I have a 91 Jimmy, 4.3 auto. When I start it up the motor rev's high even when it is warmed up.
It just started last night, could it be the fuel sensor ?

Jim

 

Where do you live?

The engine will always go into high idle until the temp sensor reads the engine temp is warm enough to drop the temp down. IAC is the device that controls idle.

 

Check for a vacuum leak. My sons 1995 had a bad vacuum line from the engine to the vacuum reservoir and it affected the heater/ac controls (only blew out of the defrost vent) and made the vehicle idle fast. Replaced the vacuum line and that restored function of the heater/ac controls and ended the fast idle.

I was told that you can use spray carb cleaner and spray around vacuum lines while vehicle is running (be careful... I would have fire extinguisher handy!!). If there is a vacuum leak, the engine will idle even faster when the carb cleaner is sucked into the intake.