I have a 2001 Chevy Silverado 1500, it has about 118000 thousand miles on the engine, lately it is taching up to over 2000RPM only when i put it in park or neutral. Once i put it into drive or any other gear, it comes down to earth at around 1000 rpm give or take. What would be things that would cause this issue?
Most of the time when this occurs it is because of a dirty throttle body. You need to take it off carefully and clean it carefully (throttle body or carb cleaner), then re-install. You'll want to pull the negative lead to the battery when you do this. Then, upon re-install, you'll find it will probably idle just fine, but will want to rev high on its own after putting it in gear. The PCM will need to recalibrate itself. To fix this, you can take it to the GM dealership and have them put a TechII on it for the throttle relearn, or you can take it out for a drive which usually consists of about 200 miles.