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1999 Chevy Silverado - High Idle - Hard shifting

I have a 1999 Chevy Silverado 1500 (5.3 V8) - 4 wheel drive - 150,000K miles that has developed a “high idle” that we can’t get resolved. This "high idle’ condition is causing the Automatic Transmission to “slam” when put in gear. Once it’s in gear, the high idle continues and sometimes will not shift down into passing gear when going up a big hill.
The throttlebody has been replaced along with the Idle Air Control Valve.
Any suggestions ?

Have the coolant temp sensor for the computer checked out.

If the sensor is faulty, and is telling the computer that the engine coolant is at -20 degrees, the computer is going to cause the engine to go to high idle to get the engine to warm up as quickly as possible.


Have you checked for a vacuum leak? Excessive air from a leak would give a high idle.

Some other ideas besides the good ones posted above. Replacing the throttle body and IAC valve didn’t fix it? hmmm … Most likely the problem is either that the computer is commanding the engine to run to fast for some reason, or there’s a path for air to get into the engine that shouldn’t. I’m presuming you’ve already checked the OBD II for diagnostic codes and there’s nothing, right?

  • What is the intake manifold vacuum at idle? Below 17 could mean an air leak.

  • Try spraying some starter fluid around the intake manifold gasket area, vacuum hoses. If the engine speeds up, you got an air leak.

  • Does the power brake booster hold vacuum?

  • Is the coolant getting up to the temperature it should? What does the dash engine temp gauge say? The engine computer will boost the idle rpm if it thinks the coolant is below the normal operating temperature.

  • Not sure if your engine sports one, but some engines (for example my Corolla’s) use an electrically operated aux air inlet valve (sometimes referred to as a VSV, which the computer turns on when the AC is on, or the headlights are on, or the rear window defroster is on. It allows a little more air into the engine, similar to holding your foot on the gas pedal, to prevent the engine from stalling at stoplights with the extra load from the alternator or AC compressor. Have your shop check that. It might be turned on all the time for some reason.

  • Using a scan tool, what does the OBD II computer say the engine rpm is? What does it say the coolant temp is? Maybe the engine is running faster than the computer thinks it is for some reason. Or doesn’t know what the coolant temp is.