Rodeo S 2WD L4-2.2L (1999)
Idle Air Control Valve: Description and Operation
Idle Air Control (IAC) Valve
The purpose of the idle air control (IAC) valve is to control engine idle speed, while preventing stalls due to changes in engine load. The IAC valve,
mounted in the throttle body, controls bypass air around the throttle plate By moving the conical valve (pintle) in (to decrease air flow) or out (to
increase air flow), a controlled amount of air can move around the throttle plate. If the RPM is too low, the PCM will retract the IAC pintle, resulting in
more air moving past the throttle plate to increase the RPM. If the RPM is too high, the PCM will extend the IAC pintle, allowing less air to move past
the throttle plate, decreasing the RPM.
The IAC pintle valve moves in small steps called counts. During idle, the proper position of the IAC pintle is calculated by the PCM based on battery
voltage, coolant temperature, engine load, and engine RPM. If the RPM drops below a specified value, and the throttle plate is closed, the PCM senses a
near-stall condition. The PCM will then calculate a new IAC pintle valve position to prevent stalls.
If the IAC valve is disconnected and reconnected with the engine running, the idle RPM will be wrong. In this case, the IAC must be reset. The IAC
resets when the key is cycled ON then OFF When servicing the IAC, it should only be disconnected or connected with the ignition OFF. The position of
the IAC pintle valve affects engine start-up and the idle characteristics of the vehicle. If the IAC pintle is fully open, too much air will be allowed into the
manifold. This results in high idle speed, along with possible hard starting and a lean air/fuel ratio.
If the IAC pintle is stuck closed, too little air will be allowed in the manifold. This results in a low idle speed, along with possible hard starting and a rich
air/fuel ratio.
If the IAC pintle is stuck part-way open, the idle may be high or low and will not respond to changes in the engine load.
