Idle Speed/Throttle Actuator - Electronic: Testing and Inspection

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

Idle Air Control Wiring Circuit:

Circuit Description:

The Engine Control Module (ECM) controls idle RPM with the IAC valve. To increase idle RPM, the ECM moves the IAC valve out, allowing more air to bypass the throttle plate. To decrease RPM, it moves the IAC valve in, reducing air flow by-passing the throttle plate. A Tech 1 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 scan tool is used to extend and retract the IAC valve valve. Valve movement is verified by an engine speed change. If no change in engine speed occurs, the valve can be retested when removed from the throttle body.
2. This step checks the quality of the IAC valve movement in Step 1. Between 900 RPM and about 1500 RPM, the engine speed should change smoothly with each flash of the tester light in both extend and retract. If the IAC valve is retracted beyond the control range (about 1500 RPM), it may take many flashes in the extend position before engine speed will begin to drop. This is normal on certain engines, fully extending IAC valve may cause engine stall. This may be normal. Step 1 verified proper IAC valve operation while this step checks IAC valve circuits. Each lamp on the node light should flash red and green while the IAC valve is cycled. While the sequence of color is not important if either light is "OFF" or does not flash red and green, check the circuits for faults, beginning with poor terminal contacts.

Diagnostic Aids:

A slow, unstable, or fast idle may be caused by a non-IAC valve system problem that cannot be overcome by the IAC valve. Out of control range IAC scan tool counts will be above 60 if idle is too low, and zero counts if idle is too high. The following checks should be made to repair a non-IAC valve system problem.

^ Vacuum Leak (High Idle) - If idle is too high, stop the engine. Fully extend (low) IAC valve with tester. Start engine. If Idle speed is above 800 RPM, locate and correct vacuum leak including crankcase ventilation system Also check for binding of throttle blade or linkage.
^ System too lean (High Air/Fuel Ratio) - Idle speed may be too high or too low. Engine speed may vary up and down and disconnecting IAC does not help. DTC 44 may be set. Tech 1 scan tool Oxygen Sensor (O2S) voltage will be less than 300 mV (.3 volt). Check for low regulated fuel pressure, water in the fuel or a restricted injector.
^ System too rich (Low Air/Fuel Ratio) - The idle speed will be too low. Tech 1 scan tool IAC valve counts will usually be above 80. System is obviously rich and may exhibit black smoke exhaust.

Scan tool O2S voltage will be fixed above 800 mV (.8 volt). Check for high fuel pressure, leaking or sticking injector. Silicone contaminated O2S will scan an 02S voltage slow to respond.

^ Throttle Body - Remove IAC valve and inspect bore for foreign material
^ Refer to "Rough, Unstable, Incorrect Idle or Stalling" in "Diagnosis By Symptom."
^ If intermittent poor driveability or idle symptoms are resolved by disconnecting the IAC valve, carefully recheck connections, valve terminal resistance, or replace IAC valve.