Idle Speed/Throttle Actuator - Electronic: Testing and Inspection

Idle Air Control (IAC) Valve Circuit Diagram:

Chart C-2B, Idle Air Control (IAC) Valve Check:

CIRCUIT DESCRIPTION
The PCM controls idle speed by moving the IAC valve pintle which opens and closes the air passage bypassing the throttle valve. A "SCAN" tool displays the commanded pintle position in counts. The greater the number of counts, the farther open the IAC valve is and the greater the commanded idle speed. IAC counts range from 0 (fully closed) to 255 (fully open.) Normal is between 15 - 20 counts.

TEST DESCRIPTION
Numbers below refer to circled numbers on the diagnostic chart.

1. The "SCAN" tool is used to extend and retract the IAC valve pintle. Pintle movement is verified by a change in engine idle speed.
2. This step checks continuity through wiring and connectors, and checks the signal from the PCM.
3. This step checks for open or shorted circuit within the IAC valve itself.

DIAGNOSTIC AIDS
An idle speed that is too fast, too slow or unstable can be caused by other conditions that the IAC valve cannot compensate enough for.

NOTE: If the air intake to the engine is restricted, the PCM will attempt to compensate by retracting the IAC valve (increasing IAC counts), opening the air passage to maintain the desired idle speed. IAC counts may increase to greater than 60. If there is a vacuum leak or the throttle plates do not fully close, the PCM will try to compensate by extending the IAC pintle. IAC counts may be reduced to zero (air passage fully closed) as the PCM tries to reduce idle speed to the desired rpm.

ALSO CHECK

^ System 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 Code 44. "Scan" and/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 output fixed above 800 mV (0.8 volt), will be a contaminated sensor, usually silicone. This may also set Code 45.

^ System rich (Low Air/Fuel Ratio) - Idle speed too low. "Scan" counts usually above 80. System obviously rich and may exhibit black smoke exhaust. "Scan" tool and/or voltmeter will read an oxygen sensor signal fixed above 800 mV (0.8 volt). Check for high fuel pressure or possibly an injector leaking or sticking.

^ Throttle Body - Remove IAC and inspect bore for foreign material or evidence of IAC pintle dragging the bore.

^ See Diagnosis by Symptom.