I have a 1990 GMC Sierra, 5.7L 4x4. I have a fast idle problem. I have replaced the TPS, IACV, Throttle body gasket, Temp switch, can hear no hissing. I recently replaced the Air Compressor. I pinched off the Vacuum line between the Brake booster and no change. When the AC is on it seems to idle ok, when i turn the AC off, it doesnt idle down... But when i start truck up it idles well... if let sit idle will pick up, of if i tap throttle it idles hi. it will change from 1st to 2nd on its own.
SOURCE: 2000 gmc rough idle
Im not sure if this can help, but my 2000 gmc sierra was idling badly. Replaced Fuel Pressure Regulator and my truck runs like its brand new. My service engine light was also on.
SOURCE: 95 GMC Sierra 4x4 350 5.7 rough idle problem
It sounds like a MAP sensor going bad. MAP is short for manifold absolute pressure sensor, it senses the manifold pressure and applies the right amount of gas with the right amount of ignition timing so the car runs and performs well if this does not work properly. It throws off the whole loop cycle for the computer, which in turn makes it run badly and get poor fuel economy. It can causes the engine to have a lack of power, combined with a loss of fuel economy. Poor take offs and sometimes an occasional stalling may occur if the MAP sensor is faulty and sending the wrong signals to the ECM.
Hope this helps and thank you for using Fixya.
Here is a link that my help you in the future.
http://www.troublecodes.net/GM/
SOURCE: right off idle truck stumbles and will die if you
its time to stop wasting money guessing and verify if parts are good/ bad. ck fuel pressure, and check the tps (Throttle Position Sensor) , see if it runs better with the EGR valve disconnected. If it does, replace it Use only AC DELCO parts they will need number off old EGR valve usually starts with 17------. NOT from ottozone or others.
SOURCE: 2003 GMC Sierra Has recently developed
You never replace anything, on a Wishing,Hoping,Pushing,Poking,diagnostic approach
You learn the vehicles Systems,Test,then Replace,if you have a failed component.
Work on the Misfire Issues.
Then you lean O2 Codes will go away.
Not a the plugs.
Think lean---Vacuum leaks,EGR Valve leak,EVAP System leaks,anything with a hose that lets air into the intake system.
Exhaust System leaks,near sensors
SOURCE: My 1995 gmc jimmy idles
Have you checked for vacuum leaks around the throttle body or intake manifold? If not try that.
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