Idle Air Control (IAC) System Check
Idle Air Control (IAC) System CheckCircuit Description
The powertrain control module (PCM) controls engine idle speed with the idle air control (IAC) valve. To increase idle speed, the PCM retracts the IAC valve pintle away from its seat, allowing more air to bypass 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 PCM 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).
Diagnostic Aids
A slow, unstable, or fast idle may be caused by a non-IAC system problem that cannot be overcome by the AC 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 system problem:
- Vacuum leak (high idle) - If idle is too high, stop the engine. Fully extend (low) IAC with the AC motor analyzer J39027-A. Start the engine. If idle speed is above 800 RPM, locate and correct the vacuum leak, including the PCV system. Check for binding of the throttle blade or linkage.
- Lean heated oxygen sensor signal (high air/fuel ratio) - The idle speed may be too high or too low. Engine speed may vary up and down, and disconnecting the IAC valve does not help. Diagnostic trouble codes P0131, P0151, P0171,or P0174 maybe set. Scan tool oxygen (02) voltage will be less than 100 mV (0.1 V). Check for low regulated fuel pressure, water in fuel, or a restricted injector.
- Rich heated oxygen sensor signal (low air/fuel ratio) - The idle speed will be too low. Scan tool IAC counts will usually be above 80. The system is obviously rich and may exhibit black smoke in the exhaust.
Scan tool O2 voltage will be fixed at about 750 mV (0.75 V). Check for high fuel pressure, or a leaking or sticking injector. A silicon-contaminated heated oxygen sensor will show an 02 voltage slow to respond on the scan tool.
- Throttle body - Remove the IAC valve and inspect the bore for foreign material.
- IAC valve electrical connections - AC valve connections should be carefully checked for proper contact.
- PCV valve - An incorrect or faulty PCV valve may result in an incorrect idle speed. Refer to Diagnosis, Rough Idle, stalling. If intermittent poor driveability or idle symptoms are resolved by disconnecting the IAC, carefully recheck the connections and valve terminal resistance, or replace the IAC.
Test Description
Number(s) below refer to the step number(s) on the Diagnostic Chart.
1. The IAC motor analyzer J 39027-A 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 resettled when removed from the throttle body.
2. This step checks the quality of the IAC movement in step 1. Between 700 revolutions per minute (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 to extend the IAC valve before engine speed will begin to drop. This is normal on certain engines. Fully extending the IAC may cause engine stall. This may be normal.
6. Steps 1 and 2 verified the proper IAC valve operation. This step checks the IAC circuits. Each lamp on the noid 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.