Azir Khan

The engine is nice and smooth when cold, however as it worms up and the revs drop the engine settles into a rough and uneven idle. Switching the AC on makes the engine idle smoothly! Engine is smooth under load at all rpms.
The car was like this when I bought it a year ago, I have changed the sparks plugs, new air and fuel filter, added injector cleaner several time with no change. I have even tried spraying WD40 in the vacuum tube leading to the idle stabilizer with no improvement.
I have even disconnected the vacuum lines and plugged the connector on the intake manifold with no improvement.
What could be the problem?

Thanks, Azir.

Rez

Is your engine properly timed? Check spark advance as that has a really big impact on idle speed. It needs to be timed revved up and then checked at idle.

The Manual 24-1 reads, 750 +/- 50 rpm at idle, from mod 1983 650 +/- 50 rpm. The Manual 28-23 reads, 23 degrees before TDC at 3000 rpm with both vacuum hoses removed. Then it should settle around to 10 at idle speed.

Garth S

Ahhh - S4's have 'digital' timing via the ECU's vs. the analogue distributor's of our primitive, older brother's - the '80's.

Azir Khan

OK, I checked the timing, fully wormed up engine, 3000 rpm I read 25 Deg aprox. and at idle it is about 10 to 11 Deg. Please remember this is an 87 S4 with ECU!