My Porsche utilizes the Bosch Jetronic Fuel Injection system. See also K_Jetronic for additional info.

My 1988 928s4 has a LH Jetronic system (the L stands for "Luftmengenmessung" in German, meaning "air flow management" and the H stands for "Hot Wire" referring the the Mass Air Flow Sensor that is used). As far as I know The L and LH systems are very similar, where the LH system outperforms the L  type by more accurate control and improved flow capacity. The LH system is operated by the  Bosch LH injection computer (the 'Brain' (part number 0-280-002-504 or 928.618.123.11 or 13)). It picks up the information of various sensors, and adjusts fuel delivery and timing to the engine. In general operation is as follows:

Whenever the engine is cold (reported by sensor) the computer runs a pre-programmed cycle, allowing a longer injector pulse, and therefore giving more fuel to air to the engine. This helps getting started and running smoothly when cold. It also activates the auxiliary air device and the cold start valve starting up the engine.

As soon as the engine is reported warm, the computer picks up the O2 sensor signal plus MassAirFlow meter reading and adjusts the fuel delivery by tuning the duty cycle for the injectors. It knows the air amount that goes in (throttle) and the O2 level (clean combustion =low CO level) in the exhaust. Using a programmed map it adjusts to match the right hysteresis.

When you start the engine, the MAF takes readings to determine altitude, temp, etc. (mass air properties for the moment) and any changes from last stored then provides a map offset factor to the fixed map.  The fixed map is what is changed when a chip is burned.

This is why the engine runs funky when you disconnect the battery must reset itself....take about 10 minutes.  Every time you make a change to the engine, it takes about 10 minutes to reset..... Yep: is is selflearning, so you can encourage it to match you need for speed and power.

The system runs on the MAF meter up to some reading (approx. .xxx volts or so) or up to about 3000 rpm with full throttle, at which point, it switches over to map running and remains on the map till the rpms or maf reading drops below that above set point.

O2 sensing/closed loop is done below some rpm...I have not found the point yet, but guess that is is around 3000 at full throttle or higher rpm at part throttle.  The MAF and O2 feed the ECU info to create the map offsets.

The system uses all throttle positions (below the set point) to get the correct offsets for the map based upon many throttle positions/engine loads below the maf meter set point/rpm set point.  The .xxx volts may be reached in many positions...full throttle at 2500 or 3000 rpm, or part throttle at 6000 rpm!

Fuel Pressure Regulation

It is a total misrepresentation of the real facts in the operating principal of the fuel delivery system and there fuel control components in the article to how the fuel delivery system is functioning (1) and how to test this system for faults (2), wrong instructions which can cause damages to the system (3) and the list goes on (4).

1. I have to disagree very much in the regulator operating descriptions in this article and I would like to emphasize that this is not the case, how it is described and will cause misinterpretations for fault diagnoses in the automotive industry. This is my most concern which forces me to have a constructive criticism particular in this fuel pressure regulator description. Secondly some parts of the article which are not so much damaging to the trade, need to be better clarified as I will do below in this statement.

2. Air entering into the fuel system caused by a damaged joint above the fuel level in the fuel pick up pipe, needs to be clarified better in real terms of the cause and the results plus some finer points to be added for clear understanding.

3. Air bubbles are visible in fuel even while under pressure via the INTER-JECT injector flow and system flow meters which are two entirely separate units, and are very sensitive to injector flow or system flow correction elements by the ECU which all INTER-JECT Service Centres use to monitor visually, as part to there normal vehicle service. It is required by the INTER-JECT service operators which are specially factory trained in this special "new" adopted servicing technique to monitor also as a must, the integrity of the electronic correcting elements of the ECU (1) and the ECU's primary sensing components used as signal and as data provider to the ECU to request the ECU to do actions (2) and the ECU's secondary action is its correcting signal output to be send to mechanically or electronically acting components to execute the requested actions by the ECU (3). Those primary or secondary components are separate to each other and are called in this statement " Helpers" or "Actuators" to and for the ECU.


The two main purpose of a Regulators is to stabilize constant system pressure and control fuel enrichment on acceleration to overcome a flat spot or hesitation and must be "synchronized" as close as possible to modern vehicle with ECU controlled Spark advance envelop!

The fuel pressure is kept always higher to the counteracting ratio of vacuum pressure in the manifold to assists for an even fuel atomisation (1) while fuel is under pressure bouncing of the pintle cone in a spread fuel pattern form (2) and the required constantly held fuel pressure behind the injector assists in addition to the internal return spring to shut off the pintle to prevent a possible pintle bounce (3).

Even in deceleration mode still the fuel pressure is kept higher against the increased manifold pressure to avoid pintle bounce.

Prove and facts are:-

This Regulator is set to required pressure permanentlyRegulator first time ever fitted to electronic fuel injected cars in 1967 on VW's have been non vacuum controlled fuel pressure regulators to be adjusted manually by a set screw and look nut to the final set NON changing pressure according to the accumulated fuel pump flow output to the required rail operating pressure to particular models of cars to obtain injector synchronization for different engine combustion sizes or the amount of cylinders. Today still in use in high performance cars and in the racing industry. Permanently set regulator in passenger vehicles which you will find in Mercedes Benz, Volvo, BMW, VW's just to name a few. They had not to relay on vacuum pull and variations of fuel pressure push at that time at all as there was no such requirement for lean burn combustion control devices like the Lambda sensor or spark advance, catalytic converters and so on, as now in this present days due to stringent emission control requirements. This was in the past which was in my time of automotive repairs & EFI trouble shooting in Germany.

Regulator pressure at rest. Engine off.Regulators with vacuum control NOW this days are in use for a quite a number of reasons including a harmonic smooth engine drivability and other enhancements. But one of the most important reason of all the others, which work hand in hand further down the track, is to provide synchronized and smooth engine momentum's on the acceleration start, and to overcome the programmed settings in the lean burn, and spark advance signal changes by the computer.
In other words; to overcome the non responsive reaction between the electronics and the fuel delivery. Electronics is very responsive, most times too responsive because of its nature. Where fuel delivery, because of its material status as liquid is lagging, ( it's to slow to match up with the speed of electronic thinking or data transfer.) To combine the two materials together, electronics and fuel to do a specific required tasks (but more then one) requires semi controls to help the lagging material which is fuel in this case to perform up to the near enough required standards to match up to the electronic requests. One of that helper roll is the fuel pressure regulator which has so many functions in one. I know you do not know about this, as this is only a very small part of the well kept secret by the INTER-JECT management in Australia and Germany, and you get to know more about as you read on.

Now today, because of many fuel related components, the (" HELPERS" as such) are used in addition to the programmed electronic data distribution which must control every possible move or action and as a paramount those must be kept in synchronization at all cost to match alongside the parameters in the requested lean burn engine characteristics, which is additionally controlled by an ("mysterious separate black box") or the ECU it self. Spark advance for instance, the oxygen sensor and number of other devices which are sending result calculations continues from the engine operating strategies right throughout to the idle segment and a number of other "finer" characteristics. One of the "HELPERS" is the vacuum controlled fuel pressure regulator.

If "NO" pollution requirement would be in force, the final set screw fuel pressure regulator would be the one in preference, which has been fitted for many years. There was no flat spot complications or mismatch in synchronization at all as the pressure was mechanically preset and nothing else from the engine compensating elements could change this set pressure as there hasn't been any!

NOW this days lots has been ask and demanded for to be changed to gain engine performance,
but please "NO", to bad economy! Good acceleration, but please "NO" flat spot! Good idle characteristics, but please "NO" to pollution or to back fire or popping into the exhaust! Controlling smooth engine momentum, but please "NO" to hunting engine on idle, so on and so on and the list goes on!

Full System pressure on take off (No Vacuum pull)Now this vacuum controlled regulator has a lot in common for this above mentioned request but basically still, foot down, regulator pressure up to overcome the first possible flat spot!
This is action (1)! Nothing else at this point!

Further along the track, so long the food is down, fuel pressure must be at all cost consisted. This is action (2) and action (3) is on deceleration used to stop possible injector pintle bounce on soft fuel cut down.

This has now three separate main purposes.

First avoid at all cost of an possible flat spot on take off, secondly avoid at all cost fuel deficiency, through the total acceleration process and the third, avoid the most important one which is caused by increasing RPM to stop any possible injector bounce. The increased fuel pressure (approximately 300 Kpa. maximum for most cars with the exemption of the VK commodore ) is also as a helper source to the addition of the internal return spring in the injector to shut the pintle off to avoid ignition splatter into the intake manifold. This is caused only by high advance spark curves, the spark is meeting with the still open intake valve on compression stroke, as tolerances are so close together, causing a backfire into the intake manifold. This can destroy the MAP sensor or other air flow sensing devices if they are equipped to that particular vehicle. Another sort of splatter into the manifold is caused if the waste spark has broken tracks into the neighbouring high tension lead or cross arc inside the distributor cap to neighboring spark distribution points zapping into the intake stroke of a cylinder with open intake valves. This has nothing do with a regulators, to be wrongly assumed to be faulty. This has nothing to do with pintle bounce, if the regulator holds constant pressure right trough the RPM ranges with a flow of at least 90 Ltr/hr.

Vacuum pull on ignition miss, air leak, or fuel problems.To continue further with the fuel pressure regulator, that they add also to the main source of causing flat spots, backfire and so on, if they are damaged, leaking, bulged out ect., or the diaphragm stretched or in the stiffness/hardness because to the location the regulator is fitted in, to be exposed direct to extreme engine heath by actually touching the engine metal or cylinder head or the decaying material of the diaphragm not allowing a responsive and synchronized pressure variations. I know, the injectors gets the blame always fist, but sorry it is not always the fist matter to look for. This is the most common mistake done to find the hard way that the problem of a flat spot still exists!

Further to this some advanced models, starting from the simple VL Commodore and models past including all late continental cars having an in-build additional circuitry, triggered by the TPS for only momentarily 2-3 seconds to a 1 and a 1/2 time injector duration increase (adding up to 50% more fuel in that set time) [Called accelerator pump circuitry and is re-programmable on some systems or as MEMCAL interchangeable] (European cars using a fine spike interaction as signal down the same wire to signal for purely timing the injection duration's on acceleration) this increase is used as the electronic safe guard to match up with the wear and tear of engines to cover-up just in case all other active data summery to not match up to the required programmed parameters, for example (to match to a possible un-tuned exhaust back-pressure) still enhance 100% to keep on track to an synchronised manner to the pressure regulator possible vacuum deficiency in case of piston ring wear, carbon on the back of inlet valves and so on. This accelerator circuitry is also used in addition to the regulator to help to allow the spark to ignite this incoming fuel mixture at all cost easier inside the combustion chamber at the right time and right moment with the right amount of this calculated air/fuel mixture which was lean just a few seconds before when the engine was at idle.

There was and is no need for those older models which are still on the road as they have been running "so wetter to better" and only, > the sky is the limit. < That's why, there was and is no need for regulators with vacuum control or other enhancements for them.

Another proof to my criticism is for example the old K-JECTRONIC vacuum controlled control pressure regulator which has been introduced because of this hesitations. Volvo 144 in particularly back in that time, but now also still used on later models for the same reason as mentioned above. The later version with Lambda control in the (KE version) has no more this type of control pressure regulator, but instead the double action vacuum controlled regulator just for the same reason as I described above in this matter of lean burn and emission control. This double action vacuum pressure regulator, (similar to the well known Euro double acting ignition vacuum advance unit) is out now in Europe for EFI cars to enhance better fuel synchronization in higher rpm ranges raising the pressure even more, but also for lower pressure when on idle, deceleration or on lean burn control with much better exhaust emissions and fuel economy. Nissan and some Toyota's using pump voltage changes timed to de-load the pump flow after 20 seconds if at idle. Soon the TPS is moved off idle, the pump spins faster providing the flow back to normal. There is no fuel pressure change at all, (only flow) as the regulator controls the pressure to operating pressure even if the flow is only 5 litres an hour or less. Pressure build up inside the small regulator doe's not necessary have to relay on volume if injectors consuming fuel on idle! But relays on it very much so to keep it there constant if injectors consuming more fuel on acceleration. The compression spring inside the regulator is factory set to match up with the maximum pump flow to avoid possible fuel accumulation as it can increase the fuel pressure higher then normal. Look out if some one fitted a K-Jetronic fuel pump into a normal multipoint system the pressure is due to the high fuel volume, this will automatically accumulate inside the fuel pressure regulator, raising the pressure also automatically to higher levels because of the narrow size of the fuel exit tubing (bottle neck (1) and (2) the spring setting and (3) fuel backlog in the supply line. Do not change any regulator if pressure is low before you check the pump fuel volume! And not again with a coffee cup as this want tell you bloody nothing. Fuel pumps which have volume deficiency due to wear/ overheated / worn brushes/ ect. can not provide accumulation force to bounce the fuel pressure up to the right setting or provide the necessary continues flow to keep the right fuel pressure stabilized.

Back to the pump voltage change!
he reason to do this, is partly pollution matter (1), and to avoid a stall of the engine whilst on idle, (2) as engine friction at idle inhibiting the smooth engine idle momentum where the ECU puts the helpers into action, " Idle Stabilizer Valve and spark advance " as on those models it requires lots of energy to drive them or keep them energized. Energy which would be wasted by the pump at idle and again in those models only. To keep the programmed active data as close as possible in the parameter envelop and by keeping emissions as low as possible without the danger of oversteering to cause OVERFUELING, or a spark deficiency whiles the ECU is changing continues injector flows at idle, it requires all the energy it can get out of the alternator to feed its helper components. It is better to sacrifice fuel flow as not needed on idle, but "NO" to loss of "Zap" to the spark plugs as the engine is lean and it requires also the "Zap" to the Idle Stabilizer solenoid to match the incoming air to the injected fuel ratio. Once the accelerator is moved, the Idle stabilizer is off its duty cycle and the fuel pump is back on full spin.

But back to the conventual vacuum assisted fuel pressure regulator, when the engine is on idle, you don't want total fuel pressure continuous at the back of the injector as it defeats the purpures of emission control! And it will waste fuel on idle if the lambda control is off loop. Another point to this is, it will dilute the engine oil with fuel and can destroy the catalytic converter by melding the core. This is also for safety reason for this as well. On engine idle, how can you know if you don't have the LB-291/2F equipment as such, what the injector flow is doing? Visually we can monitor how the 'Helpers' to the ECU assisting to perform continuous tiny injector flow corrections. It's magic to watch to see the full picture how the "helpers" to the ECU perform there duty to compensate to engine friction to stabilize the smoothest engine momentum & characteristics. Pull the wire harness off or vacuum hose of one of the "helpers" and proof for your self in a lighting flash visually as to a rebound signal bounce of the injector flow indicator if they a live or dead! Try to tap on the knock sensor with a screwdriver handle and see injector flow reductions in synchronisation to the tapping. You can not notice rpm changes at all! But the reflection of the bouncing float proves for its integrity, the quick action to counter act where it is most needed at the right time. A quick action is needed if the air-conditioner comes on, or in the same time the power steering. The injector flow will double the fuel flow amount instantly to the corresponding load factor. Mind you this is also proportionally in synchronization with the idle stabilizer valve, spark advance, and fully synchronised to the vacuum pull on the regulator which dump fuel off for lower fuel pressure (50 Kpa below normal operating pressure at least.) Due to this finer injector flow correction process, finer stabilization pulses are required to manipulate the injector solenoid in such a way that it requires less fuel pressure at the back of the injector to avoid at all cost a fuel counter pressure against the fine correcting solenoid movement.

Further which is totally overlooked to this what the regulator has in common is, that on deceleration down hill the vacuum in the manifold increases as to the corresponding engine rpm, which is absolutely normal, and this helps by pulling the diaphragm even more up allowing to dump more fuel flow back to the fuel tank. This causes an automatic rebound effect lowering the pressure even more to stop the injector from injecting fuel volume when not needed on deceleration. But the still lowered fuel pressure by this high vacuum causes a deceleration residual fuel pressure in the fuel rail by (as much of a minimum of 150 Kpa. = 1.5 bar) and is still strong enough to prevent a possible pintle bounce as well. If this is not working correctly, then the pops in the exhaust are strongly noticeable on deceleration and a hole in the exhaust or a leaky manifold gasket can only add oxygen into, for an extreme damaging exhaust detonation./s.

That's way regulator are very sensitive if they are crimped or blocked off on the outlet site as this action causes, not only damaging the diaphragm by over stretching (1), which causes the regulator not been able to de-load fuel pressure off the rail quickly and correctly (2). But also bludging the fuel pressure regulator out (3) and in term of this, it lowers the overall fuel pressure setting in the entire regulator operating segment (4).

O, no, a stretched diaphragm inside the regulator which can not de-load fuel pressure on idle (properly) and certainly not in the deceleration mode if the diaphragm is resting right up or even near against the top chamber wall and the internal spring pressure which is the only idem inside controlling the flow in a resistant primary manner as it is not vacuum sensitive. So how can it be fault free?

You can not see this diaphragm unless only by dissecting the regulator casing for closer inspection of the diaphragm. We can detect those problems in a special service application with our equipment and there is no need to destroy a possible good regulator. Please read on in the INTERNET article of mine,
Why should you not use one way rail cleaning devices as it leads to crimping off the return line after the regulator which is causing all the damages. The vehicle leaves the Workshop worse then it originally come in.


The Engine control system and it's components (L-jetronic)

1 - Fuel tank
2 - Electric fuel pump
3 - Fuel filter
4 - Distributor pipe
5 - Pressure regulator
6 - ECU
7 - Injection valve
8 - Start valve
9 - Idle-speed adjustment
10 - Throttle-valve switch
11 - Throttle-valve
12 - Air-flow sensor
13 - Relay combination
14 - Lambda sensor
15 - Engine temperature sensor
16 - Thermo-time switch
17 - Ignition distributor
18 - Auxiliary-air device
19 - Idle-mixture adjustment
20 - Battery
21 - Ignition-starter switch


 Bosch L-Jetronic EFI: The Electronic Jetronic

Robert Bosch was an interesting man. Born near Ulm, Germany in 1861, he grew up wanting to become a botanist or zoologist, but "gaps in his knowledge," as his teachers put it, kept him out of the university, and he was apprenticed to a precision mechanic at 18. He went to New York in 1884, where he got a job with a company that made various electrical devices. Two years later, he returned to Germany and started a manufacturing shop in Stuttgart with savings and an inheritance from his father. It comprised himself and two employees.

Stressing quality and integrity, Bosch built his company on a firm foundation, and it grew. A milestone was the completion of the shop's 1,000th magneto in 1896. He also cared about his employees much more than your average 19th century industrialist did. He reduced the work day to an unheard-of eight hours and paid generous salaries, which earned him the nickname "Red Bosch" among those who were both paranoid about communism and envious of his success. His answer: "I do not pay good wages because I have a lot of money. I have a lot of money because I pay good wages."

He died in 1941, but his style of enlightened management continued at Robert Bosch GmbH -- treating workers well, planning far ahead, doing plenty of homework, and jealously guarding a reputation for quality. A good example of the results is the company's family of EFI systems, on which it's now making money by the wheelbarrow. It bought the rights to Bendix patents way back when it took real foresight to imagine that such a thing might become popular, then it proceeded to develop the concept to the point where VW bought it for O.E. installation on the '68 Type III (hey, that's almost three decades ago!).

How much "Luft?"

The original D-Jetronic system used a vacuum sensor to inform the electronics about the intake situation, but it wasn't long before a more accurate means of measuring the volume of air the engine was taking in appeared: the air flow meter (also known as an air box or air vane). Using a spring-loaded rotating flap that turns a variable resistor (call it a potentiometer, if you like) this device tells the computer how much of the atmosphere the powerplant is ingesting at any particular moment.

This got the name L-Jetronic (the L stands for "Luftmengenmessung" auf Deutsch, meaning "air flow management"), although it has aliases. Volkswagen, for instance, calls it AFC (Air Flow Controlled) fuel injection. Regardless of what appellations the carmakers may use for it, if it's port injection with an air flow meter, it belongs to the Bosch L-Jet design family. Later refinements include the addition of Lambda control (oxygen sensor feedback to you), a hot-wire mass air sensor (called LH-Jetronic, this system determines the actual mass of intake air corrected for density rather than just the simple cubic volume), and Motronic, which has integrated spark advance control.

To call L-Jet popular is a gross understatement. You'll find it on slews of late-model cars, both domestic and import. The Japanese are especially fond of it -- the components may be labeled Nippondenso, but they're manufactured under a Bosch license. In fact, I'm going to use the common '86 Mazda 323 as my example where I give specific procedures and values. Others will be similar.

Short tour

No, I'm not going to bore you with a long-winded, excruciatingly-detailed description of how the system works, but a quick overview couldn't hurt.

Gasoline is supplied to the injectors by an electric pump back by the tank, and a fuel pressure regulator maintains fairly constant psi.

The injectors themselves are simply solenoid valves aimed at each intake port. They're either on or off, so the amount of fuel injected is a function of the length of time they're energized (called the "pulse width," a typical range is 1/7,000th to 1,000th of a second). An adjustable camera makes a useful illustration. At the same aperture, twice as much light hits the film at a shutter speed of, say, 1/30th of a second as at 1/60th.

An electronic control unit provides the high-speed ground that completes the circuit to the injectors, and determines the span of open time that will result in the ideal mix for the particular conditions of the moment on the basis of input from various sensors. One of the most important of these is the above-mentioned air flow meter (besides sending the volume signal, its movement closes a set of contacts that completes the fuel pump circuit when the engine is running or being cranked). Info on operating temperature, rpm, exhaust gas oxygen content, etc. is also critical to accurate fuel metering.

In early versions, a cold start valve controlled by a thermo-time switch sprayed extra fuel into the manifold for starting, but newer designs have the regular injectors doing the same job. An idle speed stabilizer, rotary idle actuator, or auxiliary air valve admits extra air during warm-up to provide decent idling.

Trouble tracking

I talked with injection training expert Andy Balmuth for some real-world advice on the system, and compiled service information from Bosch, Mazda, and other carmakers to come up with diagnostic info that should actually be helpful.

At the risk of repeating what I've preached in many other sections of this encyclopedia, the first thing to remember when you've got an L-Jet-equipped car with a performance, driveability, emissions, or no-start problem is not to automatically blame the fuel injection set-up. Just because it's present and perhaps not that familiar to you is no reason to suspect it. In fact, chances are the trouble is elsewhere because this system is extremely dependable.

So, check compression and ignition first, then look for a high, steady vacuum reading. A good way to find vacuum leaks is to use a propane-type detector (the kind with the hose, wand, and valve). Rpm will rise if the propane finds its way in through a gap.

And don't neglect the battery and charging system (including the alternator belt). Balmuth told me that on some older imports, low volts can cause excessively rich running, while a high voltage regulator setting can result in a lean condition.

Necessary clicking and psi

In cases where you're satisfied that the EFI is indeed the culprit, a good preliminary check is to listen to the injectors with your stethoscope. One's silent? Then remove its harness connector and check for voltage at one connector terminal and regularly-occurring ground at the other (one of those inexpensive "noid" lights will make this especially easy, but any regular 12 volt test light will do). If you find this combination, the injector itself is bad (resistance across the terminals of a typical specimen should be 1.5 to 3 ohms). No juice or ground means there's a broken wire in the harness branch to that cylinder. Remember, most of the L-Jets actually out there fire all the injectors simultaneously, not one at a time as in some sequential systems. By the way, if they're all silent during cranking, that's the reason the engine won't start.

Fuel pressure testing is also one of the most basic procedures. Hook up your gauge (while you're connecting it, put a rag around the fitting to catch the spraying petrol) and run the pump, which will require that you either switch on the ignition and manually move the air vane, or, on my Mazda example, jump terminals GW and B of the check connector. Look up the specs for the vehicle you're working on, but a common range is 35-40 psi. Start it up, let it idle, and the reading should drop to 28-31 psi.

According to Mazda, if you take the vacuum hose off the pressure regulator, you'll see the needle rise to between 37 and 41. Low pressure suggests a weak pump or clogged filter, while too many psi may be due to a restricted fuel return line between the regulator and the tank.

On the subject of fuel pressure regulators, Balmuth said, "They rarely go out of specs, but when they do they can cause 'ghosts' -- problems that throw off your diagnosis of other components. Also, they don't go bad as often as they're replaced." If you find a car that's hard to start when hot, suspect leak down.

Obviously, zero pressure will result in a no-start. You might as well check the pump fuse first, but if that's okay there may be another impediment to electron flow. With the key on, reach inside the air box inlet and push the flap open. If you hear the pump start running, the points in the box are okay. But maybe cranking vacuum isn't enough to move the flap because of mechanical interference, a leak in the duct between the air box and the throttle body, or a backfire protection valve (if present) in the vane that's stuck open or blown out altogether.

Cold comfort

Cold idle problems can usually be traced to the auxiliary air valve or its equivalent. The most straightforward check is to start the engine cold, and pinch the hose that runs between the regulator and the manifold. Speed should drop. Let it run long enough to reach normal operating temperature, then squash the hose again. There should be little or no change in rpm. Also, the specified resistance across the terminals of the Mazda air valve's heating element is 30 to 50 ohms.

You can do a pretty comprehensive exam of the electricals/electronics of the system by probing the terminals of the ECU connector. Of course, you'll need the specific values and instructions for the vehicle in question, which can usually be found clearly stated in the service manual.

Dying of thirst

A lean mix that causes hesitation and surging is a relatively common problem, and clogged injectors or a bad temp sensor that lets the ECU think the engine's warm when it's not are likely causes. Some cars have come from the factory with a too-lean calibration, which the engineers often address with a service bulletin. For example, an electronic module is available for late seventies Nissan Z-cars that's spliced into the coolant temperature sensor wire to alter the signal to the computer. For certain VW's with hesitation only during warm up, you can get a spacer that screws in between the sensor and the engine to slow the rate at which it heats up.

I should mention a couple of bootleg fixes that may or may not be legal depending on the situation, but are sometimes used anyway when the trouble is chronic, yet everything checks out (Rule #1 of modern auto service: Never make a modification if an authorized repair will eliminate the problem).

First, you can put resistance in the wire from the temperature sensor to trick the brain into believing the engine is cooler than it really is, which will richen the blend. A good method is to use a variable resistor, adjust it until the desired results are achieved, measure the number of ohms at that setting, then solder in a fixed resistor of the same value.

Another possible remedy is to alter the sweep of the rheostat in the air box, but caveats apply. Removing the air flow meter cover will void the emissions warranty, and it's easy to break the cover in the process (just try to get a new one). Also, a tiny adjustment can make a huge difference in the mixture -- Balmuth says .020 in. of movement may cause CO to rise 2-6%!

Flow faults

Finding clogged injectors can be done in several ways. I usually use one of those neat little "buzz box" devices that triggers each solenoid for a precise amount of time. In conjunction with a pressure gauge, you use it to compare the psi drop among injectors. If the fall-off is smaller with one than the others, you've found the problem.

Odds and ends

Finally, I'll offer some miscellaneous tips that apply to all air flow meter L-Jets:

  • Whenever you find a broken air flap mechanism, make sure you clean out all the shrapnel.
  • If you feel binding or roughness as you move the air vane slowly through its range of travel, trying to fix it will be a waste of time.
  • In cases where the air flow meter's variable resistor appears to be worn through or has burned spots, a whole new box is the only fix.
  • Resistance between terminals E2 and VS of the air flow meter should be 20 ohms with the vane in the closed position, and 1,000 ohms in the open position.
  • Never try to adjust the air vane return spring tension as you can't get it right without special equipment.
  • After you've fixed a contaminated injector situation, it's a good idea to remove the fuel filter, open it up with a pipe cutter (not a hacksaw), slice the elements with a razor blade, and inspect the innards. If you find lots of rust or other contamination, the tank should be removed and cleaned and the lines blown out. Failing in that, replace the filter more often than the specified interval.


Motec schematic pages

Motec8 ECU wiring   Motec8 wiring loom    Crank position sensor    Lambda Sensor

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