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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 P0300-P0306: ENGINE MISFIRE DETECTEDDIAGNOSTIC 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. Are any other DTCs set? If yes, diagnose those DTCs first. See DIAGNOSTIC TROUBLE CODE DEFINITIONS . If no, go to next step.
  3. Start and idle engine. Using scan tool, Review and record FREEZE FRAME/FAILURE RECORDS data. Operate vehicle within conditions present when DTC was set. Monitor scan tool MISFIRE CURRENT CYL # display for each cylinder. Is MISFIRE CURRENT # display incrementing for any cylinder, indicating a misfire currently occurring? If yes, go to next step. If no, see DIAGNOSTIC AIDS .
  4. Using scan tool, view MISFIRE HISTORY CYL # display. Does MISFIRE HISTORY CYL # display a very large value for more than one cylinder? If yes, go to next step. If no, go to step  13.
  5. Visually and physically inspect vacuum hoses for splits, kinks, and improper connections. If problem was found, repair as necessary. Did you find and correct the condition? If yes, go to step  25. If no, go to next step.
  6. Visually and physically inspect crankcase ventilation valve for improper installation and for damaged "O" rings. If problem was found, repair as necessary. Did you find and correct the condition? If yes, go to step  25. If no, go to next step.
  7. Inspect throttle body inlet screen for damage or for presence of foreign objects that may alter airflow sample through MAF sensor. If problem was found, repair as necessary. Did you find and correct the condition? If yes, go to step  25. If no, go to next step.
  8. Test fuel pressure. If problem was found, repair as necessary. Did you find and correct the condition? If yes, go to step  25. If no, go to next step.
  9. Test fuel for excessive water, alcohol, or other contaminants. If problem was found, repair as necessary. Did you find and correct the condition? If yes, go to step  25. If no, go to next step.
  10. Visually and physically inspect PCM injector grounds, power grounds and sensor grounds to ensure that they are clean, tight, and in their proper locations. If problem was found, repair as necessary. Did you find and correct the condition? If yes, go to step  25. If no, go to next step.
  11. Visually and physically inspect the following areas for vacuum leaks:
    • Intake manifold.
    • EGR adapter.
    • EGR valve.
    • EGR feed pipes.
    • Injector "O" rings.
    If problem was found, repair as necessary. Did you find and correct the condition? If yes, go to step  25. If Did you find and correct the condition? If yes, go to next step.
  12. Remove EGR valve. Visually and physically inspect valve to ensure pintle is not sticking partially open. Also, inspect EGR valve pintle and seat for carbon deposits or burrs that may interfere with pintle closing completely. If problem was found, repair as necessary. Did you find and correct the condition? If yes, go to step  25. If no, go to next step.
  13. Test for proper fuel injector operation. If problem was found, repair as necessary. Did you find and correct the condition? If yes, go to step  25. If no, go to next step.
  14. Visually and physically inspect ignition wires associated with cylinders which were misfiring. If problem was found, repair as necessary. Did you find and correct the condition? If yes, go to step  25. If no, go to next step.
  15. Install Spark Tester (J 26792) at spark plug end of ignition wire for cylinder that is indicated by MISFIRE CURRENT CYL # counters or MISFIRE HISTORY CYL # counters as having most severe misfire (largest number of counts). Connect jumper wire between spark plug end of companion cylinder ignition wire and ground. Companion cylinder is cylinder that shares same ignition coil (i.e., 1/4; 2/5; 3/6). Crank engine while observing spark tester. A spark should be observed. Is spark is present? If yes, go to step  20. If no, go to next step.
    16. NOTE: If carbon tracking or terminal discoloration is apparent at ignition coil end of any ignition wires, replace affected ignition wire and associated ignition coil.
  16. Remove and visually and physically inspect ignition wires associated with cylinders that were indicated as misfiring. Ensure wires and boots are free of carbon tracking and insulation is not damaged. If problem was found, repair as necessary. Did you find and correct the condition? If yes, go to step  25. If no, go to next step.
  17. Measure resistance of spark plug wires associated with cylinders that were indicated as misfiring (except police 3000 ohms/foot, police 600 ohms/foot). If problem was found, repair as necessary. Did you find and correct the condition? If yes, go to step  25. If no, go to next step.
  18. Remove and visually and physically inspect ignition coils associated with cylinders that were indicated as misfiring. Ensure that coils and coil towers are free of cracks and carbon tracking. If problem was found, repair as necessary. Did you find and correct the condition? If yes, go to step  25. If no, go to next step.
  19. Measure ignition coil secondary resistance. If resistance is not 5000-8000 ohms, replace malfunctioning ignition coils as necessary. Did you find and correct the condition? If yes, go to step  25. If no, go to step  24.
  20. Remove spark plugs from cylinders that were indicated as misfiring. Visually inspect spark plug electrodes for excessive fouling. Did you find and correct the condition? If yes, go to step  25. If no, go to next step.
  21. Visually inspect spark plug insulators for cracks, carbon tracking, or other damage. Inspect spark plug electrodes for .060" (1.52 mm) gap. If problem was found, repair as necessary. Did you find and correct the condition? If yes, go to step  25. If no, go to next step.
  22. Inspect for an engine mechanical condition. See appropriate article in ENGINES. If problem was found, repair as necessary. Did you find and correct the condition? If yes, go to step  25. If no, go to next step.
  23. Inspect for a transaxle TCC condition. See appropriate DIAGNOSIS article in AUTOMATIC TRANSMISSIONS. If problem was found, repair as necessary. Did you find and correct the condition? If yes, go to step  25. If no, See DIAGNOSTIC AIDS .
  24. Replace ignition control module. After repairs, go to next step.
  25. 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.
  26. 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.