Idling to high when warm?????

[Gazza]

My 911SC is starting fine, but as it warms up it starts to rev higher and higher? when its warm it will be idling at 1800 RPM, but! if i turn it off once its warm and leave it for 5 to 10 minutes, then restart it, it will then be idling fine???? driving localy is fine after doing this, but if i go say up the freeway for say 20 minutes, when i get to my first stop ( off ramp ) then stop at a stop sign or lights etc, it will be idling at 1800 again???? this is very embaressing and i look like an idiot! i do just fine looking like an idiot without my cars help! please someone help me know my problem and how to fix it, Thank you.
Gazza

PS, i am being told a few different storys, some are expensive, i can't afford trial and error, but if i have to, any advice on what to try would be apreciated, but a clear answer would help me so much, has anyone had this problem?

[Flat6pac]

You need to look first at false air leaks. Does your engine change RPM with the oil tank cap off? If it doesnt, you need to look at the hoses and the cap between the throttle body and the air flow sensor for leakage.
Spray around the intake runners at the heads with carb cleaner, see if a change in idle.
Bruce

[macssc911]

Also check yur Aux Air Valve if it does not closes once engine creates 5 to 6 inch's of vacuum on that valve the engine will idle high. You can test by disconnecting two hoses on valve and plug them.

[DRACO A5OG]

Does he have a AAV on a SC?

Check to see if the throttle linkage is not binding. disconnect at the bell housing and check for binding at the pedal and at the throttle body.

[kodioneill]

There can be lot's of issues with the SC model along with the advice from above I've seen many distributor advance plates sticking.

[Gazza]

Thanks guys i will check all that, its not the distributor advance plates sticking, thats been checked, i,ll go through the list you guys have given me so far, thanks so much for your help and advice, i,ll keep you informed. one thing i forgot to mention was that when its warming up, its " hunting " as my mechanic said, revving up and down, but then settles after a bit and then starts idling higher and higher.

[boyt911sc]

Originally Posted by Flat6pac

You need to look first at false air leaks. Does your engine change RPM with the oil tank cap off? If it doesnt, you need to look at the hoses and the cap between the throttle body and the air flow sensor for leakage.
Spray around the intake runners at the heads with carb cleaner, see if a change in idle.
Bruce

Follow the above advise. It seems that your engine was tweaked to compensate for the undetected air leak. A good way to detect these hard to find air leak/s is by simply pressurizing the system. A continuous flow of pressurized gas (1 - 2 psi.) would be sufficient to reveal the sources if any. The test takes only a few minutes. Adjusting your fuel mixture setting without verifying the absence of a significant source of air leak won't help you fix your problem. A band-aid fix is temporary and would not last long.

Tony

[jason2guy]

i have an 81, mine did the same thing as yours. the hunting is most likely mixture- too rich. the high idle issue might be your dist advance springs. the problem is that the primary advance spring is old, stretched, and is not that heavy of a spring to begin with. im not a trained mechanic but what i think is happening is when the engine gets hot the springs weaken even more and end up advancing the timing which results in an elevated idle.i found a place that has advance springs for sale. i put the new spring in my dist and the idle stays constant. could be worth a try

[Gazza]

Thanks Guys, as soon as i have a day off i,ll try doing that and the springs, i really apreciate everyones help, this is a great site! wish me luck haa

[aoncurly]

Also, try disconnecting your O2 sensor. If no change, then the sensor might be bad.