Wondering if anyone else has had a idling problem like this, I've only noticed this during winter, in the summer we didn't have any problems. On our '98 Jimmy, with 149k miles, you can let the truck warm up in the morning and watch the tach as the engine begins to slow down and see the temp rising to operating temperature. However, the engine will sit and idle at 1000 rpm after it has been warmed up and even driven many miles. Now you can shut the truck off and start it back up again and the idle will go to normal at 600. But this only happens after you shut the truck off and start it again. I replaced the temp sending unit in the left head hoping maybe that was the issue. Still having the same problem. Also my dad's '00 ZR-2 S-10 pickup is doing the thing.
On the Jimmy, I took the throttle body off and cleaned it up (had the sticking pedel issue due to carbon build up), thinking maybe the throttle butterfly was stuck open a hair. No luck. Any ideas that would keep the idle so fast?

 

Vacuum leak? Stuck or malfunctioning IAC (idle air control) valve? When you cleaned the throttle body, did you remove and clean the IAC valve and bore?

 

No I haven't touched the IAC. I was thinking it was more of a temperature related issue because it started after winter showed it's ugly head. That might be the next step, taking the IAC out and cleaning it up, it did look pretty black when I had the throttle body off. Still wondering it it's a sensor the ECM is reading that is lazy or sticking until you shut the truck off and start it again.

 

Cold temperatures can cause an already gummed up, sticky IAC to lock into place.