Chart C-2C Idle Air Control (IAC) System Check

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

Idle Air Control Wiring Circuit:

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 Tech 1 scan tool will read the ECM commands to the IAC valve in counts. Higher counts indicate more air bypass (higher idle). Lower counts indicate less air is allowed to bypass (lower idle).

Test Description: Number(s) below refer to circled number(s) on the diagnostic chart.

1. The IAC tester is used to extend and retract the IAC 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 movement in Step 1. Between 700 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 may cause engine stall. This may be normal.
3. Steps 1 and 2 verified proper IAC valve operation while this step checks the IAC circuits. Each lamp on the mode 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 system problem that cannot be overcome by the IAC valve. Out of control range IAC Tech 1 scan tool counts will be above 60 if idle is too low, and "0" counts, if idle is too high. The following checks should be made to repair a non-IAC system problem:

^ Vacuum Leak (High Idle) - If idle is too high, stop the engine. Fully extend (low) IAC with tester. Start engine. If idle speed is above 800 RPM, locate and correct vacuum leak including PCV system. Also check for binding of throttle blade or linkage.

^ System too lean (High Air/Fuel Ratio) - The idle speed may be too high or too low. Engine speed may vary up and down and disconnecting IAC valve does not help. DTC 44 may set. Tech 1 scan tool O2 voltage will be less than 300 mV (.3 volt). Check for low regulated fuel pressure, water in fuel, or restricted injector.

^ System too rich (Low Air/Fuel Ratio) - The idle speed will be too low. Tech 1 scan tool IAC counts will usually be above 80. System is obviously rich and may exhibit black smoke in exhaust. Tech 1 scan tool O2 voltage will be fixed above 800 mV (.8 volt). Check for high fuel pressure, leaking or sticking injector. Silicone contaminated O2 sensors scan voltage will be slow to respond.

^ Throttle Body - Remove IAC valve and inspect bore for foreign material.

^ IAC Valve Electrical Connections - IAC valve valve connections should be carefully checked for proper contact.

^ PCV Valve - An incorrect or faulty PCV valve may result in an incorrect idle speed.

^ Rough, Unstable, or Incorrect Idle, Stalling

^ If intermittent poor driveability or idle symptoms are resolved by disconnecting the IAC, carefully re-check connections, valve terminal resistance or replace IAC.