If a car idles erratically and surges ( that is idle speed is not steady and increases and decreases), the issue may be a buildup of carbon or fuel varnish deposits in the idle speed control valve (also known as the idle air control valve or IAC valve). The solution to this condition is to clean the valve with some aerosol throttle cleaner or engine top cleaner.
Here are ways to clean the IAC valve:
Disconnect the air intake ductwork from the throttle body.
Put on the engine, then increase and hold the idle speed to 1,000 to 1,500 rpm.
Spray the throttle cleaner or engine cleaner into the throat of the throttle body, targeting the idle air bypass port (often found on the side or top of the throttle body opening). Give this area a good dose of cleaner (about 10 second’s worth).
Turn the engine off to let the cleaner to soak into the IAC passageway.
Stay about three minutes.
Restart the engine, rev and hold at 1,000 to 1,500 rpm, and repeat the cleaning process again.
Turn the engine off again, and reattach the air intake duct work to the throttle body.
Put on the engine and rev and hold to 1,500 to 2,000 rpm until no white smoke is coming out of the exhaust pipe.
Cleaning the Idle Air Speed Control Valve withl throttle cleaner can often solve an idle issue.
If this does not work, you can remove the IAC valve from the throttle body and spray cleaner directly on the tip of the valve and/or into the ports in the throttle body. Let the cleaner soak awhile, repeat as required, then reinstall the IAC valve, start the engine and run it at 1,500 to 2,000 rpm as before until no white smoke is seen in the exahust.
If the idle speed still surges after this, the IAC valve is defective and has to be changed.
Where the old Idle Speed Control Valve does not respond to cleaning, change it with a new one.