mirousse

Here's my problem:

Car has 85k miles. All service and normal maintenance have been done on the car and nothing is left unserviceable. Apart from the following problem, everthing's normal.

When engine is warm and the car brought to a stop the idle sets steady at around 500-600 rpm. After idling for a minute or two, the rpm drops close to a stall but recovers almost instantly. It's usually a one time event in a week of driving. Still, it annoys me and I fear that it can devellop to a more serious problem.

Anybody has a clue about what it is?

Thanks.

Michel
89 928 S4 ROW

Stromius

Possible vaccum leak?

mcholdfast

I had a similar problem with my 90 ROW S4. It also had problems passing the emissions test but I'm not sure if you have that to deal with. It was running very lean and I eventually found out that there is a potentiometer on the LH that is used to adjust the fuel mixture. Once I set the pot according to Hilton's instructions in this thread, the low idle went away and it passed emissions with no problem. I hope this works for yours as well.

mirousse

Vaccum leak could be a possibility. Where should I start looking first?

The idle is not low and it is steady. It just suddently drops from time to time. I had the DME computer adaptation reset at a Porsche dealer last year. The idle was fine tuned as per the factory data.

John Speake

Is the car cat or non-cat ?

mirousse

Cat

Stromius

On testing leaks, try Dwayne's excellent intake refresh writeup. The first page contains vaccum checks.
https://rennlist.com/forums/928/4826...re-w-pics.html

mirousse

Great! Thank you Stephan.

cali4sun

I had the same issue which was corrected by replacing spark plugs. However, I would also check for vacuum leaks.....

'89 S4 GP White/Black
'76 912E Silver/Black sunroof

Objects in mirror are losing

John Speake

In that case, ignore earlier comment about idle mixture pot, it is not wired ito the LH on your car.

A vac leak would usually give too high an idle speed.

Does it idle at higher rpm while warming up, as it should, then settle down to a steady 675rpm when at normal temp ?

mirousse

Yes it does. I would say it's a normal operation until I get this momentary close to stalling drop in RPM.

John Speake

It sounds like the idle loop is well in control as it should, most of the time. This issue may be due to heat soak tending to lean out the mixture.

Does engine temp stay within normal limits when standing stationary at idle for a minute or two ?

mirousse

Temp is normal and steady. Thanks for scratching your head along with me!

What do you mean by "heat soak"?

SeanR

TPS not engaging? Had a similar issue.

John Speake

It's when you sit stationary in traffic, then the heat in the engine bay builds up, even with the fans running. Fuel gets heated up, even with the aircon running (Which feeds the fuel cooler). Whole engine car get hotter than when car is moving - "heat soaked" , Temp 2 sensor detects higher than normal running temp, this signals the LH to lean out the mixture somewhat.

Did the car behave better under these conditions when you had a diagnostic tester run the idle adaptation routine ?

Idle speed is regualted to 675rpm when engine is at normal temp. You quote your idle speed as "500-600rpm". i.e.much closer to 800rpm on the tach. Do you believe it is correct ?