touay001

Season greetings All,

I have a problem with my 1988 S4. I took the car to be spray painted a couple of month ago. I went to pick the car up and it would start but will not idle or run. Tried a few things and finally by removing the MAF connector the car would start and idle reasonably well. I managed to get a rebuild MAF (John Speake’s agent in Australia) . I replace the MAF and the spark plugs (very sooty). Started the car, it starts and idles ok but still will not run. If I press the paddle the engine will not respond. If I floor the paddle, the engine will die (I can hear lots of pooping sounds) almost as if there is a large vacuum leak. The car just had a major service at two month ago (TB/WP, full intake job, all sensors, and all hoses under the intake...) and drove perfectly well into the spay painter’s shop. I’m not sure what to do next. I’m afraid that the LH is gone as well. The person who sold me the MAF is going to lend me the LH, and the ignition computers (great service) to see if mine are toast. Any ideas of what could be wrong or what else can I do to get the car going again?
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Akram

1988 Porsche 928 S4 Auto
1981 Porsche 928 S Manual

Art20c

EZK failure is not very common as LH. If you have a chance to obtain LH and EZK loaners, start with replacing LH unit.

touay001

The strange thing is that with the MAF connected the engine will not respond to throttle or dies. Without the MAF it runs OK. Both with the old and the new MAF. I'm not really sure if it is the LH. Could it be theat the new MAF is also faulty?

Art20c

I had similar problem, and LH was the problem. This is link to my thread back then from the start to the end
https://rennlist.com/forums/928-forum/333419-how-to-test-maf.html

touay001

Thanks Art. Very interesting. I hope I get the LH soon.

Podguy

In medical school they have an expresion - "hearing zebras". New docs always diagnois rarest and most complex disease while mising simple things.

Now lets destroy some myths. LH units rarely fail. Even jump starting the car or welding will not mess them up. But for some reason this is the first thing everyone goes to when the car does not run. There are many things to check first before looking at the LH unit.

To test an LH unit the friend who is loaning you a good one is nuts. Never never put a good electronic component on a car that is having problems. If the LH or any other unit is bad it may have gone bad from another component. Putting a good LH in car with problems could easily kill the LH. Always - always put the bad component in the running car to test. This way you know two things. One the component you are testing is good or bad and you are operating only on one variable at a time.

Before you do anything else lets try to eliminate a few other things first. The car to run requires three things, spark, gas and timing. If the car runs then it is getting spark. If the valves are not bent then the belt is probably good. I will say probably here since it has just been changed the the person doing the job could have screwed it up. Besides it is wise to change the belt each time you drive the car.

So the problem points to gas. Have you checked the fuel pressure. Low fuel pressure will allow a car to start and run but not accelerate. If your pressure is not right then the car will not accelerate. You did not say what the body shop did besides paint the car, but the bodymen I know are terrible mechanics. They could have jacked up the car and pinched a gas line.

So the first place to start is with the gas flow and pressure.

Next you say it idles - but how well does it idle? If the car is running on four cylinders it will idle rough but not so rough to be immediately noticable. The S4 has two parallel ignition systems. This is because the ignition system was built as a four cylinder system and then doubled for the 928. If one system is down then the car runs on four cylinders. This will cause a loading on the cylinders not running and will first load up the cat and then catch the car on fire.

The later cars - 89 and up had a relay and sensors to keep the injection from feeding gas to the non operating cyclinders. So if a body man bumped one of the coil wires or the amplifiers on the front of the car - those things under the plastic cover on the front cross member driver's side. Before starting the car remove the plastic cover and disconnect each plug to the amplifier. You are best removing the metal spring clip on the connectors and then plugging them back in. Start the car and let it idle. Go to the front of the car and one at a time remove and replace one of the plugs. If there is a change in the idle for each plug then the ignition is working. If the car dies when you pull one of the plugs then the opposite of the ignition system is dead.

It is common for water to get into the boot of the coil wire on the distribtor cap. Corrosion will interrupt the current and shut down one side.

There are a few other things to check like all the connections and hose on the engine, but most will not cause the symptoms you are experiencing.

The best approach is to look at any possible things the body shop did to the car while it was there. Since the car was running when it went there the problem is proabaly the result of something they did.

Since you have already changed the MAF, check it again. The Plug going to the throttle switch is known to have problems with frayed wires.

Good luck and I hope you are back on the road for Christmas.

Charley B

Say what?

John Speake

QUOTE : Now lets destroy some myths. LH units rarely fail. Even jump starting the car or welding will not mess them up. But for some reason this is the first thing everyone goes to when the car does not run. There are many things to check first before looking at the LH unit. ""

They do fail, I have rebuilt many 100's of confirmed faulty LH ECUs.

Akram, send your LH ECU to Nick and ask him to check it out for you. It is most unlikely the MAF is faulty.

touay001

Thanks John. Nick was very helpful, he had offered me his MAF to get me going before he discovered that he had a rebuild one in stock. After Christmas I'll contact him.
Thanks guys for your help.

Peter F

You may want to check the cables near the MAF connector.
Mine was looking like this and touched intermittently causing bad running and hard start.


/Peter

touay001

Peter,
That's how mine looks as well. I was thinking of getting a new plug actually. Anyway, I made sure that the wires are not touching (electrical tap).

fabric

While going through some idle issues last year, I attempted to compile all the things to check. This a comprehensive list, although you may have to dig up some details on specific items I left out. I didn't necessarily document how to everything, but tried to point in the right places when possible:

http://members.rennlist.com/fabric/i...eshooting.html

John Speake

If you can get to the car, make sure the paint shop didn't undo a ground wire somewhere, disconnect something for better access etc.

touay001

Thanks Fabric, great compile.

I have summarized the engine behavior as follows:

1- The engine starts and idles (with or without MAF)
2- With MAF on, no throttle response or dies (deep popping sound from the engine)
3- With MAF off, idles reasonably well and responds to throttle (I tried up to 3000 RPM)
4- Connect the MAF while idling, instant cut off
5- Too rich, plugs are too sooty (replaced)
6- Fuel pump is working (see #3)
7- Ignition is working (#1, #2, #3)
8- If there is a large vacuum leak, the engine shouldn't behave differently when the MAF is removed (see #3)

That's all what I can think off at the moment until I can run some more tests when the shop reopens.

John Speake

Disconnecting the MAF throws the LH into a limp home mode, so you can't always draw simple conclusions from that test.

Try the O2 sensor disconnect test:
1) disconnect battery ground
2) unplug O2 sensor at 3way conenctor in footwell.
3) reconnect battery and start the car.

Does car run better from a cold start than when hot ?