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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 260 (ENGINE CONTROLS - SELF-DIAGNOSTICS)DIAGNOSTIC TESTSDTC P0507: IDLE CONTROL SYSTEM - HIGH RPMDIAGNOSTIC PROCEDURES
1999 GMC Forward Control 5.7 R, Automatic

Diagnostic Procedures

1999 GMC Forward Control 5.7 R, AutomaticSECTION Diagnostic Procedures
WARNING: This page is about a different car, the 2003 Cadillac Seville and 2003 Cadillac DeVille. However, it is still accessible from the selected car via links, so may be relevant.
  1. Did you perform the Diagnostic System Check-Engine Controls? If yes, go to next step. If no, see DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS  under SELF-DIAGNOSTIC SYSTEM.
    2. NOTE: Ensure engine speed stabilizes with each commanded RPM change to determine if engine speed stays within 100 RPM of the commanded RPM.
  2. Set the parking brake and block the drive wheels. Start the engine. Turn off all of the accessories. With the scan tool RPM control function, slowly increment engine speed to 1700 RPM, then to 600 RPM, then to 1700 RPM. Exit the RPM control function. Does the engine speed stabilize within 100 RPM of the commanded RPM? If yes, go to next step. If no, go to step  4.
  3. Observe the Freeze Frame/Failure Records data for this DTC. Turn off the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions For Running DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records. Did the DTC fail this ignition cycle? If yes, go to next step. If no, see DIAGNOSTIC AIDS .
  4. Turn ignition off. Disconnect the IAC valve. Connect an IAC Motor Driver to the IAC valve. Start the engine. With the IAC Motor Driver, command the IAC valve in until near 600 RPM is reached. With the IAC Motor Driver, command the IAC valve out until near 1700 RPM is reached. Return the engine speed to the Desired Idle Speed parameter. 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. Connect the test light between one of the IAC valve control circuits and a good ground. Start the engine. Observe the IAC Counts parameter with a scan tool and observe the test light. Command high RPM with the IAC Motor Driver until the IAC Counts parameter starts to increment. Command low RPM with the IAC Motor Driver until the IAC Counts parameter starts to increment. Return the Engine Speed parameter to the Desired Idle Speed parameter. Repeat the above 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 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 circuit and the IAC coil "A" high circuit. Observe the IAC Counts parameter with a scan tool and observe the test light. Command high RPM with the IAC Motor Driver until the IAC Counts parameter starts to increment. Command low RPM with the IAC Motor Driver until the IAC Counts parameter starts to increment. Return the Engine Speed parameter to the Desired Idle Speed parameter. Repeat the above 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 and never flashing while the IAC Counts were incrementing for any of the IAC valve control circuits? If yes, go to next step. If no, go to step  16.
  8. Test for an intermittent and/or poor connections at the IAC valve. Did you find and correct the condition? If yes, go to step  17. If no, go to step  12.
  9. Turn ignition off. Disconnect the PCM harness connectors. With a DVOM, test the circuit where the test light remained off for an open circuit, a short to ground or a short to another IAC valve control circuit. Did you find and correct the condition? If yes, go to step  17. If no, go to step  12.
  10. Turn ignition on. Test the IAC valve control circuit where the test light remained on for a short to voltage or a short to another IAC valve control 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 conditions:
    • Throttle body damage and tampering.
    • Throttle lever screw tampering, if equipped.
    • Vacuum leaks.
    • Faulty, incorrectly installed Positive Crankcase Ventilation (PCV) valve and hose, if equipped.
    • Throttle shaft binding.
    • Throttle linkage or cruise control linkage binding, if equipped.

    Remove the IAC valve. See IDLE AIR CONTROL VALVE under MOTORS & SOLENOIDS in REMOVAL & INSTALLATION - DEVILLE & SEVILLE article. Inspect for the following conditions:

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

    Did you find and correct the condition? If yes, go to step  17. If no, go to step  14.

  12. Test for an intermittent and/or poor connections at the PCM. Did you find and correct the condition? If yes, go to step  17. If no, go to next step.
  13. Disconnect the PCM. Test all IAC valve control circuits for high resistance. Did you find and correct the condition? If yes, go to step  17. If no, go to step  16.
  14. Test for an intermittent and/or poor connections at the IAC. Did you find and correct the condition? If yes, go to step  17. If no, go to next step.
  15. Replace the IAC valve. See IDLE AIR CONTROL VALVE under MOTORS & SOLENOIDS in REMOVAL & INSTALLATION - DEVILLE & SEVILLE article. Did you complete the replacement? If yes, go to step  17.
  16. Replace PCM. See POWERTRAIN CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL & INSTALLATION - DEVILLE & SEVILLE article. Did you complete the replacement? If yes, go to next step.
  17. Clear the DTCs with a scan tool. Turn off the ignition for 30 seconds. Start the engine and operate vehicle within the Conditions For Running DTC. You may also operate vehicle within the conditions that you observed from the Freeze Frame/Failure Records. Did the DTC fail this ignition cycle? If yes, go to step  2. If no, go to next step.
  18. Observe the Capture Info with a scan tool. Are there 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.