Circuit Description
The engine control module (ECM) controls the engine idle speed with the idle air control (IAC) valve. To increase the idle speed, the ECM pulls the IAC pintle away from its seat, allowing more air to pass by the throttle body. To decrease the idle speed, it extends the IAC valve pintle toward its seat, reducing bypass air flow. A scan tool will read the ECM commands to the IAC valve in counts. The higher counts indicate more air bypass, higher idle. The lower counts indicate less air is allowed to bypass, lower idle.
Diagnostic Aids
If the idle is too high, stop the engine. Fully extend the IAC valve with a IAC driver. Start the engine. If the idle speed is above 800 RPM, locate and repair the vacuum leak. Also, inspect for a binding throttle plate or throttle linkage or an incorrect base idle setting.
Test Description
The numbers below refer to the step numbers on the diagnostic table.
-
The IAC valve is extended and retracted by the IAC driver. IAC valve movement is verified by an engine speed change. If no change in engine speed occurs, the valve can be removed from the throttle body and tested. Connect the IAC driver to the removed IAC valve and turn the ignition ON. Do not start the engine.
-
This step inspects the quality of the IAC valve movement in step 2. Fully extending the IAC valve may cause an engine stall. This may be normal.
-
Steps 2 and 5 verify proper IAC valve operation. This step inspects the IAC circuit for a wiring or ECM fault.
Step | Action | Values | Yes | No | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
1 | Perform an On-Board Diagnostic System Check. Was the check performed? | -- | Go to Step 2 | |||||||||
Is a repair necessary? | -- | Go to Step 3 | Go to Step 4 | |||||||||
3 | Clean the IAC passages. Did you complete the repair? | -- | System OK | -- | ||||||||
4 | Measure the resistance between following terminals of IAC valve:
Does the resistance equal the range specified? | 40-80 ohms | Go to Step 6 | Go to Step 5 | ||||||||
Replace the IAC valve. Refer to Idle Air Control Valve Replacement . Did you complete the replacement? | -- | System OK | -- | |||||||||
Is a repair necessary? | -- | Go to Step 8 | Go to Step 7 | |||||||||
7 | Repair an open or short circuit as needed. Did you complete the repair? | -- | System OK | -- | ||||||||
8 | Inspect the IAC connector terminals and the ECM connector terminals. Is a repair necessary? | -- | Go to Step 9 | Go to Step 10 | ||||||||
9 | Repair or replace the throttle body assembly and/or ECM connector terminals as needed. Did you complete the repair? | -- | System OK | -- | ||||||||
10 | Replace the ECM. Refer to Engine Control Module Replacement . Did you complete the replacement? | -- | System OK | -- |
