RennBod

Hi all...

I friend in the local club is having massive amounts of problems with his 928S4.

If the car is left over night, the next morning (in winter) it will start almost right away and then die within a second.. after this, if he does not use the throttle, he can be there all day cranking the engine... if he uses the throttle just a little bit it will start and run (with some spluttering) but he has to keep the throttle pressed to keep it running until its warmed up... Also during this period the exhaust throws out lots of fumes.

We checked his temp2 sensor, and at cold (engine at 11 degrees with a infrared thermometer) each terminal has an equal resistance of 3.5k ohms.

Even once warm, it seems to idle low, perhaps hunting a little between 350 (almost stalling) and 500 rpm

Dissconnecting the MAF makes it run worse. But we do not know how to check the MAF any more than this.

Looking through the archives, it seems there are several causes for his symptoms, including :- sticking Idle valve, vacuum leaks, maf failure (?), LH brain going wrong, and all sorts of other problems.

I was thinking of doing the idle valve WD40 thing I have seen here, but cant even work out where it is, and where to squirt it... do I have to remove the maf to do this? where is the pipe that runs to it? is it easy to get to? (we have the air filter box all removed at the moment)

I have asked my local specialist if he can recommend anything, but he has a 928S4 in at the moment with exactly the same problem and is trying to get to the bottom of that.. and with both of these S4's with the same problem, its not even like we can exchange some parts to swap and test.

apart from this...... PLEASE HELP!!!

John Speake

Hello RennBod
You can't check the MAf with the accuracy needed, but the problem sounds more like an ECu problem.

What year is the S4, in other words does it have cats and O2 sensor ?

There is a very good 928 specialist at Poole who I can recommend, who have you consulted ?

I can help with known good parts etc.

Regards

RennBod

Hi John,

ECU problem isnt what my friend was hoping to hear... in fact it was exactly what he was afraid of!

We have been looking at this S4 for the last two weeks scratching our heads waiting for the specialist in Bournemouth to be back at work following his christmas break, so to hear that he cant look at it for 4 weeks and that he has another one in at the moment with the same kind of problem, put us into a tail spin.

Who is the specialist in Poole? If its Quatro (VW/Porsche specialist) then I doubt he will be able to find the problem, they had one of my cars there for weeks without finding a simple fault it had. If its Jon Mitchell, he is the one who is fully booked for 4 weeks, and if its the Porsche centre in ferndown they are not worth talking to!! If its someone else, please let us know!

John Speake

Hi
The specialist I was thinking of was Jon Mitchell. I can check out the LH ECU and MAF for you on my test jigs.

From earlier mail - what MY is the S4 - does it have cats ?

I will PM you - with my tel #

Regards

RennBod

Thanks John!

Its nice to know you would recommend John Mitchell, my experience with him has always been good. But you never know with garages these days.

One thing I did not remember to mention, is that the tacho (rev counter) on this car seems to act a little strange.

When the engine is running at (for example) 2000 rpm, the needle may drop down to 0 rpm, and then pop back up again.

Is this fault connected in anyway with the starting and running fault?

Just thought I had better mention it, just in case. I expect its just a bad connection on the back of the instruments or something, but for all I know it may be something more important and worth mentioning.

John Speake

OK, that is a significant observation. It may indicate a problem with the flywheel sensor.

The signal from the flywheel sensor (on top of the bellhousing, under the engine air filter) sends engine speed pulses to the EZK. From the EZK, a modified version of these pulses is fed to the tach and the LH ECU. If the LH ECU does not receive these pulses, then the fuel pump and injectors are not enrgised.

Intermittent faults with the flywheel sensor are not uncommon. I believe it is the same sensor that is used on the 944.

JP Rodkey

Just to add to John's sound advice: when is the last time you checked connectors? Sounds like either the ECU is not getting a consistent signal (usually you won't get an intermittant from the flywheel pickup - it's either working or it isn't), or the ECU isn't getting good continuity through a pinched wire or connector, or the ECU is bad.

Pull and clean the connectors at the ECU, looking for oxidation.

Good luck.

RennBod

Which ECU is which?

I notice it looks like there is one under the glove box (remembering its a right hand drive car) and I think there is another one behind the carpet in front of the door on the passenger side... Which one is the EZK and which one is the LH Brain.?

I had another look at the car a while ago, wishing for some flash of inspiration, and noticed that when the RPM's are brought above 2000 rpm, the engine does not run as smoothly as I would expect from a V8, I would even say that my 944 4 cylinder engine is smoother... is this normal? its almost as if when reved up that the engine isnt running on all cylinders.

I also suspect that there is more than one thing at fault with the car to make it like this, it just seems so strange.

Bill51sdr

Maybe it IS running like a 944... on four cylinders! This has been known to happen with S4's & I'm sure John can tell you why that is better than I...

RennBod

I am reading quite a bit about a relay on the EZK that has two LED's on it, a green one and a red one? which show if a cylinder bank is being knocked out or something... is this also the case on euro 928 S4's??? or is it just a US market thing?

If this is the case, is the relay on or next to the ECU, or on the relay/fuse box?

The car is a 1989 S4 if that helps.

also, I am reading a lot of threads about "hall sensors" are these the same as "engine position" or "crank sensors" and the same thing that John mentions about being under the air filter box on top of the bell housing?

If so, if I can borrow an oscilloscope from a friend, (I have multi meters) how do I test the sensors.

MANY thanks chaps, I really would like to work this out but I am not familuar with 928's as I am with my 944 or 911, so I am on a bit of a learning curve.

murarie

When you say the needle drops to zero and pops back again does the engine cut out for an instant....?

If yes your engine speed sensor on top of the bellhousing is defective, or the connections to it may be loose... , i had the exact same problem.

I changed the sensor and got rid of the problem.

Checkout this thread:https://rennlist.com/forums/928-forum/165006-what-could-be-wrong-engine-cuts-out.html

John Speake

"I am reading quite a bit about a relay on the EZK that has two LED's on it, a green one and a red one? which show if a cylinder bank is being knocked out or something... is this also the case on euro 928 S4's??? or is it just a US market thing?"
>>>>>>>>>>
This relay is fited to all 928s from MY89
>>>>>>>>>>>

If this is the case, is the relay on or next to the ECU, or on the relay/fuse box?
>>>>>>>>>>
There is a plate which holds both LH and EZK side by side in the passenger footwell, at the side. The relay is on that plate, with a clear case.

This protection circuit is only activated after a few minutes from a cold start, so probably isn't the problem. But worth keeping an eye open for those LEDs if it start to run on 4 cylinders
>>>>>>>>>>>>>

also, I am reading a lot of threads about "hall sensors" are these the same as "engine position" or "crank sensors" and the same thing that John mentions about being under the air filter box on top of the bell housing?
>>>>>>>>>>>
The Hall sensor is something different, the flywheel sensor that I mentioned is the same as "engine position" or "crank sensor". This sensor sounds like the problem with this car.
>>>>>>>>>>>

If so, if I can borrow an oscilloscope from a friend, (I have multi meters) how do I test the sensors.
>>>>>>>>>>
You need the workshop manual on CD, PM me off list...

Regards

Drmark

I had exact same problem with 87 S4 auto - only when cold, seemed to improve when hot, though still hunted badly. Learnt to left foot brake for a morning to stop it continually cutting out! Paul Anderson, who sold me the car, tracked it down to faulty ECU (the one in the passenger well), having excluded everything else - swap with another car fixed it and now have rebuilt original (£400) and all well.
Hope your friends is simpler. Looking at the previous listings it seems a common problem.
Cheers
Mark