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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 78 (ENGINE CONTROL SYSTEM SELF-DIAGNOSTICS - 3.8L)DIAGNOSTIC TESTSDTC P0506: IAC SYSTEM RPM LOWDIAGNOSTIC PROCEDURES
1999 GMC Forward Control 5.7 R, Automatic

Diagnostic Procedures

1999 GMC Forward Control 5.7 R, AutomaticSECTION Diagnostic Procedures
WARNING: This page does not describe the selected car, but rather 7 other vehicles, including the 2002 Pontiac Grand Prix, 2002 Pontiac Bonneville, 2002 Chevrolet Monte Carlo, 2002 Chevrolet Impala, and 2002 Buick Regal. However, it is still accessible from the selected car via links, so may be relevant.
  1. Perform Diagnostic System Check - Engine Controls. See DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS  under SELF-DIAGNOSTIC SYSTEM. After performing Diagnostic System Check - Engine Controls, go to next step.
    2. NOTE: Ensure engine speed stabilizes with each commanded RPM change to determine if engine speed stays within 100 RPM of commanded RPM.
  2. 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, and then to 1700 RPM. Exit RPM control function. Does the engine speed stabilize within 100 RPM of the commanded RPM during test? If yes, go to next step. If no, go to step  4.
  3. Observe FREEZE FRAME/FAILURE RECORDS data for this DTC. Turn ignition off for 30 seconds. Start engine. Operate vehicle within conditions for running DTC or as close to FREEZE FRAME/FAILURE RECORDS data as possible. Does the DTC fail this ignition cycle? If yes, go to next step. If no, see DIAGNOSTIC AIDS .
  4. Turn ignition off. Disconnect IAC valve. Connect an IAC Motor Driver (J 37027-A) to IAC valve. Start engine. Using IAC motor driver, command IAC valve in until about 600 RPM is reached. Using IAC motor driver, command IAC valve out until about 1700 RPM is reached. Return engine speed 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 next step. If no, go to step  11.
  5. With a test lamp connected to ground, probe one of the IAC valve circuits at the IAC valve harness connector using the Connector Test Adapter Kit (J 35616-A). Start the engine. With the J 37027-1A, command low RPM while observing a scan tool until the IAC counts start to increment. With the J 37027-1A, command high RPM while observing a scan tool until the IAC counts start to increment. While the IAC counts are incrementing observe the test lamp. Return engine idle speed to desired idle as indicated on the scan tool data list. Repeat the above procedure for the other three IAC valve circuits. Did the test lamp 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.
  6. Did the test lamp remain off, never flashing, while the IAC counts where incrementing at any of the IAC valve circuits during test? If yes, go to step  9. If no, go to next step.
  7. Connect a test lamp between the IAC coil "A" low circuit and the IAC coil "A" high circuit at the IAC valve harness connector using the J 35616-A. With the J 37027-1A, command low RPM while observing a scan tool until the IAC counts start to increment. With the J 37027-1A, command high RPM while observing a scan tool until the IAC counts start to increment. While the IAC counts are incrementing, observe the test lamp. Return engine idle speed to desired idle as indicated on the scan tool data list. Repeat the above procedure with the test lamp connected between the IAC coil "B" low circuit and the IAC coil "B" high circuit at the IAC valve harness connector. Return engine speed to the desired idle as indicated on the scan tool data list. Did the test lamp stay illuminated and never flashing while the IAC counts where incrementing during the above test? If yes, go to next step. If no, go to step  16.
  8. Inspect for a poor connection at the IAC valve harness connector. Did you find and correct the condition? If yes, go to step  17. If no, go to step  12.
  9. Turn ignition off. Disconnect PCM connectors. Using DMM, check IAC valve circuit where test light remained off for open, short to ground or short to another IAC valve circuit. Did you find and correct the condition? If yes, go to step  17. If no, go to step  12.
  10. Turn ignition on. Using DMM, check IAC valve circuit where test light remained on for short to voltage or short to another IAC valve circuit. Did you find and correct the condition? If yes, go to step  17. If no, go to step  12.
  11. Visually and Physically inspect for the following:
    • Throttle body damage and tampering.
    • Throttle lever screw tampering, if equipped.
    • Restricted air intake system. Inspect for a possible collapsed/clogged air intake duct, before and after the air filter element, restricted air filter element, restriction at the throttle body intake screen, if equipped.

    Remove the IAC valve. Inspect for the following conditions:

    • A clogged IAC passage.
    • Excessive deposits on the throttle plate.
    • Excessive deposits in the throttle bore.
    • Excessive deposits on IAC valve pintle.

    Repair any of the above conditions as necessary. Did you find and correct the condition? If yes, go to step  17. If no, go to step  14.

  12. Inspect for a poor connection at the PCM harness connectors. Did you find and correct the condition? If yes, go to step  17. If no, go to step  16.
  13. 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  17. If no, go to step  16.
  14. Inspect for a poor connection at the IAC valve harness connector. Did you find and correct the condition? If yes, go to step  17. If no, go to next step.
  15. Replace IAC valve. After repairs, go to step  17.
  16. Replace PCM. Program replacement PCM. See POWERTRAIN CONTROL MODULE  under PROGRAMMING. After repairs, go to next step.
  17. Using scan tool, clear DTCs. Turn ignition off for 30 seconds. Start engine. Operate vehicle within Conditions for Running DTC. Does DTC run and pass? If yes, go to next step. If no, go to step  2.
  18. Using scan tool, observe stored information. Does scan tool displays any DTCs that have not been diagnosed? If yes, see DIAGNOSTIC TROUBLE CODE DEFINITIONS . If no, system is okay.
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When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

  • • You smell fuel, burning insulation, or see smoke.
  • • Brakes feel soft, pull hard to one side, or make grinding noises.
  • • The engine overheats, stalls repeatedly, or misfires under load.
  • • You are missing required tools, torque specs, or safe lifting equipment.
  • • You are not confident in the next step or safety outcome.