warm engine idle surging
 

My 1986 911 has a weird problem. The cold start-up idle is a little screwed up. It's way too rich, and it doesn't stay on long enough. I live in a temperate climate so I'm not too worried about it unless it's related to the problem below. I figure the cold start idle issue is most likely a cylinder head temp sensor or the idle air control valve...I'll get to them later when I feel like busting a knuckle.

The one that's got me beat is a temporary problem I only get when the engine is at normal operating temperature, and more often on warm days. The car usually idles at around 1000 or a little less, by every once in a while, when you take your foot of the gas and coast to a stop, the engine will begin surging between 0 and normal, very rhythmically and fairly fast. The car never dies and you can usually make it stop by turning the car off and starting it again...this happens maybe once a day at most.

Any ideas?

Gordon

Either a rich mixture causing the surge, or the throttle linkage on your tranny housing is sticking (a common warm-weather problem). Check, clean and lube that linkage first before moving to the next possible solution.

Make sure the idle microswitch on the throttle body is engaging properly. As kidrock said, a misadjusted throttle linkage can cause the throttle to not completely close.

With the engine not running push the throttle open in the engine bay and let it return to idle. You should be able to hear the faint click of the microswitch. If you have to pull the throttle against the close position by hand to make it click there is your problem.

ingo

Try lowering the base idle speed.