Three things can cause a high idle speed. The most common is a vacuum leak. Next is a sticking idle air control valve. The least common cause is the Engine Computer is commanding that high idle speed in response to something.
If the idle speed comes down on its own, it is most likely not due to a vacuum leak. Still, check the rubber and plastic vacuum hoses for cracks or leaks. When the engine is still cold you can spray water over it to see if it gets sucked in through a leaking gasket.
You can remove the idle air control valve to check for carbon build-up inside, but that usually results in a low idle speed and frequent stalling. A sticking valve is harder to diagnose. You may need to temporarily remove it, the reinstall it with a thin piece of sheet metal to block the passage. If that brings the idle speed way down, or the engine won't start without holding the accelerator pedal down 1/4", suspect that valve.
If nothing else pans out, you'll need a scanner to view live data. That will show if the Engine Computer is requesting a lower idle speed without success, or if it is requesting a higher speed. Some scanners have the capability to run the idle speed up and down to see if that valve is responding properly.
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Friday, January 10th, 2014 AT 9:00 PM
