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Hanging idle on 99 Porsche 996

I have a stock, 3.4L, 6-speed Porsche 911 996 with 103,000 miles on it. Since mid-summer, when I’m rolling and press the clutch, the idle will hang around 2000 rpms. It will remain there until I slow down to 10 mph at which time it will promptly drop back to around 800 rpms, which is the normal idle speed. If it’s cold outside, it hangs even higher, sometimes at 3500 rpm.

The problem is very intermittent. It may do it everytime I drive the car for two days or it may only do it once over the course of two or three days. I’m not getting any codes with an OBD-II or a Durametric scanner. I’ve tried replacing the MAF, the TPS, the IACV and I’ve checked for vacuum leaks and binding in the throttle body, throttle cable and accelerator pedal.

I can’t get the car to do it sitting still. It absolutely has to be rolling, and like I said, even then there’s no predicting when it will happen. Sometimes it does it first thing out of the driveway, sometimes it will just start doing it on the freeway. I can’t seem to come up with anything dependent on temperature or conditions aside from the fact that the car must be moving.

I took it to a shop, and in the two days they had it, it never happened. They couldn’t find anything wrong with it.

This is driving me crazy. Can anyone help?

I don’t think it has anything to do with the problem, but it started just a week or two after I had my spark plugs changed this summer.