Thought this might help. My 94 945T (95k) has been idling low, especially with the a/c on - as low as 300 rpms. I've been keeping the revs up with my right foot mostly. It seemed to happen mostly on the hottest days. I changed spark plugs, cleaned the throttle body, and tweaked the idle speed to boost idle. But nothing seemed to work.

After doing a tons of research in the faq, I determined the IAC (idle air contol - silver cylindrical unit right under the front of the throttle body with two hoses and an electrical connection that points toward the firewall) was bad. So I sucked it up and paid $80 for one. Popped it in last night in about five minutes, started the car and ZOOM! I had to tweak the idle stop screw back to where it was supposed to be, as the new IAC was doing its job.

All is well now. Idle has leveled out. Idle speed has evened, whether running the a/c or not.

Comparing the new and old units, the older one had plenty of oily residue in it - probably because this is a turbo car - and the reciprocating assembly inside the housing seemed a bit gummed up. I wonder now if I could've tried to clean the thing with carb cleaner. I'll save it for when our 92s goes bad - and then test the carb method. Hope this helps.