Idle Air Control (IAC) System Check
Idle Air Control System Diagnosis
Circuit Description
The VCM controls idle RPM with the AC valve. To increase idle RPM, the VCM retracts the IAC pintle from the seat, allowing more air to bypass the throttle plate. To decrease RPM, it extends the IAC pintle
valve in towards the seat, reducing air flow through the IAC valve port in the throttle body. A scan tool will read the VCM 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).
IAC Valve Reset Procedure
Without Scan Tool
Start the engine for 20 seconds.
Turn OFF the ignition for 10 seconds.
Start the engine.
Check for the proper idle speed.
With Scan Tool
Connect the scan tool to the DLC.
Turn ON the ignition, with the engine OFF.
Use the scan tool in order to select the IAC Reset in Special Functions.
Activate the reset function and wait for the IAC counter to stop.
Start the engine.
Check for proper idle speed.
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. The following checks should be made to repair a non-IAC system problem.
IMPORTANT: If for some reason the for in take to the engine is restricted, the VCM will attempt to compensate by backing out the IAC pintle to maintain desired idle. If the engine has another source of air to the intake, the VCM will compensate by extending the IAC pintle, scan IAC counts may reach zero as the VCM tries to maintain desired idle.
- 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 crankcase ventilation system. Also, check for binding of throttle blade or linkage.
- Sticking throttle cable or throttle plate shaft.
- 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 P0171.The scan tool will read heated oxygen sensor (HO2S) output less than 300 mV (0.3 volt). Check for low regulated fuel pressure or water in fuel.
- System rich (low air/fuel ratio) -Idle speed too low. Scan IAC counts are usually above 80. System obviously rich and may exhibit black smoke exhaust. May set DTC P0172.The scan tool will read heated oxygen sensor (HO2S) signal fixed above 800 mV (0.8 volt).
Check for high fuel pressure, leaking or sticking injector. Silicone contaminated HO2S scan voltage will be slow to respond.
- Throttle body -Remove IAC valve and inspect bore for foreign material.
- IAC Valve Electrical Connections -AC valve connections should be carefully checked for proper contact.
- Crankcase Ventilation Valve -An incorrect or faulty crankcase ventilation valve may result in an incorrect idle speed. Refer to Rough, Unstable, or Incorrect Idle and Stalling.
- NC Compressor -Refer to A/C Compressor Control Circuit Diagnosis if circuit is shorted to ground. If the relay is faulty, an idle problem may exist. Refer to Rough, Unstable, or Incorrect Idle and Stalling.
- If intermittent poor driveability or idle symptoms are resolved by disconnecting the IAC, carefully recheck connections, valve terminal resistance or replace IAC.
An intermittent may be caused by any of the following conditions:
- A poor connection
- Rubbed through wire insulation
- A broken wire inside the insulation
Thoroughly check any circuitry that is suspected of causing the intermittent complaint. Refer to Testing for Intermittent and Poor Connections .
If a repair is necessary, refer to Wiring Repairs or Connector Repairs.
Step 1:
Steps 2 - 5:
Steps 6 - 9:
Steps 10 - 13:
Steps 14 - 20:
Steps 21 - 24:
Test Description
The numbers below refer to the step numbers on the diagnostic table.
2. This test determines if the VCM is capable of closing the IAC valve.
3. This test determines if the VCM is capable of opening the IAC valve.
4. All engine mechanical related conditions should be checked and repaired before testing the IAC system.