Idle speed problem
 

I have just purchased a 85.5 944NA. The idle fluctuates a little after driving. When first started it idles at about 1200rpm. After I get back from a short drive it idles at about 2000rpm. The idle speed doesn't jump around, it just changes after driving it. Does anyone know what could cause this and how to fix it. I am very mechanicaly inclined and like to work on my own cars if possible. Other then idle speed, the car is great. This is the first 944 I have owned and I am very impressed.

Thanks for any help.

throttle isnt sticking is it?

No, not that I know of. I was thinking of something like a small air leak maybe. Not sure though.

Sounds like the idle speed valve needs cleaning.It get's gummed up with varnish and causes the idle speed to change.I couldn't get mine off the intake manifold to clean it but i managed to disconnect the hoses and spray carb cleaner inside.Did the job for my s2cab.

First, bypass the idle stabilizer, and set the car's idle. From there, if it still won't hold the proper idle, check for vacuum leaks, and clean out/replace the idle stabilizer.

The ISV on my 951 went south late last year, I will be replacing it soon. In my case, on a cold start the idle is ~700RPM, once running for 30 seconds it stops missing, and the idle bumps up to 850RPM. From there it climbs steadily as the engine warms up until it reaches a wonderful 1600RPM. If I turn the idle set screw in at all, it won't hold an idle when cold, so it's damned if you do, damned if you don't.
Your problem sounds the same as mine, and mine is a bad ISV.

Yes, that was the problem. The Idle Stabilizer Valve was the problem. I disconnected it and the car idles perfectly. I drove the car and the idle didn't change.

What does the ISV do? Does it just raise the idle on cold starts?

Thanks for the help guys.

Basically when you come to a stop with the engine running it will stabilise the idle on the car.When they get dirty over time it causes the idle to fluctuate up and down.Mine started doing it last summer dropping down to about 700rpm then shooting up over 1400rpm.You can clean it up easy enough with carb cleaner now you've got it off, then refit it.
They're quite expensive to replace here in the Uk - i was offered one over £100 so i was glad it cleaned up ok.

Sometimes the commutator inside of the ISV can become badly scarred. Once this happens, either replace it or delete it, but if you delete it, expect idle problems.
This is the case with mine, I can't simply clean it :(

I thought the 16Vs handled idle stabilization differently than the others (closed-loop, with speed sensor feedback, like 944cabby describes, no idle speed adjustment screw). What happens in the 8V's?