The idle air control valve is responsible for controlling the amount of air that bypasses the throttle body when the engine is idling. If the IAC is faulty, it can cause the engine to idle too high or too low.
* Disconnect the hose from the idle air control valve.
* Start the engine and let it idle.
* If the engine idles down to normal, the IAC is faulty and should be replaced.
Inspect the throttle body for carbon deposits.
Carbon deposits can build up on the throttle body and interfere with the proper flow of air. This can cause the engine to idle too high or too low.
* Remove the throttle body from the engine.
* Inspect the throttle body for carbon deposits.
* Clean the throttle body with a throttle body cleaner.
* Reinstall the throttle body.
Check for a vacuum leak.
A vacuum leak can cause the engine to idle too high or too low.
* Inspect all vacuum hoses for cracks or leaks.
* If you find any cracked or leaking hoses, replace them.
Inspect the throttle position sensor.
The throttle position sensor tells the engine computer how far the throttle is open. If the throttle position sensor is faulty, it can cause the engine to idle too high or too low.
* Disconnect the throttle position sensor connector.
* Start the engine and let it idle.
* If the engine idles down to normal, the throttle position sensor is faulty and should be replaced.
Inspect the fuel pressure.
Low fuel pressure can cause the engine to idle too high or too low.
* Connect a fuel pressure gauge to the fuel rail.
* Turn on the ignition and read the fuel pressure.
* The fuel pressure should be between 41 and 58 PSI.
*If the fuel pressure is low, you should investigate the fuel system further.
