The engine idle speed is monitored and controlled by the Engine Control Module/Powertrain Control Module (ECM/PCM) which uses the Idle Air Control (IAC) valve to regulate the idle speed. The valve controls the amount of air that is allowed to bypass the intake throttle plate, which enables the engine idle speed to remain constant even though the engine loads change during idle. This is especially true when electrical consumers such as the air conditioner are used.
The minimum idle air speed is set at the factory with a stop screw. This setting allows a certain amount of air to bypass the throttle valves regardless of IAC valve positioning. A combination of this airflow and IAC positioning allows the ECM/PCM to control engine idle speed. During normal engine idle operation, the IAC valve is controlled by the ECM/PCM and properly positioned. No adjustment should be required during routine maintenance. Tampering with the minimum idle speed adjustment may result in premature failure of the IAC valve or improperly controlled engine idle operation.
Prior to testing the Idle Air Control (IAC) valve, inspect the IAC valve mounting, hoses, connections and O-rings for damage, looseness or leakage.
- Start the vehicle's engine.
- Allow to run until it reaches normal operating temperature that is indicated by one complete fan cycle.
- Detach the electrical connector from the IAC valve.
With the IAC valve disconnected, there should be a noticeable drop in engine speed. If a drop in idle speed was noted but an intermittent idle still persists, check the wiring harness for high resistance connections or exposed wires.
- A vacuum leak in the throttle body could also cause a poor idle. Check for this by placing your finger over the hole in the inner throttle body. The idle should drop. If it does not, a vacuum leak may be present.
REMOVAL & INSTALLATION
- Disconnect the negative battery cable.
- Detach the wiring harness from the IAC valve.
- Remove the two retaining bolts.
- Remove the IAC valve and discard the old seals or gaskets.
- Clean the gasket mating surfaces thoroughly.
- Using new seals or gaskets, position the IAC valve on the throttle body.
- Install and tighten the retaining bolts.
- Connect the wiring harness to the IAC valve.
- If raised, lower the vehicle.
- Connect the negative battery cable.