This is from a fellow 928 driver here in MN. He's on the road, so I'm trying to help him out. Here's his email:

He drives a 1990 S4.

On a cold start it starts right up. On a warm start it hesitates and won't start until I push the trottle (getting a little smoke). Now at idle in drive and park I get a pulsation in rpm and if the AC is on I get stalled regularly when at idle rpm. Engine ruffness in park. Once going runs great ac or no.

Last time it stalled I got a green indicator on (it went off before I could read it but had a symbol that looked like a differential and axel with three letters). I have just driven the car 3000 miles cross country. It has a fresh timing belt and water pump 1500 mi. ago. A tune up and $4000. worth of maint. before the trip. Oil changed today. Ideas?

 

Two things come to mind.

1. Bad throttle position switch. The normal 10 second test is to listen for the 'click' but sometimes you can hear the click, but this test isn't always reliable.
2. Idle stabalizer valve. May need to try the WD40 trick.

Oh... one other possibility, the Temp II sensor (the passenger-side temp sensor on the coolant crossover), but I believe that would tend to cause hard starting when cold, but it could play a factor I guess.

The green light was his PSD (active limied slip) coming on. Nothing bad there.

 

May have a leaking injector that provides to rich a mixture at idle when warm and also leaks down making it over rich on hot starts.

 

Doubt it. The fuel would remain and woul still be there on cold starts.