Diagnostic Procedure

Perform Diagnostic System Check - Engine Controls. See DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS under SELF-DIAGNOSTIC SYSTEM in appropriate SELF-DIAGNOSTICS article. After performing Diagnostic System Check - Engine Controls, go to next step.

NOTE: Ensure engine speed stabilizes with each commanded RPM change to determine if engine speed stays within specifications. See RPM VARIATION LIMITS table.

Set parking brake and block drive wheels. Install scan tool. Start engine. Turn all accessories off. Using scan tool RPM control function, slowly increment engine speed to 1700 RPM, then to 600 RPM. Then to 1700 RPM. Exit RPM control function. Did the engine speed stabilize within specifications during the above test? See RPM VARIATION LIMITS table. If yes, see DIAGNOSTIC AIDS . If no, go to next step.
Turn ignition off. Disconnect IAC valve harness connector. See WIRING DIAGRAMS article. Connect IAC System Motor Analyzer (J 37027-A) to IAC valve. Start engine. Using IAC system motor analyzer, command IAC valve in until about 600 RPM is reached and then command IAC valve out until about 1700 RPM is reached. Return engine RPM to desired idle as indicated on scan tool data list. Did the engine speed steadily decrease to near 600 RPM and steadily increase to near 1700 RPM when the IAC valve was commanded in and out? If yes, go to step 5. If no, go to next step.
Did you observe an excessively high idle condition that could not be controlled with the IAC motor driver during the last test? If yes, go to step 11. If no, go to step 12.
Using a test light connected to ground, probe one IAC valve circuit at IAC valve harness connector. See WIRING DIAGRAMS article. Start engine. Using IAC system motor analyzer, command low RPM while observing scan tool until IAC counts start to increment, then command high RPM until IAC counts start to increment. Observe test light while incrementing IAC counts. Return engine to idle as indicated on scan tool data list. Repeat procedure for other three IAC valve circuits. Did the test light remain ON, never flashing, while the IAC counts where incrementing at any of the IAC valve circuits during the above test? If yes, go to step 10. If no, go to next step.
Did the test light remain OFF, never flashing, while the IAC counts where incrementing at any of the IAC valve circuits during the above test? If yes, go to step 9. If no, go to next step.
Using Connector Test Adapter Kit (J 35616-A) connect a test light between IAC coil "A" low circuit and IAC coil "A" high circuit at IAC valve harness connector. Using IAC system motor driver, command low RPM while observing scan tool until IAC counts start to increment, then command high RPM until IAC counts start to increment. Observe test light while incrementing IAC counts. Return engine speed to desired idle as indicated on scan tool data list. Repeat procedure on IAC coil "B" low circuit and IAC coil "B" high circuit. Did the test light stay illuminated, never flashing, while the IAC counts where incrementing during the above test? If yes, go to next step. If no, go to step 17.
Check for poor connection at IAC valve harness connector. Did you find and correct the condition? If yes, go to step 18. If no, go to step 13.
Turn ignition off. Disconnect PCM harness connectors. With a DMM, test the circuit where the test light remained off for the following:
- An open circuit.
- A short to ground.
- A short to another IAC valve circuit.
Did you find and correct the condition? If yes, go to step 18. If no, go to step 13.
Turn ignition on. With a DMM, test the circuit where the test light remained illuminated for the following:
- A short to voltage.
- A short to another IAC valve circuit.
Did you find and correct the condition? If yes, go to step 18. If no, go to step 13.
Visually and physically inspect for the following:
- Throttle body damage and tampering.
- Skewed TP sensor signal.
- Throttle lever screw tampering (if equipped).
- Vacuum leaks.
- Faulty, incorrectly installed PCV valve and hose (if equipped).
- Throttle shaft binding.
- Throttle linkage or cruise control linkage binding (if equipped).
Remove IAC valve. Inspect for the following:
- Clogged IAC passage.
- Excessive deposits on throttle plate.
- Excessive deposits in throttle bore.
- Excessive deposits on IAC valve pintle.
Did you find and correct the condition? If yes, go to step 18. If no, go to step 15.
Visually and physically inspect for the following:
- Throttle body damage and tampering.
- Throttle lever screw tampering (if equipped).
- Restricted air intake system.
- Excessive deposits on throttle plate.
- Excessive deposits in throttle bore.
- Remove IAC valve and inspect for excessive deposits on IAC valve pintle and in IAC valve passage.
Repair as necessary. Did you find and correct the condition? If yes, go to step 18. If no, go to step 15.
Check for poor PCM harness connections. Did you find and correct the condition? If yes, go to step 18. If no, go to next step.
Disconnect the PCM harness connectors. With a DMM, test all IAC valve circuits for high resistance. Did you find and correct the condition? If yes, go to step 18. If no, go to step 17.
Check for poor IAC valve harness connection. Did you find and correct the condition? If yes, go to step 18. If no, go to next step.
Replace IAC valve. After repairs, go to step 18.
Replace PCM. Perform PCM relearn procedures. See PROGRAMMING in appropriate SELF-DIAGNOSTICS article. After repairs, go to next step.
Reconnect any disconnected components. Use the scan tool to clear any DTCs that may have set. Turn OFF the ignition for 30 seconds. Operate the vehicle within conditions to verify the repair. Did you verify the repair? If yes, go to next step. If no, go to step 2.
With a scan tool, observe the stored information in Capture Info. Does the scan tool display any DTCs that you have not diagnosed? If yes, see DIAGNOSTIC TROUBLE CODE DEFINITIONS in appropriate SELF-DIAGNOSTICS article. If no, system is okay.