Idle Speed/Throttle Actuator - Electronic: Testing and Inspection
Chart C-2B - Idle Air Control (IAC) Valve Check:
Chart C-2B Wiring Circuit:
Circuit Description:
The ECM controls idle RPM with the IAC valve. To increase idle RPM, the ECM retracts the IAC pintle, allowing more air to bypass the throttle plate. To decrease RPM, it extends the IAC pintle valve, reducing air flow through the IAC valve port in the throttle body. A scan tool will read the ECM commands to the IAC valve in counts. The higher the counts, the more air allowed (higher idle). The lower the counts, the less air allowed (lower idle).
Test Description: Number(s) below refer to circled number(s) on the diagnostic chart.
1. The Tech 1 is used to extend and retract the IAC valve. Valve movement is verified by an engine speed change.
2. This test checks all wires, connectors, and the 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. Idle air control will be above 60 counts, if idle speed is too low. If idle speed is too high, IAC counts will be "0".
^ 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 DTC 44. The Tech 1 and/or voltmeter will read an oxygen sensor output less than 300 mV (.3 volt). Check for low regulated fuel pressure or water in fuel. A lean exhaust, with an oxygen sensor output fixed above 800 mV (.8 volt indicating rich exhaust to ECM), will be a contaminated sensor, usually silicone. This may also set DTC 45.
^ System rich (Low Air/Fuel Ratio).
Idle speed too low. IAC counts usually above 80. System obviously rich and may exhibit black smoke exhaust.
The Tech 1 and/or voltmeter will read an oxygen sensor signal fixed above 800 mV (.8 volt).
Check:
- High fuel pressure.
- Injector leaking or sticking.
^ Throttle Body - Remove IAC and inspect bore for foreign material or evidence of IAC pintle dragging the bore.