Chart C-2C: Idle Air Control: Notes

The ECM controls idle RPM with the IAC valve. To increase idle RPM, the ECM retracts the IAC plunger (pintle), allowing more air to pass around the throttle plate. To decrease RPM, it moves the IAC plunger in, reducing airflow around the throttle plate. A "Scan" tester 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).

1. Continue with test, even if engine will not idle. If idle speed is too low, "Scan" tester will display 80 or more counts, or steps. If idle is high, it will display zero counts. Occasionally an erratic or unstable idle may occur. Engine speed may vary 200 RPM or more up and down. Disconnect IAC. If the condition is unchanged, the IAC is not at fault.
2. When the engine was stopped, the IAC plunger retracted (more air) to a fixed "park" position for increased airflow and idle speed during the next engine start. A "Scan" tester will display 100 or more counts.
3. Be sure to disconnect the IAC valve prior to this test. The test light will confirm the ECM signals by a steady or flashing light on all circuits.
4. There is a remote possibility that one of the circuits is shorted to voltage, which would have been indicated by a steady light. Disconnect ECM and turn the ignition on. Probe terminals to check for this condition.