The problem is most likely in the Electronic Idle Speed/Throttle Actuator or it's circuit.
The Idle Air Control (IAC) motor controls air flow through a passage connected to both intake manifold plenums. The PCM controls engine idle speed, by adjusting the IAC motor to compensate for engine load or ambient conditions
The IAC varies the amount of air that is allowed to bypass the throttle blades. It does this by extending or retracting the IAC pintle into the idle air passage. The further the IAC pintle protrudes into this passage the slower the idle speed, and the less the IAC pintle protrudes the greater the idle speed.
The PCM calculates proper idle speed using information from throttle position, vehicle speed, and coolant temperature sensors and brake, and neutral/safety switches.
Chances are good that the Idle Air Control Motor itself could be bad. But you will need a scan tool to properly test the circuit. I would recommend giving the throttle body a good cleaning before doing anything else. It may just be gunked up.
Friday, April 30th, 2010 AT 1:00 AM