The idle speed on my GTS was ok, 650 rpm o so, but at some point I noticed that the car was pulling quite hard when idling for a traffic light. From that moment on Saw idle speed usually on 800rpm, and when running in neutral sometimes climbing up to 1200 rpm.
I've done checks to see if the cable linkage still moved back to the stop position,
I've checked for anything pushing the pedal, like the carpet
I've checked the error codes in the LH and EZK
I've checked for any obvious leaks in the air intake system (so far as possible)
I've done a few idle stabilization tests to make it stabilize at 650 rpm again
The only thing that made the LH convince that 650 is the right idle speed was:
- loosen the ground strap of the battery so that the 12v drops on the car
- wait for 30 minutes
- re connect the ground strap again
Worked superb. This makes you wonder what goes on in the ECU memory and what may have caused this high rpm.....
One other typical cause: I've seen a 1991 928s4 with a tap into the breather line coming from the V going to to the oil filler. A "T" model device, capped by a rubber cap. It was broken and leaked badly. The engine was drawing so much air that it made the idle gradually climb up to 1200 rpm. Re-connecting fixed the idle instantaneously as expected. Remarkable is that when you disconnect the battery to reset the ecu, it seems to work for a little while, and then the problem of high idle returns.