Starting Problems Trouble Shooting
 

I ran across this and though it might be helpful in my starting problem.

Troubleshooting Porsche 928 Starting Problems

These notes apply to the LH equipped 928 cars.

For any car, there are 4 essentials for the engine to run - fuel, spark, timing and compression of the air/fuel mixture.

All LH equipped 928s operate on a similar principle - a sensor (flywheel speed sensor) detects engine speed and crank position from a toothed gear at the rear of the engine. The flywheel speed sensor output is fed to the EZK/EZF (ignition) ECU. This gives the EZK the information it requires to time the spark.

A modified version of the rpm signal is fed from the EZK to the LH ECU (and also the rev. counter). When the LH (fuel injector ECU) receives this rpm signal it will energise the fuel pump relay. The rpm pulse also times the injector signals from the LH ECU. Information on engine air intake mass (load) comes from the MAF (mass airflow sensor. From rpm and laod data, plus engine temperature and other sensor paramters, the LH ECU determines the correct fuel injector opening time.

3 relays are vital for the car to start - fuel pump relay, EZK relay, LH relay. The fuel pump relay is a common source of problems, as it passes a high current to the fuel pump (and also the O2 sensor on later cars).



Method

1) Does the car have fuel ?

2) Do you hear a regular clicking noise from the engine when the igntion is switched on, but the engine not cranked? If so, this is a sign of LH ECU failure (see LH ECU FAQs). If no, proceed as below.

3) Is there a spark ? If not, swap out the EZK relay for a known good one. If still no spark, check if there is no spark at each distributor. If spark at one only, then check out final amplifiers. For a Euro S2, check that both distributors are turning when the engine turns. There is a belt coupling them which can break Then suspect flywheel speed sensor or its connections. This sensor is located under the engine airfilter, on top of the engine.

4) If there is spark, but no start, then check that the fuel pump relay is being energised when the engine is turning. Alternatively, you can "jump" the fuel pump relay. But take great care to link the correct pins on the relay socket, or damage can be caused to the LH ECU. The contacts to be linked are 30 and 87. When these are linked, the fuel pump should be heard to run, even with the igntion switched off. If the car now starts, then remove the link and fit a known good fuel pump relay.

5) If there is still no start, then change the LH relay for a known good one.

6) If still no start, then LH failure is a possibility if car ran well before the no start. If car stopped suddenly at speed, and now will not start, consider the following possibilities - a) Cambelt snapped. Remove both intake airtubes and check if both camshafts are still turning. b) Check fuel pressure at the test point on the fuel rail. A fuel pump can fail if fuel tank level is low and/or very hard sustained cornering momentarily starves the pump with fuel. Note that there are often two pumps, the external one, and often an in tank pump. Both may be damaged through fuel starvation.

7) If there is fuel, but the car does not start, bear in mind that a 928 engine can be easily flooded with fuel if cranked repeatedly with a no start. Remove a spark plug to check if it is wet. If wet, try removing the fuel pump fuse and with accelerator pedal fully depressed crank until the engine fires. Then replace the fuel pump fuse, and again crank to start with throttle depressed.



JDSPorsche can check out your LH ECU, EZK ECU and MAF for a nominal charge if you believe any of them are responsible for a non start.

check compression

The motor is in very good condition and has very low miles on it. I think the problem is electrical in nature. It was running fine but did have a few hard starts when hot. I am going to do more testing on it. I do have a compression gauge if necessary. I am getting spark from both distributors.

Do you have the CD's?

The full 85/6 EZK/LH test plan is in there. (edit: EZF / LH )

Send me your email and I'll send a copy of the plan.
It might help you narrow-down the issue.

Thanks, I will send you a personal note from here.

Well, I have the new plugs all in and tested and got spark on both distributors. I tried to start but, a little pop but not much. I will check for any more flooding later and run some relay tests, etc., as outlined above.

A Porsche mechanic in town told me that even on good engines the rings have to have oil on the cylinder walls to seal correctly. He said some people have pulled the car in the driveway for a wash and then can't get it restarted and the problem was low compression. It's just one of the 4 things you need to start it so I thought it would be good to check especially with a flooded car that could have washed the cylinder walls off with fuel. Gunar

Yes, this is all true, if there is a really bad flooding of the motor. I can easily run a compression test. However, I just tried to start it with fuel relay out and it ran for maybe a few seconds and quit. It almost felt like it was going to run. I put the fuel relay back in, went to start, nothing, I did feel the fuel relay operate. I need to determine if fuel is at right pressure, is LH relay operating, etc. I am learning. Right now I am a little frustrated so need to sit back and analyze the situation.

just a quick note on compression,
if you have a doubt about it then the easy way is to put the socket on the crank bolt and spin the engine ,
if you can feel each compression stroke,
IE there is resistance then you should have compression to start the engine.

OTOH if the engine spins easily with not much resistance then the compression may be low or the timing of the cams is off.

Note on an engine that has sat for a long while the compression may be low and will usually increase once it has been run and all of the parts are meshed together

Thanks Stan,

It sat maybe since Dec. 1 about the same as last year when I did the T-belt, after which it started immediately. I am going to investigate it more today. Ok on turning the motor over by hand. I have new plugs in, pulled the fuel relay, it fired and almost ran but quit. I reinstalled the relay, nothing on starting, relay did operate. Getting spark on both distributors. I bridged the relay once, pump ran. Not sure what fuel pressure is. Going to check LH and EZh relay operation. Also see if it flooded again.

To reset alarm, lock, then unlock, the car.

Take time out and set to TDC and check both inspection holes for cam gear alignment.

Sounds like you are close to running. Can't be much of a problem. Temp II replacement may be in your future.

Oh THANKS on resetting the alarm. I was reading the manual on the "S" key deal. I have two "S" keys which came with it. Did not know about lock and unlocking!!! It said nothing in the manual. Thanks much.

I did do the T-belt last year! Checked tension again at 2K miles. All looked normal.

Many thinks for those documents, I was reading some last night. Better than some I had on trouble shooting.

QUOTE=Landseer;5857350]To reset alarm, lock, then unlock, the car.

Take time out and set to TDC and check both inspection holes for cam gear alignment.

Sounds like you are close to running. Can't be much of a problem. Temp II replacement may be in your future.[/QUOTE]

[/QUOTE]

Ya need closing brackets on the word QUOTE, it's the HTML guy in me. I think Landseer has it nailed as I always hear about that Temp II Sensor in the later cars, like yours.

When I refresh them ( and have pocket money) best course of action is replace all sensors premptively.

I checked it (temp II ), at 47 F engine temp it read 4K on pin 1 and 3.7K pin 2. I may just replace it. I put a temp probe on the motor.

According to the WSM

Degrees F Resistance-ohms
32 4.4-6.8k
59-86 1.4-3.6k
104 .9-1.3k
140 480-720
176 250-390

Notice as the engines warms up the resistance decreases. I would pull the sensor and using a heat gun or hair dryer warm it up to see if the resistance changes. Both pins should read essentially the same thing. The signals from the pins go to different places. It does appear though that your temp sensor is reading higher resistance for the actual temperature which gives a very cold engine signal which would cause the computer to richen the mixture.

Yes, it is suppose to change and the resistance has a variation for sure. For the cost I think I will order a new sensor. However, I did measure it back when the motor was hot and RUNNING and it did show a much lower resistance at higher motor temp. I just wonder what the actual designed tolerance is. Usually resistors are in 1% 5%, 10%. I assume this is what is called a thermistor or variable resistor with temperature.

Pin 1 seems to control the EZH feed and Pin 2 the LH module from what I have read so far. LH for the injectors.

Thanks for the info.

928 sputters but wont start
 

Hi,
I have read and learned much from other posts but still have a problem. My 86.5 928 automatic wont quite start. I have checked and replaced the relays. I cleaned and resealed the fuel injectors and it wont start. It has spark and will run for a few seconds if I spray starting fluid into the maf entry point but that is it. Any ideas on something else I may not have looked at or tried yet would be most appreciated

Corona Ahh a clue is the fuel pump getting warmer,
what that means is its seizing and the 12 Volts is heating it when the switch is engaged.
replace the fuel pump and the fuel filter .
order 3 new sealing washers and the hard pipe from the pump outlet to the filter.
also drain off some of the fuel from the tank and smell it,
if it smells old then you should consider flushing out the tank prior to, installing the new pump.
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