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

Chart C-2B:

Chart C-2B Wiring Diagram:

CIRCUIT DESCRIPTION:

The ECM controls engine idle speed with the IAC valve. To increase idle speed, the ECM retracts the IAC valve pintle away from its seat, allowing more air to pass by the throttle bore. To decrease idle speed, it extends the IAC valve pintle towards its seat, reducing bypass air flow. A "Scan" tool will read the ECM commands to the IAC valve in counts. The higher the counts indicate more air bypass (higher idle). The lower the counts indicate less air is allowed to bypass (lower idle).

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

1. The scanner is used to extend and retract the IAC valve. Valve movement is verified by engine speed change.

2. This test checks all wires, connectors, and ECM.

3. This test checks for shorted or open IAC valve coils.


DIAGNOSTIC AIDS:

^ A slow, unstable idle may be caused by a system problem that cannot be overcome by the IAC.

^ If for some reason the air intake to the engine is restricted, the ECM will attempt to compensate, by backing out the IAC valve to maintain idle speed. This may result in IAC counts above 60. If the engine has another source of air into the intake, the ECM will compensate by extending the IAC valve. The IAC counts may be zero as the ECM tries to maintain idle speed. If idle speed is to high IAC "Scan" counts will be zero.

^ 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 (0.3volt). 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.

^ Refer to TESTING PROCEDURES/DIAGNOSIS BY SYMPTOM/STALLS-ROUGH,UNSTABLE OR INCORRECT IDLE.