With Powertrain Control Module (PCM)

Idle Air Control (IAC) System:

The diagnosis of idle Air Control (IAC) can be found in Idle Air Control (IAC) Valve. Refer to C Charts. Under Computer and Controls/System Diagnosis/Procedures. Testing and Inspection

If the IAC valve is disconnected or connected with the engine running, the idle RPM may be wrong. The IAC valve may be reset by disconnecting negative battery terminal for 10 seconds then reconnect battery cable, turn ignition "ON," engine "OFF" for 5 seconds then turn ignition "OFF" for 10 seconds.

The IAC valve affects the idle characteristics of the engine. If it is open fully, too much air will be allowed in the manifold and idle speed will be high. if it is stuck closed, too little air will be allowed in the manifold, and idle speed will be too low. if it is stuck part way open, the idle may be rough, and will not respond to engine load changes.

^ System too lean (high air/fuel ratio) - Idle speed may be too high or too low. Engine speed may vary up and down, disconnecting IAC does not help. May set Diagnostic Trouble Code (DTC) 44. Scan tool or voltmeter will read an oxygen sensor output less than 300 mV (0.3 volt). Check for low regulated fuel pressure or water in fuel. A lean exhaust with an Oxygen Sensor (O2S) output fixed above 800 mV (0.8 volt) will be a contaminated sensor, usually silicone. This may also set a DTC 45.

^ System too rich (low air/fuel ratio) - Idle speed too low. Scan counts usually above 80. System obviously rich and may exhibit black exhaust smoke. Scan tool and/or voltmeter will read an Oxygen Sensor (02S) signal fixed above 800 mV (0.8 volt).