Shark_gts

A friend of mine has been driving a '90 S4 we did up a few years ago for him. We had to swap out the transmission & changed differentials (to keep the PSD)recently. After adjustment she drove fine after that. That was about a 6 weeks ago.

Two weeks ago he started having some idle issues. I thought it would be the typical ISV at first, he brought it in. Nope this sounded different.

Symptoms:
Starts fine from cold. Runs OK.

Within a few minutes as temperature builds to about 75 DegC, all hell breaks loose. Stumbles & stalls, won't idle smoothly, won't accept throttle input (dies), sort of like a bad MAF. Exhaust smells like some kind of paint thinner too!

Survive this zone and at about high 80's or so to full operating, the engine smooths out. It drives OK, good power. But there is the occasional miss here and there and there burp in the idle at times. Not bad, but I know not 100%. I betting most wouldn't detect any issues at full operating temp.

I've checked Temp sensor II at the computer plugs. It is within the spec range listed on the manual for the temp the engine is at. I have not swapped in a new Temp II.

We swapped in a rebuilt the MAF, a good LH computer and EZK computer. Idle contact is good. Checked for any leaks in the manifold area and could detect none.

Hmmm.. any ideas?

AO

Sounds like maybe the throttle position switch may not be making contact at idle. That's all I can think of off hand.

SwayBar

Sounds like the O2 sensor could be bad.

Shark_gts

Unplugged the O2 and no difference. I could test the O2 though. I just never thought an O2 would absolutely cripple an engine.

Idle contact is OK, or at least makes on throttle closed.

heinrich

fuel fouled? filter?

Bill Ball

Someone recently had a crazy short in the Hall sensor shielding that caused the car to run like crap, a lot worse than you would expect from 6 degrees of retardation, so the short was doing something else. You need to complete the Spanner/Hammer tests. You can do them as you are doing with a meter at the LH and EZK plugs.

dr bob

Another lister recently reported some similar issues when the WOT switch was incorrectly reporting throttle open all the time. Car went to full-load rich in closed-loop mode, ignoring the oxy sensor among other things. Test at the LH plug, same way you test the idle switch.

Just a thought.

Shark_gts

Another clue, during the warm up when the "episode" occurs within that temperature window (75-85), I unplug the MAF she runs OK in the limp mode. 5 minutes of that and it's up to temp, and will run half-way decent, certainly driveable.

I did not check WOT.

I have to figure out is she going rich or lean during the fault period. My guess is she is going too rich based on the smell and overall feel of it. I plugged in a new temp sensor II, not connected to the engine manifold (i.e. lower temp) and it ran the same - like crap - which would put it too rich.

I'm going to bring the car to my house where I can work on it over the weekend.

Thanks for the feedback..codes would be nice. I have a new Snap on Solus, scans most anything but didn't ask about the early Porsches.

Tom. M

possibly bad exhaust temp sensors? running on 4 cylinders...

Later,
Tom

TheoJ

SharkGTS,
all things were already mentioned but one was left out. Maybe everyone is scared to say. Run the engine hot and then check the exhaust for CO reading. If it is very high, way above limits, and temp2 is ok, MAF is ok, ...... you're maybe looking at a defective LH ecu. Typical phenomena is that the car runs ok when cold, but the ecu is unable to identify if the engine is hot. The engine then goes from 400rpm and stumbling, suddenly to 1200rpm, and drops down to 400 rpm to eventually die. Let it cool down completely and it seems ok again.
You may smell the unburnt HC from the overly-rich fuel in the exhaust....? Maybe even a CAT that is red-hot?

I would first connect the 928 diagnostic tool to the car's diag bus to see what is going on.

John Speake

It sounds like an O2 snesor problem to me as well.... the car goes onto the O2 loop when water temp gets to about 75C

Unplugging the O2 sensor is not good enough. You have to fool the LH better than that. Disconnect battery ground strap. Then unplug O2 sensor. Then reconnect battery ground.

Start car.

Let us know how you get on.

AO

Listen to Herr Speake. He's very knowledgable on the subject.

bd0nalds0n

I thought he already mentioned swapping LH units with known good?

Besides, what's to be scared about? I'd be a lot more excited to learn my problem with good cold/bad warm running was LH-related than thrust bearing related (which, in this case, has already discounted by virtue of the car running well at normal operating temberature).

Garth S

Any hiccups shown by the diode lights of the ignition monitoring relay as the car warms up?

worf928

Nothing to add other than a second for the motion to hook up a tool to the 19-pin diag port.