Oxygen Sensor in a 928s4 my 1988:  928.606.128.01  Bosch part nr: 0 258 003 048

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At 09:37 AM 11/11/02, David Carelli, 84 Euro wrote:
>After about 2 weeks, I think now I'm over my head... Bought a universal 3 wire O2 sensor, climbed under the car, the O2 sensor on the car has only one wire, and its not even hooked up, where does this wire finally go to? and where do I hook up the other 2 wires? and is this O2 sensor even needed since it wasn't ever hooked up and ran fine.

An oxygen sensor is a DC voltage generator. When it is hot enough, it generates a voltage between zero and one volt DC, depending upon the amount of free oxygen in the exhaust stream.

A one-wire sensor uses the exhaust, then the chassis for the ground side of the circuit.

A two-wire sensor has a dedicated ground wire to ensure that the tiny voltage isn't impaired by a bad ground on the exhaust.

A three-wire sensor adds an electric heater to bring the sensor to operating temp more quickly. One wire is the positive DC voltage, one wire is the power for the heater, and one wire is the common ground.

The sensor on the '84 USA cars is a three-wire type. The sensor output wire is usually shielded, with the output going to terminal 24 on the LH ECU, and the shield going to ground on terminal 23. The power wire goes to a 12 vdc supply on X5, and the ground wire goes to ground.

The oxygen sensor is used as a trimming device - that is, it slightly changes the values that would otherwise be used to control the fuel flow. The closer the programmed maps are to reality, the less effect the oxygen sensor has.

The programmed maps are usually a bit richer and a bit more conservative. The oxygen sensor input is used to trim the mixture closer to stoichiometric (chemically exact) combustion in order to lower emissions, improve mileage, and maintain performance.

>Now at the same time
>when I finally fixed my starter solenoid wire only getting 9 volts problem, I tried to start the car, got nothing but sputter, strong gas smell. it seems to be filling my MAF up with tons of gas. gas mist pouring out of the MAF. did I screw something up when I was looking at the starter relay per Wally's instructions ?

The starter relay shouldn't affect the fuel system at all. Your problem could be either too much fuel or not enough spark. You will need to determine which.

If the problem is too much fuel, pull the vacuum lines off of the fuel pressure regulator and dampener units, and rub your fingers over the fittings. Then smell your fingers - any fuel odor is cause for suspicion of a split diaphragm.

Of course, the air is supposed to be going in the MAF, not coming out. Have you stripped teeth on the timing belt?

Wally Plumley
928 Specialists

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My last o2 sensor came from Toyota and cost ~10 dollars. Lasted nearly four
years.

The only advise I would offer is if you go with a universal and need to splice, splice inside the car and far away from the exhaust system. BTDT.

I replaced that with an OEM unit from 928 Specialists and have noticed that the car drives smoother. That's the only way to describe it. It accelerates smoother and goes from 70-110 like it's riding on glass. Just a seat of the pants review.

Good luck.

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At 04:37 AM 12/28/02, Theo Jenniskens wrote:
Hi,
any experience with fuel economy in relationship to the O2 sensor quality?

It is said that the Lambda Sensor gradually gets contaminated and/or drifts from specs. This informs the LH computer to change the mixture... and make rich effectively. That will have a nasty effect on the fuel economy of the car in the full range that the LH controls (dynamic map).

How different are the O2 sensors anyway? It seems to me that the O2 sensors are in a few types: 1wire, 3wire, 4wire.
1 wire is unheated, 3 wire is heated, 4 wire is heated with extra signal ground.
My 928s4 1988 has a Bosch: 0 258 003 048 (different models 928 have different number) Porsche: 928 606 128 01.
Some say that any generic Bosch 3 wire sensor can be made to fit the car.

It seems that the Bosch 0 258 003 xxx indicates a 3 wire, and the xxx is the cable version.
Difference is only the connectors and cable length. Is this correct?

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The following is rumor and hearsay - I have no sure knowledge on the subject.

Supposedly:
All oxygen sensors develop a DC voltage between zero and one volt when exposed to varying oxygen levels in a hot exhaust stream.
The heated sensors start operating more quickly from cold, and operate more reliably at low gas flow and power level conditions.
The primary difference in new, uncontaminated sensors is their response time. If this response time does not match that programmed into the ECU, you will get larger swings in mixture control. All lambda-controlled systems swing around stoichiometric, but one with a poorly-matched sensor has larger, un-damped swings.

We have had owners report that they installed a generic (cheap) sensor with poor results. Again, I have no reliable knowledge in the area.

I don't think that I will try to save a few bucks when I have to replace my oxygen sensor, I can see how some people might be tempted, given the cost involved.

The following are facts.

Porsche and Bosch obviously feels that it is important, given the differences in the sensors specified.
'80 - '82 P/N 911.606.123.00 $63.22
'83 - '84 P/N 928.606.124.00 $201.84 - ouch!
'85 - '86 P/N 928.606.128.00 $113.85
'87 - '95 P/N 928.606.128.01 $135.00

And no, I don't know what the differences are. That is what Porsche and Bosch say that the various models need.

Wally Plumley
928 Specialists

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the generic Bosch sensor works fine but must be soldered to the existing leads... It is the wire and connectors which make one 928 specific .

Jim Bailey
928 International

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>Is it required to have the O2 Sensor hooked-up?
>
>I thought it was imperative to be installed for the brain to map outside of WOT or deceleration.

The system can run in either "open loop" or "closed loop" mode.

In open loop, the fuel/air ratio is set according to a pre-programmed map burned into the EPROM's, as modified by the air flow sensor, temp sensor, etc.

The system is in open loop when the oxygen sensor is not yet hot enough to give a reliable signal; when the full-throttle switch signals the ECU that the throttle is open; when the idle switch signals the ECU that the throttle is closed.

In closed loop, the ECU uses the map and the sensors as in open loop, but the mixture is further refined based upon input from the oxygen sensor. If the oxygen sensor signal is not present, the system simply reverts to open loop mode.

Wally Plumley
928 Specialists

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what about an alternate O2 sensor? These are supposed to be changed out every 60k miles or so.  The 968 uses the Ford Temp sensor (or something like that) Bosch part number 13913. Universal type 3-wire. $34.99 on sale at Advance Auto Parts. I  don't mind splicing wires. Has anyone used an alternate sensor for the 928?
Harvey
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All 3-wire Bosch O2 sensors are basically the same thing except for the length of wire, the connector at the end, and in some cases the metal cover over the sensor head (which makes little difference). So when you see a discrepancy of up to $100 between sensors, know that Bosch is happily gouging you. If there was some competition in the O2 sensor market, they would not cost more than $50 for any car.

A universal sensor is nice, but it's hard to find the connector for the other side. What I do with all my cars is to buy a Ford Mustang 3-wire O2 sensor (~$35, part #13942), then go to a wrecking yard and pick out some connectors that fit it (these connectors btw are very high quality waterproof ones and are super easy to connect/disconnect). Then I chop the old connector off the stock 928 sensor and make myself an adapter harness. This way I can use the mustang sensor in the car without messing with the car's harness and can always put a stock one back in if necessary.

I've had at least 4 cars (including the 928) running on these sensors for years without problems.

Dan
'86 928S 5-spd w/LSD
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If your car came with a 3 wire sensor, you'll want to replace it with a 3 or 4 wire sensor. Universal versions of both are available, but the best deal I've found is a 4 wire Bosch for VW Jettas #16121. It was $21.96 at NAPA. For Bosch O2 sensors, white is heater (polarity doesn't matter), black is sensor signal, and grey (not in 3-wire) is sensor ground.

To splice it into your car, just twist and solder the wires then put some shrink wrap over the connections. It would be preferable to have the splice inside the car. If you use a 4 wire, you can run the sensor ground to a ground point on the frame or splice it to the heater ground (test polarity of white wires first). If using a 4-wire, the closer the sensor ground to the ECU, the better the signal and mixture control.
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Matt
1984 928 S2 5-speed LSD