Musche

Over the winter I freshened up some stuff on the 89 turbo. I replaced both temperature senders and cleaned their wiring harness ends, cleaned battery cable connections, added some grounds between the engine and firewall, switched to a lower temp thermostat, replaced vacuum lines, and various running gear changes. My problem is that once the car starts, it idles fine. After a few minutes, if you start to drive and use the throttle, the idle jumps to around 2000 rpm and won't come down right away. I can turn of the ignition and car will start and idle normally but if you blip the throttle, the high idle speed remains. After about 15-20 minutes of driving, the idle will return to normal. I can jump the pins in the diagnostic connector to disable the ICV and idle will retun to normal which leads me to believe it is working properly, just that something is telling the DME to increase idle. The AC has been removed and temp gauge needs to be around the 1/4 mark for things to return to normal. I'm beginning to think I have a brand new faulty DME temp sensor. One of my things to do is to put the old one back in and see if this cures the problem. If anybody knows resistance values vs. temp I could check it with the ohmmeter and the pyrometer. Any other ideas are appreciated.

Oddjob

Failing throttle position switch can cause intermittent high idle. Try disconnecting the TPS while the car is idling high and see if that changes the idle/throttle response. If the TPS is not telling the DME that the throttle is closed, the engine will hunt around for a steady idle and can float around 2-3k rpms.

But youre on track questioning the DME temp switch and the ISV. Those and the TPS are likely candidates to cause an idle problem.

Otherwise if the car is running really rich (for some reason - chip, FPR, etc), O2 sensor - the DME may play some games with the idle. But I think thats a bit of a stretch.

Musche

Oddjob, you were dead-on with your diagnosis...Thanks. I pulled the TPS and tested as per the manual. The potentiometer part tested good, but terminals 4 and 6 for the microswitch tested open at all times. Makes sense...if the switch wasn't closing the DME thinks the throttle is slightly open. I tore it apart and found a cold solder joint from the switch to the terminal connection. Repaired it, cleaned the pot and epoxied it back together. Car runs fine but I ordered a new TPS just to be safe.