38.00 for the universal? And, are the wires color-coded the same as the original or do I have to guess at how to splice the wires together? Or is there another route?

Russ McMullen Jr., 85.5 944na

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Subject: RE: O2 sensor

From: cdwhite cdwhite@Concentric.net

To: McMullen@Concentric.net, Russ russ@so.co.lee.fl.us

I used a 5.0 mustang three wire O2 sensor. Bosch of course. The price varies by which year mustang. The only difference is the length of the wires. The wiring job is pretty straight forward. There are two white wires and one black on the new O2 sensor. Same on the Porsche harness. Just cut and paste.. oh I mean solder. The wires on mine did not like being solder, just keep cleaning and trying, pencil type soldering iron may not be enough watts.

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I just purchased the Bosch 13913 Ford Mustang Sensor. According to the sales guy the only difference between three wire sensors is the wiring harness/connector? If this is true, the higher price of "universal" replacement units must be a result of supply and demand due to lack of consumer awareness.. Given the high price of the direct replacement for our cars, the connectors sure are worth their weight in gold!

Does anyone have info on what a three wire O2 sensor signal should look like? I would like to scope it out to make sure I have correct O2 sensor operation with the 13913 unit. Thanks, Ezra

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As I understand it, the only difference between three wire sensors is the wiring harness. As such, you should be able to use any three wire Bosch or equivalent O2 sensor. You will have to splice your old wiring onto the new sensor. Russ will be posting some info soon, I am not sure what he found out. I have installed the 13913 sensor, but will not be able to do a road test until next week when my wheels return. Ezra

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On Tue, 26 Aug 1997 23:19:21 +0200 Konstantin Kotitsas wrote:

>The 951 has the 0258003012 12V. Is there any equivalent for this O2 sensor like >the 13913 unit for the 944 NA?

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All the information (and more) you ever wanted to know about O2 sensors. I wish I could say I was this brainy expert on O2 sensors, however, the majority of this information came from other lists, the library and chats with Bosch and Borg Warner, as well as with local shop owners and other Porsche owners, and some experiments using my Porsche as the guinea pig. And thanks Ezra for your assistance.

HOW DOES AN OXYGEN SENSOR WORK

The oxygen sensor is a measuring device that determines the oxygen content of the exhaust gas. The oxygen content reacts with the sensor to produce a voltage output. Basically, it is a chemical generator, constantly making a comparison between the oxygen inside the exhaust and the air outside. If this comparison shows little or no oxygen in the exhaust, a voltage is generated.

The output of the sensor is usually between 0 and 1.1 volts (for our purposes). All spark combustion engines need the proper air‑fuel ratio to perform correctly. This being 14.7 parts of air to 1 part fuel (stoichmetric). The O2 sensor will constantly change in an effort to maintain stoichmetric. When the engine has more fuel than needed, all available oxygen is consumed in the cylinders and the gases leaving through the exhaust contain almost no oxygen. The sensor sends out a voltage greater than .45 volts to the control unit. If the engine is running lean, all fuel is burned and the extra oxygen leaves the cylinders and flows through the exhaust. In this case, the sensor voltage goes lower than .45 volts. The most common output range is .2 to .7 volts.

The sensor does not begin to generate output until approximately 300 degrees Celsius and is at optimum performance around 600C. At this time, a closed‑loop control of air‑fuel mixture is created. Prior to this, the sensor is active, (open‑loop mode). The control unit only uses information provided by the O2 sensor in the closed‑loop mode. Mid‑point voltage is approximately .45 volts. This is at a point where there is neither a rich nor lean condition. A fully warmed up sensor WILL NOT spend any time at .45 volts. The control unit sends out a bias voltage of .45 volts through the O2 sensor wire. If the sensor is not warmed up or if the circuit is not complete, the control unit gets a signal from the sensor of .45 volts. Since the control unit knows this is an inappropriate value, it believes the sensor is not ready. It remains in open‑loop operation and uses all sensors except the O2 sensor to regulate air‑fuel mixture. Any time an engine is operated in an open‑loop mode it will have a rich condition. Obviously this results in lost power and poor fuel economy as well as other fuel‑related problems. The O2 sensor is constantly in a state of transition between high and low voltage. In technical terms the crossing of the .45 voltage mark is called cross count. The higher the number of O2 cross counts the better

the sensor. The faster the cross counts, the better and quicker ability the control unit has of adjusting the air‑fuel mixture.

DETERMINING IF THE O2 SENSOR IS BAD

O2 sensors do wear out; however, not nearly at the rate manufacturers would have you believe. Many good to excellent O2 sensors are replaced unnecessarily. Most manufacturers recommend testing the O2 sensor every 20,000 miles and/or replacing every 25,000 to 60,000 miles. Research has shown that O2 sensors routinely last 50,000 to 70,000 miles and, if the engine is well maintained, will last for the life of the vehicle. I believe that testing the O2 sensor every 30,000 miles as a precautionary measure would be adequate or at any time you suspect problems with the sensor.

Indications of possible O2 sensor failure are; hesitation problem either hot or cold, higher than normal fuel consumption and usual tune up steps have not corrected the problem, rough idle, "hunting" idle (such as when the engine is cold the idle is a steady 1000 rpm for a brief period, then takes a sharp dip until the engine almost stalls, then back to 1000 rpm... and continues to cycle through in this manner), misfiring and poor engine performance. Additionally, if left unattended, a malfunctioning O2 sensor could burn or loosen up your catalytic converter resulting in an increased expense. Remember these are not

positive indicators of an O2 sensor malfunction, they just bring up the possibility of a problem. Vacuum leaks and ignition problems are also common fuel robbing problems.

Damage to the sensor will result any time auto repairs are made and a silicone gasket sealer is used that is not labeled "oxygen sensor safe" or something similar, if this sealer was used in any area connected to the crankcase. This includes, valve cover, head gasket, oil pan, or other seal or gasket that contacts engine oil. Leaded fuel will ruin the O2 sensor in a very short period of time (several hours). If your Porsche has been running rich for a long period of time, the sensor may have become plugged up with carbon deposits or even destroyed.

Undercoating, antifreeze or oil, soundproofing, or any other similar product that comes into contact with the outside of the sensor will destroy it.

1‑WIRE OR 3‑WIRE SENSOR

1‑wire sensors are usually the cheapest and depend strictly upon exhaust heat to activate them. The sensor element must get very hot to develop any voltage and low exhaust temperatures result in a reduction or absence of sensor output, (thereby giving you an open‑loop). Tests have shown there is not enough heat at idle to keep the sensor in a closed‑loop mode. The output voltage will begin to decay in a matter of seconds at idle. Compensation is sometimes made in 1‑wire sensors by adjusting the trip point of the sensor based upon the temperature of the cell. This allows operation even if the sensor has cooled off to the point that the voltage is much lower than it should be. Additionally, 1‑wire sensors are usually mounted at some point directly on the exhaust manifold so as to be exposed to the hotter gases.

3‑wire sensors or heated sensors are used to heat up the sensor as soon as the engine is started, although heat is not supplied continuously. They are usually located in the exhaust pipe or the catalytic converter. Normally 1 lead supplies 12 volts, another will be ground and the third will be the sensor output signal lead. It is not advisable to use a 1‑wire lead to replace a 3‑wire lead unless you do not care how your vehicle runs when cold and can afford to purchase a new catalytic converter every year or so.

TESTING PROCEDURES ‑ ON CAR

The engine must be fully warm, Disconnect the sensor plug from the wiring harness (round pull apart plug located next to the firewall near the speed and reference mark sensor plugs). Since the sensor output wire must be connected to the control unit to obtain a reading, you will need to use a jumper wire to connect terminal 1 (top male terminal) on the sensor plug and terminal 1 (top female terminal) on the control unit plug. Attach the positive lead of a DC voltmeter to the O2 sensor output wire (or jumper wire). Attach the negative to a good engine ground or negative terminal of the battery. Use a voltmeter that has an

acceptable value of 1,000,000 ohms/volts or more on the DC voltage.

Most, if not all, digital voltmeters meet this requirement. I have been told that very few analog (needle‑type) voltmeters meet this requirement. I have also been told not to use the digital voltmeter as, although it will not damage the sensor, the inherent sampling rate may mask the actual voltage and/or its cycling nature. Both meters worked fine for me.

Set your meter to read 1 volt DC. Turn the key to the on position but do not start the car. You should see a change in the voltage on the meter. If not double‑check your connections. Make sure your leads and wires are out of the way of belts, etc. and start the engine. Run the engine for a couple of minutes to get the O2 sensor warmed up and into closed loop mode. You will know when you have a closed loop, as the voltmeter will register several rapid cross counts per second. You are looking for the voltage to go above and below the .45 volts. Below .45 volts indicates rich and above .45 volts indicates lean. If you see less than ..2 volts and more than .7 volts, your sensor is fine and the testing is completed. If the cross counts are SLOW, then your sensor needs to be replaced. If the voltage reading is steady high, low or in the middle, you may not have the sensor in the closed‑loop. Run the engine above 2000 RPM for several more minutes. If you still have a fixed or no voltage, you will need to remove the sensor for further testing.

If you are not getting any voltage and your Porsche has been running rich for awhile, it is possible the sensor is carbon‑fouled. Sometimes it's possible to clean the sensor while it remains in the vehicle. Disconnect the sensor plug from the wiring harness and warm up the engine. Create a large enough vacuum leak that will cause the engine to slow down (any vacuum lines will work). The extra heat should burn off the carbon deposits. Retest the sensor. If you still get a fixed or no voltage, the sensor is probably bad. Additionally, you will need to find the cause of this rich mixture or your replacement sensor will go bad. If you are getting a steady low voltage reading the problem could be associated with an exhaust leak. In any case, I would recommend testing the sensor off vehicle.

Several individuals advised me of a simple test. If you disconnect the sensor plug from the wiring harness with the engine warmed up and running and, when disconnected it makes a difference how smoothly the engine runs, then the sensor is good. No change does not mean the sensor is bad as other fuel injection could be present.

When working on the sensor, should you accidentally short out the sensor output wire, do not be alarmed as this normally does not harm the sensor, it merely grounds the output voltage to zero. Once you fix the wire, the circuit should operate normally. DO NOT apply voltage to the sensor.

TESTING PROCEDURES ‑ OFF CAR

Disconnect the sensor plug from the wiring harness. Raise the vehicle and support it securely on jack stands. Spray some liquid wrench, WD 40, etc., carefully around the base and threads of the sensor. Unscrew the sensor from the exhaust using some care and patience (warm the engine, if necessary to facilitate the removal, being careful not to burn yourself on the hot exhaust parts). If you can use a wrench, great. If not, I would highly recommend the use of O2 sensor socket ($10.00, pep boys, auto zone, etc.). Make sure you do not damage the threads or break off the sensor. With the sensor removed examine the shield over the sensing tip. A black sooty coating indicates a rich mixture; A brownish tan coating indicates lead contamination and a powdery white coating indicates silicone contamination.

You will need a propane torch, vise grips (or you can clamp the sensor in a vice) and your voltmeter. Clamp in vice or use vise grips to hold sensor. Connect the negative lead of the voltmeter to the casing and the positive to the sensor output wire, terminal 1. Set the propane torch on high and use the inner blue flame directly on the tip of the sensor. You should see the voltage climb to .6 volts with 15‑20 seconds, then on to .8 volts in the next 45 seconds. If not the circuit is open internally or there is severe lead contamination. If it reaches these tolerances, remove the flame. The voltage should drop to less than ..2 volts in less than four seconds. If not the sensor is silicone fouled. If the sensors is maintaining the proper tolerances, heat for two minutes to observe any voltage drop. Sometimes, the sensor's internal connections will open under heat. In this case the sensor is bad. If everything checks out up to this point and you are getting rapid cross counts as you move the flame back and forth, the sensor is good. Put some anti‑seize compound on the THREADS ONLY and replace.

Any O2 sensor that will have a reading of .9 volts or more when heated and read .1 volt or less with one second of flame removal and will pass the two minute test is GOOD!

RECOMMENDATIONS

I received several replies concerning what other Porsche owners are using or have used. Some stated they shell out the money and purchase the correct Bosch sensor. Others have used the universal Bosch 3‑wire and have not reported any problems. Still others have used the Bosch part number 13913 (I believe this is the number used by Bosch to market their sensors under the Precision Tune logo). I also discovered that Bosch routinely changes their part numbers so this could be one of their newer part numbers. 13913 is a Ford sensor and fits 84‑86 Mustangs/T‑birds, 85‑86 Continentals/Town Cars and other Ford products.

The Ford sensor is also being used by several BMW owners as well as Volkswagen Jetta owners. The 10 digit number for the 13913 sensor is 0258003913. The 10 digit Bosch number for 944 is 0258003011. Price difference approximately $100.00. It should be noted that different sensors are set with different trip points, voltage settings and lambda settings as the heat builds in the sensor to facilitate either rich or lean bias depending on the application. Virtually all 944's should have the following settings at 600C:

VOLTS LAMBDA

.05 1.02

.25 1.00

.50 1.00

.75 1.00

.80 .96

.90 .90

.95 .80

Now let's say you purchase a standard 3‑wire Bosch sensor (E971‑9F472‑AA ‑ Bosch changing numbers again).

VOLTS LAMBDA

.25 1.02

.375 .99

.50 .95

.625 .92

.75 .88

.875 .85

1.00 .82

As you can see there is a difference between the two sensors. Obviously this will effect the performance of the vehicle, even though a change of ..1 lambda is quite small when you consider the normal range of a fuel system is from approximately 12.0:1 to 16:1 (air fuel ratio)

Optimal engine performance occurs at lambda 1.000, which equates to stoichmetric.

Bosch manufactures their sensors to come into lambda 1.000 at

approximately .3 volts. The curve is reduced at approximately .85 volts. Rich or lean bias is changed depending upon application.

FINAL THOUGHTS

Purchasing the correct O2 sensor for your particular make, model and year is ultimately the best way to go. However, it would be beneficial to obtain the Bosch part number from your current sensor and cross‑reference this number. One of the Jetta owners did this and found his Bosch part number was identical to the Ford number (13913) ‑ savings for this owner ‑ $90.00. Should you decide to use the Ford sensor or a universal 3‑wire, you will have to cut, solder and use heat shrink tubing to splice your old sensor plug to the new sensor. Make sure the connections are extremely clean and use at least an 80‑watt soldering gun.

Subject: O2 Sensor documentation

From: "McMullen, Russ" russ@so.co.lee.fl.us

I had included a small diagram showing the terminals for the sensor plug. For some reason it did not get posted. If you look at the sensor plug the top terminal (#1) is output signal, terminal 2, (lower left terminal) is voltage and terminal 3 (lower right terminal) is ground. If you look closely, you can see the numbers stamped next to the terminals.

Subject: Re: More about Holleys

From: hotrod@dixie.com (The Hotrod List)

> O2 Sensor setup, etc <

I drilled a hole in the cast iron exhaust manifold right before the outlet flange, and reamed it out until an 18mm anti‑fouling spark plug adapter was a hammer fit. I ground down the adapter until its inner end was flush with the interior wall of the manifold, and then welded it in place.

The O2 sensor I used was a freebie 1‑wire type. A 3‑wire is better for reasons I'll go into in a bit, but costs more. I picked up ground locally using a valve cover bolt through a loop soldered in the end of the ground wire, and connected the O2 sensor to its mate, solder and shrink‑wrap. I used some automotive "trailer wire" (the 4‑conductor ribbon used for hooking up auxiliary lights etc.) which seems to be taking the heat and grease reasonably well. The reason for taking a local (engine block) ground is to minimize any error induced by body/block return currents and any voltage offset that might develop from loads like A/C, lights if you used the dash as ground.

I am using a spare analog VOM to read O2 volts; it seems to be working well enough, even though I've been told that the O2 sensor needs a very high load impedance. This meter has about 100Kohms (50K/volt, 2.5V scale) and I think it's about on the edge of affecting the sensor output accuracy; if I switch to the next lower scale I get a slightly lower value reading indicating Zload sensitivity to me.

The reading ranges from 0 to 1V (approx), with 0 indicating lean and 1V indicating rich. 0.5 is supposed to be stoichemetric. When warm I seem to be pegged rich, or close to it, except for a "hole" during throttle transitions.

The one‑wire sensor depends on exhaust heat to activate it; the element has to be pretty hot to develop any voltage and low temperatures result in a reduction or absence of output. I find that there's not enough heat at idle to keep the sensor fired up. If I idle for more than a few seconds the output voltage starts to decay. The other thing is that it takes a while to start reading. For a tailpipe test rig I made, a 3‑wire was a necessity since by that point there's not enough heat ever.

For a permanent installation I think the best solution would be to use a small panel meter with a single‑supply op amp as a unity gain buffer and a calibration resistor to get 1V full scale. A shunt resistor from amplifier output to ground might help the amp in the low end. I tried cobbling up a bar graph display using an old LM3914 I had in the parts‑is‑parts tin but it seems to have developed an attitude. For some reason Radio Shack has stopped selling these although they still sell the bar graph displays. (James W. Swonger)

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Subject: O2 sensor, unplugged

From: RossEdmond@aol.com

My 944na failed emissions last week due to a faulty O2 sensor, so I went out and bought the Mustang 3‑wire unit, a soldering iron, etc. However, when I went to put in the new sensor, I found that the old unit is seized into the front exhaust pipe. . .

With the old (*@&$!!#) sensor unplugged, the car passes emissions and seems a bit peppier. What are the disadvantages of the status quo? Will I cause damage to the motor? How much will my gas mileage suffer? Have I stumbled upon a cheap means of increasing HP? Ross Edmond, '87 944na

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Take the car out for a short drive, then try removing the sensor with the exhaust system good and hot. The metal will have expanded with heat, possibly allowing the sensor to unscrew. This worked for me the last time I changed a sensor on my Golf. Gregor 944 2.7

Subject: O2 sensor (yes again)

From: Qassim Moolla qmoolla@gpu.srv.ualberta.ca

The O2 sensor appears to be ill though. If you hook up a digital voltmeter to the output pins, the reading stays at 0.69 volts and does not cycle through the 0.2 to 0.7 volts as befits a healthy sensor (or so I was told).

Now the questions:

1. Will a non‑functioning O2 sensor affect/retard boost?

2. What else can I look for, assuming the sensor is not the culprit?

NAPA has a generic O2 sensor, p/n OS‑203‑SB (not Bosch but made to fit 84/86 Mustangs and TBirds or 85/86 Lincoln Continentals with 302 engines) all for the princely sum of $45 CDN). I will not know if it fits the 951 until I take my own out of the car next week. (dealer sensor @ $287 CDN)

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Subject: Mustang O2 sensor in my 944, update

To: qmoolla@gpu.srv.ualberta.ca , russ@so.co.lee.fl.us

From: "Ezra D. Hall" ehall@btv.ibm.com

Ok Folks, it has been two months since I replaced my O2 sensor with the Bosch 13913 Ford Mustang Sensor. I have waited on posting my results as I wanted some time to make sure everything was working well.

Vehicle: '86 NA 944 236,000 miles, '87 engine, K&N, Aftermarket Chip

Gas Mileage:

o Bought the car last fall, the O2 sensor was clogged or otherwise not working and was disconnected. ~17mpg with it disconnected, no better with it connected (and the car didn't run well either).

o With the new 13913 sensor I now get ~26mpg with similar driving (recent trip through Maine on back roads with lots of passing at WOT)

Power:

I can't say there is any power loss as WOT, but at partial throttle positions, I feel a stepping as engine power increases. This stepping behavior is not present with the O2 sensor disconnected. I have yet to try the stock chip, could be an artifact of poor mapping with the aftermarket chip (installed by the PO, I wouldn't have wasted my money).

Overall, I would have to say it was a successful replacement. I do not have the equipment to test emissions or Lambda ratio, so the engine may not be operating in closed loop mode at the correct Lambda ratio. In Russ McMullen's post at the end of August, he stated that various Lambda sensors have different outputs at a given Lambda ratio. I believe Russ included a table for the stock unit, but not for the Mustang unit. If I knew how far off the curves were, I could possibly tweak the mixture bias to compensate.

I would recommend using the Mustang unit as a replacement to save some money. It only requires some minor soldering to connect the stock wiring to the new unit. If anyone has questions, don't hesitate to send me a note.

Ezra Hall, ehall@btv.ibm.com

Subject: Edelbrock air/fuel ratio monitor

From: hotrod@dixie.com (The Hotrod List)

I just bought the Edelbrock air/fuel ratio monitor and hears what I discovered. The sensor is a Bosch PN E971‑9F472‑AA (a lot of numbers stamped on it, I think this ones the Bosch part number). I believe this to be a standard three wire conventional O2 sensor though admittedly I do not have the manufacture's literature. The "little black box" is nothing more than little and black. Inside there is a surge suppresser, a filter capacitor, one needed and one redundant resistor to control the LEDs brightness, and a LM3914 Dot/Bar Display Driver. The design is straight out of the application books with no creativity. Did I mention the lack of input signal conditioning/filtering to remove engine/ignition noise? Given the speed of the LM3914's comparators, this is a problem and certainly a problem in my implementation.

The electronics clearly runs the sensor in the voltage mode drawing only a 25nA biasing current for the LM3914's internal buffer. The sensor voltage and air/fuel ratio have the following corresponds:

volts air/fuel Lambda

(open circuit)

=====================================

.250 15.0:1 1.02

.375 14.5:1 0.99

.500 14.0:1 0.95

.625 13.5:1 0.92

.750 13.0:1 0.88

.875 12.5:1 0.85

1.000 12.0:1 0.82

From what I have read in the SAE Transaction pertaining to O2 sensors, this relationship is "bull shit". :( I can not believe this mode of operation would let the sensor go as low as .8 lambda let alone have this type of linearity.

I would have at least expected Edelbrock to use a current mode of operation. My next step is to change Edelbrocks "little black box" into a current mode of operation and call NGK in hopes of finding an affordable UEGO.

John S. Gwynne, jsg@magnus.acs.ohio‑state.edu

‑‑‑‑‑‑‑‑‑‑-----------------------------------------------------------------------

[Cyberdyne makes an in‑dash A/F indicator that lists for $29 in Summit's catalog. Don't pay a dime more. Many of these things are labeled with wide range A/F ratios. Since the standard O2 sensor covers a narrow range around stoich, this labeling is fraudulent. As long as standard production O2 sensors are used (UEGOs sensors by themselves cost several hundred bux), all the displays must work the same. If you want an analog readout, a simple analog meter with an op‑amp buffer does the trick. The voltage range from the sensor is 0‑1 volt at high impedance so gain isn't necessary; only buffering.

For references, see the archives plus SAE's "Sensors and Actuators" publication from the last few years. More info than you can stand. Remember though, lead‑free gas only. JGD]

emory!dcmdc.dla.mil!xgg3511 (Roger Hensley)

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Subject: Air / Fuel Guage results & Thanks.

From: hotrod@dixie.com (The Hotrod List)

Just a word to say thanks to all those people that helped us out on our Air/Fuel meter question I posted a few weeks ago. We bought a couple of Cyberdyne Air/Fuel Gauges from SUMMIT for $30.99 and installed a BOSCH O2 sensors at each collector on my friends (Andy) Datsun 240Z SCCA ITS race car.

During warm ups at Grattan raceway on 04 September both gauges read two bars too rich (what ever two bars mean). The car was brought in and the carbs leaned out. Back on the track the gauges read center or stoch. and Andy's times improved down the straight and throughout the course. Saturday Andy qualified second in class behind a 280Z. On Sunday the gauges were still reading stoch and Andy won the South Bend regional race being chased by the 280Z. On Monday he qualified for the Western Michigan regional races 3rd overall and 1st in class in the rain. The Cyberdynes were now reading two bars too lean; however, we did not readjust the carbs because the race was scheduled to be run later in the day and the forcast was for clear skies. I figure the cooler temps were making the air a little denser thus the reason for the lean condition. The guess was correct. During the race the skies were clear and temp was warming up to what it had been Saturday and Sunday. Andy won the Monday race again being chased by the 280Z.

We learned a lot using the Cyberdynes. They aren't perfect but do give an adequate indication of what is going on. By the way we used 75% HI OCTANE unleaded fuel and 25% leaded Racing Fuel. It didn't seem to bother the O2 sensors. Anyway Thanks a Bunch!

Roger Hensley

Patriot Motorsports

[The guys down at Ga Tech F‑SAE team have been using lambda sensors with leaded fuel for quite some time. They were fortunate to have had GM donate a whole box full of sensors to the effort. They tell me the sensor will last for a few hours of dyno running. The first indication of poisoning is sluggish response. Sounds like yours should be good for a couple of events. JGD]

Posted by: emory!dcmdc.dla.mil!xgg3511 (Roger Hensley)

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Subject: Re: custom O2 gague (& others)

From: hotrod@dixie.com (The Hotrod List)

In message m0omSNo‑0000JaC@dixie.com, you write:

>These questions deal with the air/fuel mix sensor (like Cyberdyne's in the >summit catalog): ‑ where does the sensor need to be located, i.e. real close to >the engine (reporting on just one cylinder (and hoping it is 'typical')), or can >it be put down on the collector, so it would report on how the cylinders (as a >whole) are doing? How does the placement affect the accuracy/linearity (or what >else?)

----------

Sensor location is important for the non‑heated EGO sensor. Too close to the engine and it will become too hot and shorten the life expectancy. Too far way and the sensor may not stay warm enough to work at idle. Location is not a problem for the thick film heated sensors (TFHEGO), and placement in the collector is fine.

==========

>Does the sensor come with a full spec sheet (i.e. what the voltage or resistance >is for different a/f ratios, or can one be obtained?

>Which leads me to the thought that brought on the questions: I want to make a >small 'gage' for a/f ratios (i don't like the look of the Cyberdyne unit), with >5 lights or LED�s on it:

> 1) red ‑> way too lean

> 2) yellow ‑> fuel‑economy cruise (slightly lean)

> 3) green ‑> stoich

> 4) yellow ‑> acceleration (slightly rich)

> 5) red ‑> way too rich

>As far as I know (which may not be much:) ), all of these gauges for under $150 >that claim to cover such a wide A/F range (specifically on the rich side) ARE >bullshit... I bought the Edelbrock air/fuel monitor (see post end of March/93 >in the archives ‑‑ ece.rutgers.edu) and traced out the circuit. It uses a TFHEGO >in the voltage generation mode which can really only indicate 2‑states >(rich/lean). Needless to say that I was not happy and you should not waste your >money on that one.... Before you buy one of these, run down to that auto parts >store and get a generic 3‑wire sensor.

>To build a wide‑range Air/Fuel meter, you need something like a UEGO sensor >(again see the archives) that would use a oxygen cramming method or oxygen bias >method (see SAE journals). This can cover the range .6<lambda<1.6. I checked >into buying one of these from Horiba, but they wanted $900. I now know Ceramic >Sensor Co. makes this for them, but I don't know how much it would be to buy >direct (I would guess $500). John S Gwynne, Gwynne.1@osu.edu

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Subject: Honda LAF sensor

From: hotrod@dixie.com (The Hotrod List)

The wide‑range O2 sensor used in some of the Hondas is called an LAF sensor in England, UEGO over here. There's a chart in the October 1991 Car Design & Technology which shows the response curves of a conventional Lambda O2 sensor and the Honda LAF sensor

The LAF's response isn't linear, as depicted on the chart. It starts at about ‑5v at 10:1 A/F, moving up sharply to about ‑1v at stoich, then progressing slowly up to maybe +2v at 25:1.

Posted by: emory!chaos.lrk.ar.us!dave.williams (Dave Williams)

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Subject: Re: Determining AFR from O2 Voltage readout

From: jws@billy.mlb.semi.harris.com (James W. Swonger)

A non‑heated sensor will behave as described if driving a low impedance load. This is due to the low exhaust temperature at idle, which reduces the current available from the cell. There will be an error induced, which is less into a higher impedance DVM than a resistive input such as a VOM (I have used both and

observed this directly). The bar graph indicator may have an input resistance low enough to make this noticeable. On my van, I could see the sensor output voltage indication drift downward at about 100mV/sec after dropping onto idle. Eventually

it would hit the floor, just "go out". Revving it for a few seconds would light it back up, and cruise was enough to keep it hot. With a very high input impedance meter, the indication remains steady even at idle.

You can substitute a heated sensor to get around the problem of inadequate exhaust temperature; three wire Bosch sensors can be obtained cheap at the AutoZone or whatever, if the future high school graduate behind the counter can be made to understand the concept of reading a parts list. I used a heated generic sensor to make a slip‑on tailpipe sniffer.

You could also try using header wrap to maintain exhaust temperature up to the point where the sensor is located.

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new air/fuel ratio meter

>Accel

>8700 Brookpark Rd.

>Cleveland, OH 44129

>(216) 398‑8300

http://www.cyberauto.com/info/o2sensor.htm

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Subject: Re: O2 sensor

From: "Christopher Hanlon" cwhanlon@eai.com

To: angel1@peace.oaktree.net

The o2 sensor is on the pipe coming from the turbocharger (on the driver's side).

At first it seems rather far away from the exhaust valves, but it makes sense to have it that far away. It is at a place where all the exhaust waves are practically canceled out (which you want with a turbo), so the o2 sensor receives a steady flow of exhaust. christopher hanlon

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Subject: Oxygen Sensor Fixes Jerky Acceleration

From: "Peltier, James D (NM75)" james.peltier@das.honeywell.com

Date: Wed, 16 Jul 1997 08:27:33 ‑0600

A couple weeks ago I wrote that my 86 944 was not accelerating smoothly in the mid RPM range at partial throttle. After testing every ignition component, I checked the throttle position sensor that had been suggested by someone on the list. They were all fine. One person suggested that I check the oxygen sensor in the exhaust. So as a last resort I unplugged the sensor from the engine wiring harness in the engine compartment. I went for a test drive with it disconnected; it was smooth and pulled stronger than ever.

The Bentley manual for my old BMW 325I has a very good section on oxygen sensors. BMW suggests replacing them every 30,000 miles. It also describes how to test the sensor using a multimeter. I have not performed the tests yet, but I hope I just need to clean it, because oxygen sensors are expensive.

Writing about expensive: I have started buying all my parts from my local Porsche dealer here in Albuquerque, because they have been cheaper than Automotion, Tweeks and Performance Products.

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Subject: Re: power surging

To: STUA68A@prodigy.com (DR JAY R WEISKOPF),

From: Jeff Mayzurk jeffm@eonline.com

At 10:09 AM ‑0500 6/23/97, DR JAY R WEISKOPF wrote:

>Have 1988 944 Turbo S, excellent of boost and full boost performance; however, >has gradually developed surging‑like response at partial throttle...car has >quite a few modifications... Thought this was related to O2 >sensor...disconnected it, and the performance again became AWESOME...replace it, >but some surging still persists...any ideas?

----------

Sounds like a bad oxygen sensor. When you say "replace," do you mean with a new oxygen sensor, or only that you reconnected the old one? If the latter, you need to buy a new one.

The sensor definitely has an influence on your mixture at part‑throttle and idle, so it could be the cause of your surging problem. At WOT the mixture goes full‑rich anyway, and any input from the O2 sensor is ignored. ‑Jeff

Subject: 02 Sensor test.

From: Farzaan Kassam fkassam@direct.ca

>As I am still running the Ford O2 sensor, what kind of O2 sensor testing did >you do?

----------

We tested whether to have them plugged in or not. On stock type of chips, O2 unplugged produced power and brought the boost in faster. On APE cars, O2 disconnected caused lower power and much lower torque. Farzaan.

Subject: RB Racing Air/Fuel Meter info...

From: Davidjalai@aol.com

To: BadBob951@aol.com, ck930@hotmail.com

A couple of list members wanted to know which air flow meter I used and how to install it. I use a RB Racing digital fuel/air ratio meter. RB Racing does a lot of land speed record motorcycle stuff & serious hi‑per hot rod projects. The main thing I like about the meter is that the LED's are calibrated for each particular set of operating conditions. The LED's are also color‑coded to let you know exactly what the engine is doing ‑ after all it is hard to Read & Drive at the same time! The meter reads from 17.1:1 (lean) all the way up to 12.1:1 (rich). The LED's progress from green ‑ thru yellow ‑ to orange ‑ and lastly to red LED's. I know for instance that with AutoThority's V2.7 Chips & a test pipe, that when I accelerate hard, the meter shows the first orange (13.8:1), then the second orange LED (13.2:1) glows once you are on full boost. Then the first red LED just starts to glow. This meter, I think shows the operator on the fly what the fuel mixture is by color of the LED. You do not have to look at a chart or read the printed scale on the gauge while you're driving! You can contact RB Racing for more info at: 310‑515‑5831, they are located in Gardena, CA.

The meter installs fairly easy. I just hook it up temporary by building a small electrical harness ‑ power leads to the car's battery, and one wire to the O2 sensor. I tapped into the O2 sensor where the sensor plugs into the engine main harness ‑ to the rear of the intake manifold by the oil filler cap. There is a three-wire plug ‑ round in shape. The two white wires are power supply for the heating element ‑ you want to tap into the Black wire ‑ O2 sensor feed. I used a "scotch‑lock tape so I could later remove it. I just hook everything up and route the harness under the hood, by the battery, around the passenger window/door. Davidjalai

Subject: RB Racing's RSR Air/Fuel Meter

I have been getting a few questions regarding my results from using a Air/Fuel Meter. I have a 1986 944 Turbo, stock, with 160K miles, only mods are: AutoThority Stage II chips V2.7, a test pipe, K&N cartridge air filter, and a new O2 sensor. I used a air/fuel meter from RB Racing ‑ they make a lot of custom turbo kits for motorcycles. Some of their customers even go out to the salt lakes to attempt land speed records. RB Racing sells a air/fuel meter to calibrate their custom programmable fuel injection systems. The thing I like is the meter comes with a detailed 7 page "tuning manual" on how to read the meter. The manual is written for turbo charge engines perspective ‑ but is useful for NA's also. If any one would like a copy let me know.

The RSR air/fuel ratio meter displays digitally via ten L.E.D.'s your exact air/fuel ratio. There are ten LEDs divided into four color divisions: (Left to Right: Lean (17.1:1) to Rich ( 12.1:1)), three green, four yellow, two orange and two red LEDs. You can tell at a glance what the air/fuel ratio is by looking at the color of the LED. The chemically "correct" point at which the most complete combustion takes place which is 14.7 parts of air to one part fuel by weight. This is also refereed to as "stoic" or 14.7:1 air/fuel ratio.

My results with the RSR meter. On hard acceleration the meter goes up to the first LED, ie 12.7:1 air/fuel ratio. On part throttle or steady state throttle the meter reads last yellow (14.4:1) & first orange (13.8:1) or around 13.2:1 air fuel ratio just what it should be.

Now for my car, I do not see how more fuel could be use full. In fact when I installed a higher fuel pressure reg. last year ‑ The meter indicated one LED. Higher readings all the time. On full throttle accel the meter went all the way to the last red LED or over 12.1:1 air/fuel ratio. As you recall I observed a slight increase of power seat of the pants wise (who knows may be I felt the engine not revving up quite as fast?) but at part throttle the higher fuel reg. killed power! Davidjalai@aol.com

Subject: RE: O2 Sensor

From: ATTDCS!TMBWPO1!sjagernaut@dcstm1.attmail.com (Stephen Jagernauth)

To: mleeber@swspectrum.com (Mark Leeber)

The idle on my '89 951 is jumping from 700RPM to 1100 RPM like someone is pumping the accelerator. This just recently started and I understand that it could be a bad O2 sensor. I also understand that the O2 sensor is connected to the wiring harness at the back of the intake manifold. I was going to disconnect the sensor as a test but there are two electrical connections held by a stalk mounted to the rear of the engine. Which one is the O2 sensor, the rearmost or the one closer to the front of the car? Mark, '89 951

----------

It sounds like a vacuum leak or false unmetered air. Check all your vacuum hoses. With the motor running, spray some carburetor cleaner on hoses. When your idle stabilizes for a moment, the spray has temporarily stopped the hole and you've found your leak. Due to high underhood heat, hoses may become brittle or porous over time.

BTW, you don't need to remove the O2 sensor to test it. Use a digital VOM and a pin to probe the wire in the pigtail. It should read 0.2V cold and should oscillate quickly from 0.1 to 0.9V after the engine heats up and goes closed loop. If it's dead, it'll probably be stuck at 0.2V. If it changes slowly, it's 'lazy' and should be replaced. Steve Jagernauth

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Subject: Re: O2 Sensors

From: "Ezra D. Hall" <ehall>

To: THE GOMBERG FAMILY wildw5@flinet.com

It was a bit difficult to remove the old O2 sensor. I used lots of WD40, and a large crescent wrench. There really isn't any room for an O2 sensor socket unless you drop the exhaust.. If the large wrench hadn't worked, I would have tried heating it with a torch. The wiring wasn't too hard to remove, you might want to pull a string or wire through while removing the wiring harness to make re‑routing easier.

I clipped the 13913 leads as close to the supplied connector as possible to maximize the length of the leads. I left a small length of wire leading from the old O2 sensor just in case I ever decide to do something with it in the future. I suggest staggering the length of the wires such that when the wires are soldered together, the joints do not touch. This prevents potential shorts from occurring if the heat shrink tubing I used to cover each joint happens to split or otherwise loose integrity. I pulled back the silicon/fiberglass heat resistant shroud as much as possible while soldering, and slid it back over the splices that I made when done. I used an 80Watt Weller soldering iron and high quality rosin core solder, make sure the wires completely wet with solder, you don't want to do re‑do it later due to a cold solder joint... Oh, I think there were two black wires and a white wire, or vice versa. The two wires of the same color can be connected either way, polarity does not matter. Ezra

Subject: How O2 Sensors Work.

To: lumipor@prtc.net, ehall@btv.ibm.com ,

The O2 Sensor only works or feeds into the ECU (Electronic Control Unit) once the sensor warns up ‑ that usually takes 45 ‑ 120 seconds. Once the sensor is hot enough for the precious metals to generate a voltage signal the ECU will use this signal to tune the amount of fuel being fed into the engine. A sort of on the fly tune‑up. When the O2 sensor is disconnected or upon full throttle (WOT) the ECU goes to full rich preprogrammed condition. This is why a rough idle, due to a bad O2 sensor, will clear up when the sensor is disconnected.. The ECU will go to full rich, and the power and fuel mileage will suffer. Well the basic function of the O2‑Sensor is to monitor the oxygen content of the exhaust gas and it works producing a voltage output signal that aids the control unit in the determination process of a correct air/fuel mixture ratio thus controlling the pulse of the fuel injectors. All systems working correctly and a closed loop situation is met. Various factors may have a negative effect on this sensor. When there is a vacuum leak or an exhaust leak before O2 sensor, the O2 sensor can see a lot of extra unmetered air, the ECU reads this as a lean mixture (when it is not) and adds more fuel ‑ by increasing the injector duty cycle. If there is a vacuum leak, the O2 sensor sees extra air and ECU will richen the fuel mixture. When there is a exhaust leak, between compression pulses, some unmetered air is sucked into the exhaust and travels on down to the O2 sensor. Same effect ‑ The ECU will richen the fuel mixture. This could indicate that you have a leak in your exhaust system between your header and O2 sensor, possible suspect being that exhaust test port tube that runs up into the engine compartment at the rear of the manifold. If you do have such a leak, air is being sucked into your exhaust stream causing your O2 sensor to sense, you got it, more O2. The ECU would then increase injector pulse width to richen the mixture causing the rough idle.. Is this the little tube w/ the gray cap sealing its end? I was under the impression that the little cap gets some exhaust pressure (i.e. positive pressure, not vacuum) thus I would not expect it to suck air and present the 02 sensor w/ extra 02. Is that so?

Otherwise it seems that the lack of the cap could confuse the oxygen sensor. A exhaust leak on a turbocharged car can confuse the O2/ECU. The CO test pipe (little tube w/gray cap, to the rear of the exhaust headers) can cause a pressure leak under boost (because it's between the wastegate & cylinder head) and also cause a vacuum leak under part or closed throttle (idle) conditions. On my 951 last year the CO test pipe shattered along the seam ‑ causing a leak. BTW ‑ the CO test pipe cost something like $160, I simply removed the old pipe and installed a spare end cap nut (part #928‑110‑475‑01, for $2) from the fuel filter

area I had in my tool box. Since I have a test pipe instead of a cat, I can still set & adjust my CO%. DavidJalai@aol.com

Subject: O2 Sensor

From: "THE GOMBERG FAMILY" wildw5@flinet.com

Installed the Bosch 13913 oxygen sensor today ($48.99 NAPA Auto Parts). Removal of the old sensor was easier than expected. 22mm crescent wrench ‑ no problem. Good advice from Ezra Hall re: attaching a string to the plug so that, when I pulled it down from the engine compartment, I had an easy route back. The parts are physically almost identical and the numbering is similar. The soldering of the wires was no problem ‑ again per Ezra Hall, the "joints" were staggered to avoid bare wire contact. Installation is somewhat difficult due to the tendency of the three foot wire and plug to want to coil which provides an energy force that wants to unscrew the plug as you screw it in. Finally, "balled" the wire up so that I could turn the wire and the plug.

To recap, I have an 87 944S and I have been complaining for more than a year about idle related problems (stalling after high speed trips); excessive "hunt" when decelerating quickly...etc. Last week I simply unplugged the O2 sensor and the problems abated and the car ran great.

With the Mustang sensor, the performance is not nearly as bad as it was, but not as good as with the sensor unplugged. I do not believe this is due to using this sensor, but rather suggests other related problems (car has 114,000 miles). I have not had the time to measure gas mileage, but expect an improvement.

Subject: re: 85.5 944 emissions

From: 944 turbo gt7300c@prism.gatech.edu

>As for Auto Atlanta telling you that there's no adjustment to help, I'm >surprised and somewhat disappointed since there *is* an adjustment screw that >affects the CO levels. Actually, there's two of them! One screw that could >possible affect your CO setting is a 7mm bolt that sits near the throttle body. >It's set in what looks like an epoxy resin and it changes how much air bypasses >the throttle plates. This, combined with the throttle plate screw, can be used >to vary the CO content while keeping the idle approximately the same.

>The second screw is located on the air flow sensor and is described (with a >picture) in the book, "Bosch Fuel Injection & Engine Management" Chapter 4, >section 7.1 (a very informative book, by the way). The book describes this >second screw as a "mixture screw". Turning the screw clockwise will richen the >mixture, while turning counterclockwise will lean the mixture...

----------

In VA, an emissions tests has to be done every time a car is bought in order for the DMV to process the title, tags, etc. After buying my first 944, it promptly failed the emissions test because of excessive CO levels, and I immediately took it to my shop. They opened the hood, and adjusted a screw while monitoring CO levels from their own equipment. No labor charge because the whole thing took less than 3 minutes. I confidently went back to retry my emissions, and I passed with flying colors: .01! For a first‑time Porsche owner, this quick and free fix was really assuring! Ben

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Subject: re: oxygen sensor

From: "eyecare1" eyecare1@datastar.net

I got it at Autozone. They pulled up the 944 on the computer. Two sensors were listed. The OE one, which is the same but has the complete lead and connector was not in stock. This is the one that you can get from Automotion for $198. Carquest had this one for $75. The second one was listed as universal. Made by Bosch in Germany, assembled in US. Joe

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Subject: O2 sensors

From: "Gomberg's" wildw5@flinet.com

To: BadBob951@aol.com

I am one of those who used the Bosch 13913 three-wire sensor from NAPA ($59.95). It does require splicing the old connector to the new sensor. I must say I haven't noticed extraordinary results, good or bad. I guess its working, and it was cheap!! Steve Gomberg, 1987 944S

Subject: Re: O2 Flow Meter? Where is it?

From: "Lee C. Archie" larchie@lander.edu

To: "Bonkoski, Lawrence K. (IA)" BonkosL@ia‑arng.ngb.army.mil

Generally speaking, if the only symptom is a rough cold idle, then the troubleshooting procedure would be as follows in the following order for Motronic injection:

(1) Vacuum leak‑‑check for leaks (which you've done). Check also PCV and crankcase connections and evaporative emission connections.

(2) Idle system‑‑test auxiliary air regulator and idle speed stabilizer

(3) Engine temperature sensor faulty‑‑test sensor

(4) Air‑flow meter faulty‑‑test air‑flow meter

(5) cold start system faulty‑‑test cold start valve and thermo‑time switch

(6) Fuel injectors faulty‑‑test

(7) Insufficient fuel pressure or delivery‑‑test

Before testing the fuel injection system, you should probably test the following basic engine functions:

(1) timing

(2) fuel filter

(3) electrical‑‑disconnect the battery‑alternator wire and start engine; if the idle is not rough, then bad alternator diodes or a faulty regulator are causing voltage spikes which give the control unit false information and cause the engine to miss.

Mr. Haynes is pretty clear on all of the above tests. If you have a specific problem not covered by Mr. Haynes, then the list will be glad to provide more information. Lee, larchie@lander.edu

Subject: O2 Sensor Good News!!

From: "Donna and Jason from Home" jasonmaynard@worldnet.att.net

For anyone who suspects that their O2 sensor is bad and causing hesitation, rough low speed cruising, and intermittent power like I experienced with my 944S, read on.

I Just got back from a high speed, high G romp through some backroads and wanted to share my good news regarding my O2 sensor woes.

In my previous submits, I was wondering about the O2 sensor for my 1988 944S from the dealership for $224 versus the Bosch #13913 I picked up at my local Napa store for $48.99. Turns out the one the dealer says if for 1987‑88 924S and 944S (part number 0‑258‑003‑011) is the same sensor as the Bosch retail one (part number 13913). The difference is in the length of the wire(s) and

the type of connector.

I first took my old sensor out of the car and bench tested both the new and old one using instructions I found at http://www.cyberauto.com/info/o2sensor.htm . The new Bosch one performed exactly how the information said it should. My old one did not. Very slow response, and it never got above .3 volts, even when glowing red hot, whereas the new one went to 1.15 volts in 18 seconds. The first thing I notices was how little effort was required to remove my old sensor from its flange, and then that there was a little bit of corrosion on the threads. Also I noticed that the bulb of the old sensor was clean and white/gray, but most of the slots in the bulb were closed with corrosion.

So, I took a small steel wire brush the size of a toothbrush and cleaned the sensors threads and the bulb slots. Then, I took a pocket knife blade and made sure the slots were clearly open exactly like the new Bosch one. I then cleaned the contacts in the connector and the male prod inside the connector was roughed up with a tiny piece of aluminum‑oxide sandpaper. I also noticed that the male part of the connector had a little slit in the end. I took the pocket knife blade and spread it open just a tad so it would get a good connection with its corresponding mate still under the hood. I repeated the bench test. To my GREAT surprise, my old sensor performed EXACTLY like the new Bosch one. I then did the "2‑minute" test as the instructions describe and it performed perfectly. I repeated all the tests 10 times just to be

sure. That baby was glowing red!!

I noticed that when I removed the new ones plastic protective cover, there was anti‑seize on the threads. So, I put anti‑seize on the newly cleaned threads of my old one and reinstalled it. I tightened it securely then did the pocket knife trick on the male parts of the connector end that was still under the hood. I put dielectric grease on the connector, plugged it back in... then... I fired her up.

After letting it warm up like I always do, I took off. I drove easy until reaching operating temperature... and they the tach needle went SouthEast (as it so easily does in a 944S). And again and again and again. It ran perfectly! I lugged it up hills in 5th, and everything in between. No more hesitation or stumbling. I easily reached 130mph on a long country back road that I could only ever get to 120mph on before (no one was around of course).

So, I put the new Bosch sensor back into the box untouched and promptly returned it for a refund.

I have 88,000 miles on my 944S and likewise on the old sensor. I would recommend this experiment to anyone before shelling out the dough for a new one, especially if they want to kick out the $200+ for the factory one instead of splicing wires on your old connector.

I was so happy with the results, I thought there HAD to be someone out there who was experiencing the same symptoms as I was. If anyone has any questions I would love to help out. Jason Maynard, 1988 944S, Zymol Guards Red

Subject: Hesitation.

From: Farzaan Kassam fkassam@direct.ca , 5/29/97

>3. during a smooth normal acceleration, I can feel very slight hesitation in >engine as I accelerate, but in very hard and aggressive acceleration I don't get >any hesitation and my speed increases very smoothly ‑ is this related to my >cable needing some adjustment ‑ I don't know what else could cause this, because >my car behaves the same way even after a major tune up service

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It sounds like your O2 sensor. Stand in front of your car, open the hood, look at the intake manifold. Beyond the intake manifold is a connector on a holder. Pull the plug apart, that's the O2 sensor plug. Take the car for a boot, let it warm up and see if the surging is gone. My O2 sensor does that, probably needs to be replaced.

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Subject: O2 Sensor, 6/27/98

From: Farzaan Kassam fkassam@direct.ca

>Is disconnecting the O2 sensor on an '89 951 with Stage 2 chips >a good idea for more HP or will the car run worse?

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On a stock chipped car, disconnecting the O2 sensor does not affect the power output, but it does reduce a lot of turbo lag. It also gets rid of that annoying oscillation as the computer tries to maintain a set A/F mixture.

On a Stage II APE equipped car, we consistently lost 5‑10 hp at the wheels with the O2 sensor disconnected. Why? I can't answer that. The O2 sensor is supposed to be ignored when at WOT and above 3000rpm, so I don't understand why it would be lower with it disconnected. Perhaps residual rich mixture causing problems.

A disconnected O2 sensor also gives the car a much smoother idle. Bottom line is, stock chips or stock based chips (such as mine), disconnect the sucker, aftermarket APE chips, leave it connected.

Subject: Re: <951> O2 sensor, 6/27/98

From: Larry Bershtein, Cyntex, Inc. lbershte@idsonline.com

The O2 sensor has no impact on HP. The DME does not look at it during full throttle (over 66 degrees throttle travel) operation. Given that the engine is perfectly happy, makes minimum emissions and maximum fuel economy with the O2 sensor hooked up, and has no impact on performance, you should leave it hooked up.

Subject: Re: Air/Fuel mixture sensor, 7/6/98L

From: Derrek Khajavi, Huntley Racing dkhajavi@flash.net

The O2 sensor has a 1 volt range. It is very difficult to measure as you are indicating but we sell an air/fuel ratio meter which is just as accurate as the $200 ones available for $40.

Subject: Re: O2 sensors, 10/2/98L

From: Derrek Khajavi huntleyracing@home.com

Actually the sensors are basically the same but without the fancy Porsche harness. If you are going to change over though I would get the 4 wire. The fourth wire is a ground signal wire that you run separately to the negative terminal of the battery. This extra signal wire is to keep the signal clean and helps the unit stay accurate over the temperature extremes of your motor. Our price is $70 on the four wire and we no longer sell the three wire for obvious reasons.

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Subject: Re: cannot remove oxygen sensor, 11/22/98L

From: DAVE superd@direct.ca

Buy a cheap 7/8-combination wrench. Cut the handle of the wrench so that 1 inch of it remains on the box end. Disconnect the O2 sensor wire and slip your now very short box end wrench on to the O2 sensor. Position the wrench handle so that it sticks out away from the exhaust pipe, hold it like this in one hand and tap on the wrench handle with a hammer in the other hand. The small impacts seem to unsieze it, works every time.

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Subject: RE: 944 O2 sensor question, 3/26/99L

From: Jack Vines JackV@ambassadors.com

The third wire, usually black, is the actual metering wire to the computer. The two usually white wires are for the preheater element to get the 02 sensor up to operating temp faster on cold starts. Without the black wire connected, your computer thinks your 02 sensor has failed and goes into default mode.

Subject: Re: O2 sensor

From: Douglas Briggs drbriggs@ix.netcom.com

This is not exactly true. I don't know why the A/F gauge drops for some people; mine doesn't.

The voltage to the O2 sensor is only to provide 12V to heat it up quickly so the emissions info is obtained quicker. Single wire sensors take longer to come up to temp because it relies on the exhaust to heat it up.

The Lambda element draws no power at all. In fact it varies continuously from 0 to 1V when hot.

ref.

"The lambda sensor is essentially a small battery that generates a voltage signal based on the differential between the oxygen content of the exhaust gas, and the oxygen content of the ambient air."

_Bosch Fuel Injection & Engine Management_, p.3-21.

Subject: O2 Sensor, 5/25/99L

From: John Schneible schneible@juno.com

Your car has a 3-wire o2 sensor. 2 are white (they are the heaters, you don�t need to worry about them) and one black. The black one is the signal wire to the computer.

If you have a voltmeter this is an easy process. Unplug the o2 sensor (there will be a plug somewhere along the wire) and connect the positive side of your volt meter to the unplugged end leading down to the sensor (black wire), attach the negative side of the meter to ground. It does not matter if you unplug the heaters or not.

Run the engine (it wont hurt it to run with the sensor unhooked for the time it takes to run the test) and watch the readings on the meter.

When the car is warmed up the meter will probably read around .450 volts at idle. Accelerate the engine to 3000-3500 rpms and let it drop back to idle.

The 02 sensor�s range should be from .05 volts to .850 volts. If the sensor is good you should have no trouble seeing this range through random increases and decreases in engine rpm. If it won�t move thru the entire range, its no good.

Subject: Re: Oxygen Sensor, 8/7/99 951

From: "Derrek Huntley Khajavi" huntleyracing@home.com

You won't hurt anything by running with it disconnected but you will be too rich and likely using way too much fuel. My suggestion is to replace the O2 sensor with a new unit as soon as you can. You can buy the factory unit for lots of $ or you can get one of the generic units for less money which require wiring in. I am of the belief that if it needs to be changed it should be upgraded if possible. So when we sell O2 sensors we sell the EGO1 4-wire which is more accurate than the 3-wire that our cars had stock. The EGO1 is $70 and there are several 1 wire, 2 wire, and 3 wire generic available between $20 and $100.

Subject: Re: O2 sensor, 9/9/99L

From: "Vaino Narma" vaino@narma.com

I just order the three-wire Bosch Universal for my 951. I don't have the part number in front of me. I paid $69 for it from www.europart-sd.com. I'll let you know how it works out. They have the actual Bosch Porsche OEM part for $135.

Subject: RE: O2 sensor test, 9/20/99L

From: Clifton Hipsher clifh@microsoft.com

This could be a real easy fix, just test the O2 sensor:

1. Disconnect the O2 sensor. Connector is on the top center of the firewall.

2. Connect the positive (+) lead of a Digital voltmeter to the sensor lead.

3. Connect The negative (-) lead to a good ground, like the negative battery terminal.

4. Set the DMM to read 0-2 vdc.

5. Start the engine and let it warm up.

6. As the engine warms up, the DMM should begin to display the output of the O2 sensor.

7. With the engine warmed up, the O2 sensor should have an output of at least .5 vdc. If not, the sensor is bad. Hint: Let the engine idle for a bit after it is warmed up (I always wait for the radiator fans to kick in.). O2 sensors have to be hot in order to function properly. Also, the reading may "jump around." This is because the DME is in "open loop" control, and is using a conservative input of .45 vdc to compensate for no signal from the O2 sensor.

Subject: Inexpensive Air/Fuel Ratio Monitors, 9/25/99L

From: "F.R. Wilk" frwilk@email.msn.com

I found an interesting Web page on inexpensive Air/Fuel Ratio Monitors.

http://idt.net/~gdonovan/o2gauge.html

You can by in with Cyberdyne 10 LED bar graph with 3 colors for only $33.

Uses existing Lambda sensor.

Installing an oxygen sensor gauge.

By Gary S. Donovan

Oxygen sensor gauges are a good way to tune any automobile. I have been using one (K&N) on my 1970 Plymouth GTX for 8 years now and it makes carb tuning a snap! How can you go wrong when there's a gauge staring you in the face? Beats trying to read plugs or guessing by a country mile. More important is it will keep you from burning up your Turbo car when your cranking da' boost!

How they work

An O2 sensor generates voltage based on the oxygen in the exhaust flow. Using this data your car's computer modifies the base pulsewidth of your injectors for maximum gas mileage and emissions. During wide open throttle however, the O2 sensor is ignored and the computer uses a factory preset pulsewidth for the injectors. Your gauge displays this voltage output in a bar graph: each "bar" represents .1 volts. .5 volts is a perfect 14.7 to 1 fuel ratio- perfect for max gas mileage that is. For max power, 12-11 to 1 is recommended (8-9 bars) Note: O2 sensors work in the 0.0 to 1.0 volt range.

Ok, I want one; who sells them?

Intellitronix/Cyberdyne ITC-TH7009/7009 $30

Note: has nice 10 LED bar graph with 3 colors. (round)

K&N 85-2442 (round) or 85-2439 (square) $50

Note: round is 2 color, square has just red. 10 LED

HALMETER AF30 $140

Note: 30 LED graph is nice but worth $140? (square)

Edelbrock EDL-6593 $126

Note: with O2 sensor and only 7 LED's (square)

I have 2 Cyberdynes and 1 K&N, I'm happy with both.

Oxygen sensors: more than one kind? and why?

83-86 1 wire sensor

87-88 3 wire sensor

89-present 4 wire sensor

In the beginning, there was the one wire O2 sensor; and it was good. BUT there was a problem with the sensor in regards to warming up- it was too slow. Oxygen sensors don't start working till 600F. So the engineers went back to the drawing board and came up with a new O2 sensor- the THREE wire. It had a heating element and a ground wire so that the sensor heated up right quick! BUT there was one more problem. The sensor itself was grounded by the turbo housing/exhaust manifold and it was throwing the sensor off by almost a tenth of a volt by excessive resistance. SO back to the drawing board again and the FOUR wire sensor was born. It had a ground just for the sensor!

1. one wire for sensor output (black)
2. one wire for sensor ground (gray)
3. one wire for 12V heating element (white)
4. one wire heating element ground (white)

That's why on some cars with one or three wire sensors the bar graph won't read 10 bars even when fuel is practically rolling out the tailpipe. They are losing that last .1 volt in the exhaust housing and/or wiring. I converted my 86 Turbo Z and my 85 K-car to the 4 wire sensors so that my O2 gauges were reading more accurately.

Installation

1. Pick a spot where you can easily see it.

2. Run power from a key on connection (the radio fuse is fine)

3. Run the ground directly to the firewall or even the battery!
Do not attach to any other ground wire or bracket!

4. On 84-87 you can tap the O2 sensor signal wire right off the logic module inside the car; it's pin number 18 of the logic module BLUE connector, it's the only SOLID BLACK wire.

5. 88-present you will have to tap off the wiring under the hood near the sensor. For 3 wire sensors there are 2 white wires and 1 black- it's the black wire. For 4 wire sensors there are 2 white wires, 1 grey and 1 black- it's the black wire.

I leave it up to you on how to connect your wiring; soldering is best, good tape and heat shrink tubing is also a real good idea here.

Modes of operation

1. Key on, engine off; 1 bar or less (duh)!

2. Engine running first 1-3 minutes; bar will slowly climb to 9-10 bars (.9 to 1.0 volts) after the computer sees .9 volts, it will start O2 feedback. If it's VERY cold out it will not begin O2 feedback till the engine warms up a bit regardless what the O2 sensor is reading.

3. Engine at normal operating temp; at cruise bouncing up and down fairly quickly. At idle will be about the same, maybe a shade slower.

4. 3/4 throttle, normal temp, 4-7psi/boost; will rise to 6-8 bars (.6 to .8 volts), no bounce.

5. WIDE OPEN THROTTLE!, any temp, 1-14psi/boost; 9 bars (.9 volts), no bounce.

Troubleshooting

1. Gauge blank, no lights of any kind; check power, check grounds.

2. Have rich/lean/air fuel light up, but no bar graph; bad connection to O2 sensor, bad O2.

3. Bounce very sluggish at cruise and idle; O2 sensor on its way out, replace.

4. Only reads 8 bars (.8 volts) or less at WOT; poor ground to gauge, poor ground O2 sensor, lean fuel mixture, bad sensor. Note: See below on sensors.

5. Bar graph rises to 10 bars (1.0 volts) and stays; voltage leakage in O2 wiring or bad sensor. Note: Oil leakage onto sensor plug will do this.

How to tell if my grounds or a sensor is a problem (low bar graph readings)

Start the car and let it idle. Pull off the inlet hose at the throttle body and enrichen the engine by spraying a fuel into the throttle (BE CAREFUL!). Carb cleaner or some propane (not lit I hope) from a shop torch works fine. Practically stall the engine out, have someone observe the gauge when you do this. It should spike to 9-10 bars. If not:

1. 9 bars with a 1 or 3 wire sensor is fairly normal- see sensor section.

2. Less then 9 bars; Poor ground to gauge or sensor going bad. A bad sensor will also show itself as sluggish operation (slow bounce).

This test will eliminate your fuel system as a cause of a low gauge reading because you are supplying the extra fuel instead of the car.

A note from a reader...

You might want to mention a repair technique that works more than half of the time. Sooner or later someone will touch the orange O2 gauge wire to the 12V positive source by accident. Then all 10 lights stay lit all the time, after it's hooked up properly.

To fix it, you just need to touch all 3 gauge wires together (engine off, and power disconnected) and it'll work normal again when you hook it up and turn on the power.

I've done this about 6 times, and it worked about 4 times.

No, I'm not an idiot! LOL! I only wired a gauge wrong ONCE. The second time, I was going to do a quick underhood test of a gauge. I had connected the ground already, and the orange wire slipped out of my hand and swung down right into the positive battery post! The other 4 times were all cases where other people messed up, and I fixed most of them... Gus Mahon

Subject: Re: 4-wire O2 sensor, 11/1/99 951

From: "Tom M" tmgee@iname.com

Let's see, if memory serves, the factory connector has 2 white wires and a black. The 4 wire O2 sensor (from Huntley) has 2 white wires, a black and a grey. Connect one of the white wires on the sensor to EITHER of the white wires on the factory harness. Connect the other white wire on the sensor to the other white wire on the factory harness. (The white wires power the sensor's heater. The heater has no polarity so it does not matter which white sensor wire gets connected to which white harness wire.) The black sensor wire goes to the black wire on the factory harness. The gray sensor wire should go directly to the negative battery terminal. (If you have an ARM1, the brown wire from it should also go directly to the negative battery terminal.) I was lazy at first and did not connect these wires directly to the battery and just grounded them instead. When I got around to running wires to the battery, I could see a difference on the ARM1. (In addition to bringing the ARM1 readings up about 1 light on average, it also fixed a strange problem I had wherein my ARM1 was reading lower when my headlights were on. Now, I get consistent readings with my headlights on or off.) The dithering centers around 1, but actually stays below it at idle more often than above it.

Subject: Re: Exhaust temps, 11/24/99L

From: Dave superd@direct.ca

I've seen this phrase "lean is mean" mentioned before. I'd really like to know how. It's a fact, an engine will produce more power when tuned on the rich side (about 12.5:1) of stoichiometric (14.7:1). Our cars fuel mixture is held by the oxygen sensor to a fuel / air ratio of 14.7:1 This is done for emissions reasons. When you plant your foot on the throttle, the wide open throttle switch takes the oxygen sensor off line and injection proceeds on a preprogrammed richer strategy.

To Quote The Bosch Motronic technical instruction manual:

"Spark- ignition engines develop their maximum power at 5-15% air shortage (Lambda =.9) Maximum fuel economy occurs at about 20% excess air (Lambda = 1.15). Perfect idle operation takes place at about Lambda = 1, and good transitions at about 20% air shortage (Lambda = .8)"

In regards to good CO readings, The ideal reading of pre-catalytic CO for emissions would be 0.5% CO. This is required for NOx reduction in the cat. A good reading for maximum power output in my opinion would be about 2 - 3% but this reading will change significantly with amount of ignition advance the engine is running / will tolerate.

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Subject: exhaust gas oxygen analyser, 12/7/99L

From: "Martin Taylor" martin.taylor@clear.net.nz

Hey everyone a few of you asked me about these kits a while back, here are the kit details off the web page If you are interested go to this page

http://marketplace.xtra.co.nz/cgi-bin/dse.storefront and type EGO analyser in the search category, then click on the product for more details. The product is being discontinued as most cars in this country don't have an O2 sensor, nothing old at least. I think the prices are in NZ dollars, 1= 0.5 US$

Monitor car's engine performance through the Exhaust Gas Oxygen (EGO) sensor. Interfaces with your PC to display results.

Features: This low cost analyser combined with any IBM PC and some special software can provide a large amount of information on your cars performance. This circuit is designed so that it connects to the exhaust gas oxygen sensor that is located on the engine manifold. The sensor produces a voltage that is proportional to the oxygen percentage in the engine�s exhaust. The unit will then monitor this voltage to determine whether the engine is running rich or lean and even how tightly the engine control unit is controlling the engine. The kit will be supplied with all components, hardware, PCBs, case pre-punched front panel and software.

Cat No. K4214

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Subject: [951] Re: O2 sensor bosh part #, 4/6/00

From: "Jason Burkett" jason@paragon-products.com

951.606.135.00 or Bosch #13012. Or you can go with the generic Bosch 3-wire unit and splice on your connector.

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Subject: [951] Re: Which is the signal wire on the 02 sensor, 4/27/00

From: John Anderson blackbox@san.rr.com

Its the black wire on a stock 3 wire Bosch unit.

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Subject: [951] Oxygen Sensor, 7/1/00

From: "Claus Groth" bora450@gte.net

I have some important information for many of you. Bear with me for a minute while I give you a little background.

I have an '86 951 that has been running great for the 2-1/2 years that I have owned it. It had 72,000 when I purchased it and now has 123,000.

This is my daily driver and I also time trial it. To better understand how the car performs, I recently installed a VDO boost gauge, oil temperature gauge and a 10-LED air/fuel ratio meter. The air/fuel ratio meter indicated midrange at idle/low speed, somewhat lean at highway cruising speeds and lean (off the scale) when under load. The LEDs would slowly turn on or off as the indication shifted. DAMN! I'm running lean at the track. Why haven't I blown a head gasket?

After thinking about this for a couple of weeks, I decided to replace the O2 sensor. Picked up a three-wire for a Ford Taurus (#13950) and spliced the wires to the Porsche sensor leads.

The gauge indication now moves up and down once per second at idle and about three to four times per second at higher exhaust flow rates. I didn't know that it was supposed to move like this! And it shows the exhaust going richer when accelerating, not leaner.

The old O2 sensor has a tan buildup on it and the slits in the housing that allow gasses inside the sensor were almost completely blocked. That accounted for the sluggish gauge reaction. As far as I knew, the car was running great and I couldn't tell that there was anything wrong. It may just be my imagination that there is a little more power but I have got to believe that the closed loop system has to be more precise now.

My suggestion is to replace your oxygen sensor earlier than I did. I don't know how long they are supposed to last, but apparently not 123K.

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Subject: Re: [951] Oxygen Sensor, 7/3/00

From: Blake Suhre bsuhre@yahoo.com

The O2 sensor in your 951 is a UGO or "switching" type sensor. It acts as a switch that essentially tells you if you are rich or lean of stoichiometric (approximately 14.7 A/F for typical pump gas). If your DME is working it should switch from lean to rich as you are seeing - slower at idle, faster off-idle. At or near WOT your should see the sensor output stop switching and go rich. This is because the engines are typically calibrated rich of stoich. At high loads to produce more power and protect the catalyst from melting down.

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Subject: RE: Oxygen Sensor, 7/5/00

From: Clifton Hipsher clifh@microsoft.com

You're correct about the new sensor. What you're seeing on your mixture gage is "O2 sensor dithering" caused by the DME's reaction to the O2 sensor's output.

From what you describe, before you replaced the sensor, the DME had determined the sensor was bad, and was using a "canned" or pre-programmed input. Typically this is about 0.451 Volts DC, and you can measure this at the male end of the O2 sensor lead, with the sensor disconnected of course.

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Jim Lill jpl@vectorbd.com http://www.vectorbd.com/users/jpl

Thu, 30 Oct 1997

I had previously posted a Bosch part number for the "cheapest" 3-wire I could find. The Consumer Parts Store P/N is 13953 and that cost is $44. I now have the jobber P/N, 0258003953 (looks like a Bosch #) and that cost was $35.95 dealer, $63.85 list.

Here's the install/adaption info: http://www.vectorbd.com/peugeot/archive/97/0080.html

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Subject: Re: Ego 1 vs. Stock O2 sensor, 10/13/00

From: Blaszak Precision Motorsports mblaszak@kos.net

Look elsewhere for your hesitation problem. Check all of the sensors and connectors. Also the TP sensor and connector. The 4-wire sensor is identical in operation to the 3-wire sensor. Two wires are for the heater circuit, one for the signal, and a dedicated ground wire for the signal circuit. That is where the difference is. The 3-wire system must get its ground from the exhaust pipe that must get it from the exhaust bolts and gaskets. So the 4 wire is better due to better grounding. However this can be duplicated with a ground wire to the exhaust pipe near the O2 sensor. I prefer the flex braid type as they do not burn off.

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Subject: [951] The Magical Inexpensive Bosch 3 or 4 wire O2 Sensor, 10/20/00

From: Ray Rfrx7@aol.com

You can buy a "4-wire" version of our O2 sensors for a 94 Taurus for the whopping cost of $39 at www.partsamerica.com AKA Kragen.

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Subject: [951] Re: was: boost problem. now: O2 sensor in ROW cars, 10/25/00

From: Donald R Langley Donald.R.Langley@aero.org

The car will run without the O2 sensor. However, the computer will not be able to "fine tune" the fuel mix without it. It will have to rely on the DME pre-programmed mixture that is determined by RPM, throttle position, temperature, and air flow. You also may not be getting the best fuel mileage....not that it matters.

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Subject: RE: [951] Re: boost problem, 10/24/00

From: "Christopher White" whitechristopher@earthlink.net

Actually it uses the O2 sensor for quite a bit more than just idle levels. On the turbo cars it will use the O2 sensor to control the mixture up to about 5 lbs of boost. This is a general statement as it is dependent on the throttle position to decide if the mixture should be richened for acceleration.

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Subject: Re: [951] Re: The Magical Inexpensive Bosch 3 or 4 wire O2 Sensor

From: "Jason@Paragon" jason@paragon-products.com 10/25/00

I'm not sure of the Taurus connection but Bosch produces what we call a "generic" or "universal fit" 3 or 4-wire O2 sensor. Pricing is actually pretty comparable between the two versions. We normally sell the 3-wire version for $38. It does have a connector on it but you just snip it off and attach your Porsche connector.

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MESSAGE: (#4833) 4-wire o2 sensor, 11/26/00

AUTHOR: Phil Coomber philfix@home.com

Hello all, I purchased a 4 wire o2 sensor to replace the temporary one I installed during the summer. It's a Bosch 13223. I ohmed out the wires and found the two white ones are for the heating element. Does anyone know which of the remaining two wires is for ground and which is signal? I suspect the black is ground and the gray is signal, but I'd like to know for sure before I install it the wrong way.

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MESSAGE: (#4838) Re: 4 wire o2 sensor, 11/26/00

AUTHOR: David Floyd

With the Huntley 4 wire, 2 whites are heater, black is signal output and gray is signal common.

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Subject: 3-wire oxygen sensor for under $40 - BOSCH #13913, 11/29/00

From: "FR Wilk" 944@technologist.com

The BOSCH #13913, 3-wire oxygen sensor, was just $38 at AutoZone. It has a Ford connector on it. You chop it off and splice it to your old wiring and connector. It is made in such high volume that the price stays low.

I was getting price quotes of $160 to $220 for the original Porsche part number (also made by BOSCH). That is over $120 more just for the correct connector? Life data - the substitute sensor is good for 50,000 miles while the stock Porsche sensor will go longer up to 60,000 miles.

Also, BOSCH #11027 is the universal 1-wire oxygen sensor available for about $25.

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Subject: Re: Setting Idle CO level with Oxygen Sensor, 12/3/00

From: Dave SuperD@direct.ca

If the engine is in good shape, an O2 sensor reading of 0.5V usually represents a CO content of about 0.7% and that should be very close to a 14 to 1 air fuel ratio. Note though, if you set the airflow meter with the sensor disconnected, you'll want to set it on the rich side so that there is room for the DME to trim it leaner. I think that with the sensor off, the DME goes to full rich trim, you don't want to set it lean when it's supposed to be in the rich headroom zone if you know what I mean?

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From: "Rusty Cullens" zac3@bellsouth.net

Subject: Re: 3-wire oxygen sensor for under $40 - BOSCH #13913, 11/30/00

We have the same Bosch sensor for $30.85 brand new.

Rusty Cullens

Wholesale Parts

http://www.rustyparts.net

1-800-741-5252

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Subject: Re: 951 gas mileage FYI, 12/28/00

From: DON ISTOOK istook@airmail.net

Barry Lenoble wrote:

<< I'm pretty sure the o2 sensor on my car is shot. However, since it's predominately a PCA club race car, I don't know if it's worth fixing. What do you think, should I fix it, or will it do nothing for me, as the car is usually driven at full throttle or full brakes? >>

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Barry, I am assuming that it is a turbo......if so, you should be fine not running the oxs sensor...also assuming that you are using a chipped ECU. The turbo engine at full boost loves the extra gas. If it is not a turbo, I would check the oxs sensor...if it functions, I would run with it hooked up.

Here is what happens, in my lowly opinion....a proper oxs sensor can produce between about .02 to 1V DC, depending upon the ox content of the exhaust gas. The ECU reads this figure...if it is lean (lower voltage), it compensates by dumping extra fuel...this make the mixture rich, which produces higher voltage...the ECU then reads this and cuts back on the amount of fuel...making a leaner mixture, etc. ..never ending cycle...to produce the proper mixture (which makes for about .5V).

With the oxs not hooked up, the ECU does not get the approx .5 V it using for stabilization. It senses that the engine must be running too lean and compensates by increasing the injector time per cycle to richen the engine up...the mixture continually stays rich....fine for a turbo car that is on the track.

We start the 2001 race season off at Daytona with the Motorola Cup the first weekend in Feb. with our Audi S4. This Audi will pull 993's down the straight at Daytona. We will do the full Motorola 10 race schedule and compete in selected Speedvision events when we can. The car is not as competitive in Speedvision because of rule differences in the class that we run in Motorola Cup (GS) versus Speedvision GT.

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Subject: Re: Two problems to solve... 3/13/01

From: "FR Wilk" 944@technologist.com

Mike Oberle wrote:

<< 1 - Mild surging with part throttle opening between 2000-3000 RPM., engine hot or cold.

2 - When I drop throttle the idle dips to 700 RPM then "hunts back to 900 RPM, engine hot or cold. >>

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Both are classic signs of a sluggish O2 sensor. It still works but reacts slowly. How old is yours? They are easily contaminated by silicone and many thread compounds. Contaminate jumps off your spark plug threads and into the air and vapor deposited on your O2 sensor causing irreparable damage to your poor O2 sensor.

You can replace it with a BOSCH #13913, 3-wire oxygen sensor, for about $40 if you crimp your old cable and connectors to it. Otherwise, $200 plus.

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Subject: RE: Oil Usage, Running Rich (LONG track observations) 4/10/01

From: "Mohn, Jerry" jmohn@ci.chanhassen.mn.us

The sensor you want is Bosch part #13913. (3-wire)

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Subject: Re: air/fuel meter & O2 sensor use, 7/27/01

From: Blaszak Precision mblaszak@kos.net

No, at present you have 3 wires. 2 are for the heater circuit and can be discounted. The third is the signal wire. Your car presently pulls the reference or ground line for this signal through your exhaust system to the chassis and then the (-) to the DME. By adding the dedicated ground line you get a better continuity that does not fluctuate with temperature. That is why I said save your money from a 4 wire and install a cheaper 3 wire and put a ground strap to it. It does the exact same thing, regardless of what any vendor of the more expensive part will tell you. I have a scope and have checked the output. Believe me it is the same!

On a later car, you may have a computer that has provisions to take in the 4 wire sensor signal (2 dedicated signal wires) directly, but our computers do not have that provision. Remember it takes 2 wires to have a current flow. If you only have one, the chassis is the other!

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Subject: [951] RE: Causes of indicated lean running when not actually lean???

From: Huntley Racing huntleyracing@home.com 11/5/01

The O2 sensor is not calibrated to the same inputs as the ARM1 and as it heats up to it's operating temp is out of the voltage range to light the second blue light of the ARM1. There are about 50 different BOSCH O2 sensors out there but only a couple that will match the ARM1 inputs.

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Subject: RE: Disconnect Oxygen sensor? 2/21/02

From: "Clifton Hipsher" Porsche944@carolina.rr.com

In a nutshell, the O2 sensor monitors the exhaust gasses for the presence of free oxygen.

Too much O2 means the fuel/air mixture is lean, too little O2 and the mixture is rich. The O2 sensor develops a small DC voltage that is roughly proportional to the amount of O2 in the exhaust gasses. This voltage ranges from about 0.2 to 0.9, with 0.5 being roughly equivalent to a "proper" fuel/air ratio of 14.7:1.

The engine management computer (DME in a Porsche) monitors the O2 sensor output and adjusts the fuel injector duty cycle to maintain a 14.7:1 fuel/air ratio.

This is fine for US specification cars, yielding adequate performance, good fuel economy and low emissions.

However, in a racing environment, a fuel/air ratio of 14.7:1 is too lean, limiting peak engine horsepower. To compensate, tuners reprogram the engine management system to provide a mixture of approximately 12:1.

Should you disconnect the sensor, or should it fail, the DME will use a "default" setting of 0.451 volts DC, which is just below the desired 0.5 volts. This default setting is used to insure that the fuel/air ratio is slightly rich and never goes lean to protect the engine from detonation or knock.

The major downsides are poor fuel economy, carbon build up in the cylinders and exhaust system, failure to pass emissions testing, and eventual failure of the CAT due to contamination by unburned fuel and hydrocarbons. NOTE: This will also cause the exhaust to have a very sharp odor.

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Subject: RE: Oxygen sensor question, 2/26/02

From: "Clifton Hipsher" Porsche944@carolina.rr.com

The big question is whether or not the DME is actually using the sensor's output. For starters, the DME has a "bias" voltage of about 0.45 volts without a sensor connected. If the sensor's output does not exceed this "bias," then the DME "assumes" the sensor is either bad or not present and uses the 0.45 volts. This causes the engine to run rich, protecting the engine from "ping," or detonation.

This is not a catastrophic situation in and of itself, but left unresolved, the excess hydrocarbons in the exhaust gasses will over time clog the cat.

Since you have over 100K on the car, and it is running rich, I'd recommend replacing the sensor.

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Subject: Re: O2 sensor for 88 951, 5/9/02

From: "FR Wilk" 944@technologist.com

<< Anyone know the Bosch part # that will replace the OEM and will this sensor work exactly the same? Also, is there a procedure on replacing it. >>

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http://early944.tripod.com/o2.htm

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Subject: Re: O2 sensor for 88 951, 5/9/02

From: "Chris Luckett" clluckett@earthlink.net

It has been recommended by a Bosch engineer in this thread to not use butt connectors to splice the O2 sensor.

http://forums.rennlist.com/forums/ultimatebb.php?ubb=get_topic&f=15&t=006207

"OK I thought I would put in my two cents because I am a development engineer for Bosch Oxygen Sensors. First of all let me check on that replacement universal 3 wire as it may not have the correct ceramic for your application. Believe it or not there are different ceramics and more importantly different heater powers. The heaters in the sensors come in two different varieties of 12 and 18 watts. Lower power heaters will take more time to light off (time it takes for ceramic to heat up and become conductive). More than likely it is a 12-watt heater and the ceramic may be ok. If you wait until Monday and tell me the year, model of your Porche I will know exactly what it requires and I will let you know if you can use the universal with no problems.

The protection tubes and ceramics play a big role in where the sensor switches its voltage when the mixture changes from rich to lean. Most of Porsche's sensors tend to be lean switching sensors and using the wrong universal (there is more than one universal) could effect catalyst life. Also, it is important to use the universal connector system. Do not use butt connectors when hooking these things up. The sensor must replenish the reference air inside in-order to create the oxygen gradients necessary for voltage generation. The sensor acts as a battery in a rich gas environment (long story). The reference air is replenished through the wire harness. Anyway I could go on and on and bore you to death but let me know about your vehicle and I will see what I can find for you. OK?"

"When you buy a universal sensor they are supposed to give you a newly developed posilock sealed connector system. We have recently released these parts. There is a universal that has been released which is an ideal replacement for the 944/968 series vehicles. However it has not been cross-referenced and sold for European applications. But what I can do is find what American vehicle the sensor is sold for and let you know. Basically I'll tell you (Monday or Tuesday) to go to your auto parts store and buy a universal for let's say a Ford Windstar and it will be the correct ceramic/heater and similar protection tube and you will receive the universal connector system (patent pending). I have done some work on this for my personal use so I will let the list know of my findings. Myself and a buddy have already installed these sensors on our 968 and 951 respectively. As far as air replenishment goes, inside the ceramic (thimble shape) is a chamber of reference air (ambient air) and a platinum conductor. During rich operation there is an absence of air in the exhaust stream. This creates an oxygen gradient across the ceramic. On the outside of the ceramic is another platinum conductor (power grid - Bosch patent) the gradient causes ionic conductivity across the zirconia ceramic). If the sensor was in ambient air and was at operating temperature the voltage output would be approx. 0 volts."

"I apologize for the late reply but my computer locked up before I finished. This is what I know. The 13913 is an adequate sensor however it does not include the universal connector kit as discussed before. The released universal sensor that will work for the 944 Turbo and 968 is the 15725(I will have to check the other vehicles tomorrow). This sensor has the correct heater, ceramic, and protection tube and should be cheaper than the 13913 because you are not paying for the Ford connector. This sensor has been cross referenced and a similar part (different number) is sold in Europe. But because the volumes in the US are low this universal has not been cross-referenced in the states and therefore only the OE is available. I am not sure what cross-referencing scheme some aftermarket suppliers are using in supplying the "fits all" three wire. In this case the part seems to be OK. But again ask your auto parts store for the 15725(part # 0258005725) and I will inform the list of the other makes and models this sensor will fit. Remember that Bosch does not monitor all vendor sales and when looking in my aftermarket sensor book there are no universals listed for Porsche in the US, therefore I do not know where the aftermarket vendors are getting their info. If anybody knows let me know (I don�t work in sales)."

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Subject: O2 Sensor for 88 951 15725 or 13913/53, 5/13/02

From: "Ron Dion" rwd@intergrafix.net

I've located a source for the new 15725 universal sensor. Best price is about $20 more than the 13913 or 13953. Will it be worth the xtra cost? Anyone currently using the 15725 have any input on this subject?

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Subject: Wideband Setup, 8/1/02

From: "Tom M'Guin" Tmgee@iname.com

Anyone know anything about this set up:

http://www.diy-wb.com/info.htm

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Subject: Re: Wideband Setup, 8/1/02

From: "David Floyd" kfloyd@cafes.net

www.techedge.com.au This is where to buy assembled units.

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Subject: Re: Wideband Setup, 8/1/02

From: Doug Donsbach dldonsbach@tds.net

Look at http://www.egortech.com for starters.

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Subject: [951] Re: Fuel Pump fuse, 8/10/02

From: "Tom L." porsche951@comcast.net

This was happening to me too........the fuel pump fuse kept immediately blowing each time I tried to start the car. The oxygen sensor heating element is on the same circuit as the fuel pump. A year ago, I spliced in a new generic O2 sensor & over time the heat shrink tubing had rubbed thru causing a short which would blow the fuse instantly. To test, unplug the O2 sensor and see if the fuse stops blowing.

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Subject: [951] Re: Unplugged 02 Sensor, 8/11/02

From: "Derrek Khajavi" huntleyracing@cox.net

The O2 sensor is used to maintain 14.7:1 Air/Fuel ratios during idle, light load to moderate load. When the car is under high to full load the O2 sensor is no longer used to assist in mixture control in 944 based cars.

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Subject: Re: O2 sensor, 12/3/02

From: "Ron Dion" rwd@intergrafix.net

<< I failed California smog and will replace the O2 sensor as it appears to be sluggish. What did you learn about the 15725? Did you use it, and if so, where did you find it? >>

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Yes I installed a 15725. Purchased at AutoZone in Pa. I can't attest to emissions testing since rural Pa. doesn't require it yet. Comes with a nice splicer also. Hope this helps. Sorry for the delay, busy with the holiday.

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Subject: RE: Running rich and O2 sensors, 10/26/02

From: " Clifton Hipsher" Porsche944@carolina.rr.com

George Gundesen gundesen@telus.net wrote:

<< I have been following the thread on this subject with interest because my exhaust tip is pretty black and leaving residue on the bumper above it. It also smells somewhat, 'almost like oil burning', but I have no leaks. No blue smoke but I suspect it is running rich. I haven't checked the plugs. >>

<< Q: If the O2 sensor is working properly can it go out of rich/lean adjustment? Is there a manual mixture adjustment somewhere? I understand the DME has different internal settings but I have read these have little, if any, effect. >>

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The quickest way to figure out what is happening is to connect a digital volt meter to the O2 sensor.

Here is what you'll need:

Digital Volt meter.

6" "jumper" wire with alligator clip on one end and about 1" of insulation stripped from the other end.

Set the meter to read 2 volts DC. Connect the negative lead to a good ground (I use the negative battery terminal), and connect the jumper lead to the sensor output wire. To do this, unplug the O2 sensor connector at the center line of the fire wall. Insert the stripped end of the jumper wire into the female (O2 sensor) side of the connector, and then plug the connector back together.

Connect the positive meter lead to the alligator clip.

Set the meter to read 2 volts DC.

Start the engine and watch the meter. As the O2 sensor heats up, the meter should start to indicate. Once the sensor is up to its normal operating temperature, the meter should vary or "dither" across 0.5 volts.

If the sensor reads high or low and does not dither, disconnect the sensor and take a reading from the male (DME) side of the connector. This should read about 0.45 volts, which is the value the DME uses when the O2 sensor output is not valid. If you do not get a good reading here, you may have a problem in the harness.

If you get a good reading (0.45 volts), then check the sensor output while varying engine RPM. As you open the throttle, the sensor output should increase (indicating a "leaner" condition), and then steady out at some value. From this value we can get a rough idea of what the DME is trying to do.

If the reading settles out above 0.5 volts, the fuel/air mixture is lean. If the reading settles below 0.5 volts, the fuel/air mixture is rich. HINT: 0.45 volts is the DME "fail safe" for an invalid O2 sensor input. This forces the DME to maintain a rich mixture to insure the engine does not detonate, or ping.

If you see the sensor is not varying, then the sensor is bad.

Sensors can fail for a number of reasons, but the biggies are age and contamination. Over time the internal components age, resulting in longer and longer response times, until finally the sensor fails. Typically you would see this as a slow decrease in fuel mileage with an attendant build up of carbon deposits on the plugs, piston tops, and combustion chambers.

Contamination by engine coolant or unburned hydrocarbons (rich mixture for a long time, or excessive oil burning) will typically cause a more immediate failure with a sudden decrease in fuel mileage.

Beyond the fuel quality switch in the DME, the only other adjustment is for the CO (carbon-monoxide) level. This is done at the Air Flow Meter with the O2 sensor disconnected and the engine at idle, and requires an exhaust gas analyzer (emissions test equipment).

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Subject: Re: Running rich and O2 sensors, 10/26/02

From: Dave dc944@telus.net

Clifton Hipsher wrote:

<< As you open the throttle, the sensor output should increase (indicating a "leaner" condition), and then steady out at some value. From this value we can get a rough idea of what the DME is trying to do. >>

<< If the reading settles out above 0.5 volts, the fuel/air mixture is lean. If the reading settles below 0.5 volts, the fuel/air mixture is rich. HINT: 0.45 volts is the DME "fail safe" for an invalid O2 sensor input. This forces the DME to maintain a rich mixture to insure the engine does not detonate, or ping.

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An oxygen sensor typically has an output that varies between 0 and 1.0 Volt. A lean mixture is indicated by a reading below 0.5 Volts and a rich mixture would be represented by anything above 0.5 Volts. A good way to remember this is simply: "Low is lean"

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Subject: RE: What happens if you keep the o2 sensor unplugged? 12/29/02

From: "George Beuselinck" georgeb@944ecology.com

Of course, you could go to your nearest Autozone and "rent" a set of O2 sensor removal tools. By "rent", I mean that they charge you a $15 deposit and refund that deposit when you return the tools. If you like the tools enough (I have, on several occasions), you can keep them and they keep the deposit...

This turns out to be a good method of obtaining tools that you cannot find anywhere else.

Unfortunately, they do not have the Porsche timing belt tensioner tool.

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Subject: Re: What happens if you keep the o2 sensor unplugged? 12/29/02

From: "John Hajny" REDL944@aol.com

I had a high speed miss in my 84 years back. Unplugged the O2 and it ran perfectly. I have run without it now for years. It's a track car now, always runs great, no bad gas mileage, runs a little richer by the tailpipe being black, but I don't care. Only this year did the cat plug up after 6-7 years of use. It was time for it to go anyway! 150k miles and still runs like new!

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Subject: RE: What happens if you keep the o2 sensor unplugged? 12/30/02

From: "Under Pressure" underpressure@attbi.com

Sorry for the interjection, but I think you do not fully understand the policy of Auto Zone and other similar parts places. In no way is George suggesting ripping these guys off.

These places have a free "rental/use/loaner" policy that encourages folks to rent the needed tool for their project and keep it if they desire.

The rental fee is actually the purchase price of the tool, if and when you return the tool you get your "rental" fee back, which results in a free "loaner" program. As George mentioned, there are times where you find the tool quite valuable and you just keep it - After all, you "did" pay for it in advance.

George used an example where the cost of the set of O2 tools cost, say, $15.00 - But I assure you the deposit amount for the tool you plan to "borrow" is the full purchase price of that particular tool.

Here is a link to see the details of Auto Zone's "Loan a Tool" program for yourself...

http://www1.autozone.com/servlet/UiBroker?ForwardPage=/az/in_our_stores/loan_a_tool/loan_a_tool.jsp

A few excerpts;

"There's nothing like having the right tool for the job."

"That's the idea behind AutoZone's free Loan-A-Tool service. It's the most complete selection of seldom-used, expensive-to-own specialty tools that will help you do the job right. We offer more than 60 different tools for all sorts of jobs - steering and suspension work, air conditioning jobs, engine repair, and more."

"Using our Loan-A-Tool service is easy. Just leave a deposit at the store when you pick up the tool. When you're through, just bring it back, and we'll refund the deposit."

What the linked page does not tell you is that the deposit amount is the full purchase price of the tool - Return the tool, you get your deposit back, keep the tool and Auto Zone will keep your deposit and you effectively paid full price for a used tool. Incidentally, you can return the used tool anytime, within reason, and just trade it in for a new one.

Again, it is not my opinion that George was trying to lead anyone into any scam-like activity - He was simply pointing out that there are places that "offer" free use of some hard to find tools.