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Home >> Buick >> 2003 >> Century Base >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls - System Testing & Engine Component Tests - 3.1L >> Idle Control Systems >> Idle Air Control System >> Diagnostic Procedure
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Diagnostic Procedure

  1. Perform Diagnostic System Check - Engine Controls. See DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - 3.1L CENTURY, GRAND PRIX & MALIBU article.
    2. NOTE: Ensure engine speed parameter stabilizes with each COMMANDED RPM change to determine if the engine speed stays within 100 RPM less than the commanded RPM or within 150 RPM more than the commanded RPM.
  2. Set parking brake and block drive wheels. 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 100 RPM less than the commanded engine speed or within 150 RPM above the Commanded RPM? If yes, see DIAGNOSTIC AIDS . If no, go to next step.
  3. Turn ignition switch to OFF position. Disconnect IAC valve harness connector. Connect IAC Motor Driver (J 37027-1A), or equivalent, to the IAC valve. Start the engine. Using IAC system motor driver, 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.
  4. Did you observe an excessively high idle condition that could not be controlled using the IAC motor driver during the last test? If yes, go to step  11. If no, go to step  12.
  5. Connect a test light between one of the IAC valve control circuits and a good ground. Start the engine. Observe the IAC Counts parameter using a scan tool. Command high RPM using the IAC motor driver until the IAC Counts parameter starts to increment. Command low RPM using the IAC motor driver until the IAC Counts parameter starts to increment. Return the engine speed to the Desired idle speed parameter. Repeat procedure for the other 3 IAC valve control circuits. Did the test light remain ON, never flashing, while the IAC Counts were incrementing for any of the IAC valve control circuits? If yes, go to step  10. If no, go to next step.
  6. Did the test light remain OFF, never flashing, while the IAC counts where incrementing at any of the IAC valve control circuits during the above test? If yes, go to step  9. If no, go to next step.
  7. Connect a test light between the IAC coil "A" low control circuit and the IAC coil "A" high control circuit. Observe the IAC Counts parameter using a scan tool and the test light. Command high RPM using the IAC motor driver until the IAC Counts parameter starts to increment. Command low RPM using the IAC motor driver until the IAC Counts parameter starts to increment. Return the Engine Speed parameter to the desired idle speed parameter. Repeat procedure with the test light connected between the IAC coil "B" low control circuit and the IAC coil "B" high control circuit. Did the test light stay illuminated, never flashing, while the IAC Counts were incrementing? If yes, go to next step. If no, go to step  17.
  8. 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.
  9. Turn ignition switch to OFF position. Disconnect PCM harness connectors. Using a DVOM, 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 control circuit.

    Did you find and correct the condition? If yes, go to step  18. If no, go to step  13.

  10. Turn ignition switch to RUN position. Using a DVOM, test the circuit where the test light remained illuminated for the following:
    • A short to voltage.
    • A short to another IAC valve control circuit.

    Did you find and correct the condition? If yes, go to step  18. If no, go to step  13.

  11. 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. Check for the following:

    • Clogged IAC valve 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.

  12. Visually and physically check for the following:
    • Throttle body damage and tampering.
    • Throttle lever screw tampering (if equipped).
    • Excessive deposits on throttle plate.
    • Restricted air intake system. Check for possible collapsed or clogged air intake duct, before and after the air filter element, a restricted air filter element or restriction at the throttle body intake screen, if equipped.
    • 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.

  13. 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.
  14. Disconnect the PCM harness connectors. Using a DVOM, 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.
  15. 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.
  16. Replace IAC valve. After repair, go to step  18.
  17. Replace PCM. See POWERTRAIN CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in appropriate REMOVAL & INSTALLATION article. Program PCM using correct vehicle software and calibration. See POWERTRAIN CONTROL MODULE under PROGRAMMING in SELF-DIAGNOSTICS - 3.1L CENTURY, GRAND PRIX & MALIBU article. After repair, go to next step.
  18. Reconnect any disconnected components. Use the scan tool to clear any DTCs that may have set. Turn ignition switch to OFF position 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.
  19. Using 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 under SELF-DIAGNOSTIC SYSTEM in SELF-DIAGNOSTICS - 3.1L CENTURY, GRAND PRIX & MALIBU article. If no, system is okay at this time.
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