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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 139 (ENGINE CONTROL SYSTEM - SELF-DIAGNOSTICS)DIAGNOSTIC TESTSDTC P0300: 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 is about a different car, the 2003 Pontiac Vibe. 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, perform diagnostic system check - engine controls. See DIAGNOSTIC SYSTEM CHECK - ENGINE CONTROLS  under SELF-DIAGNOSTIC SYSTEM.
  2. Install scan tool. Start engine. Monitor TOTAL MISFIRE CURRENT COUNT parameter on scan tool. Did scan tool indicate any misfire counts? If yes, go to step  5. If no, go to next step.
  3. Use scan tool to clear DTCs. Place vehicle into CHECK MODE, using scan tool. See SERVICE BAY TESTS  under SELF-DIAGNOSTIC SYSTEM. Operate vehicle within freeze frame conditions as specified. Monitor TOTAL MISFIRE CURRENT COUNT parameter on scan tool. Did DTC P0300 set or did scan tool indicate any misfire counts? If yes, go to step  5. If no, go to next step.
  4. The fault is not present. Are there any DTCs stored that have not been diagnosed? If yes, go to DIAGNOSTIC TROUBLE CODE DEFINITIONS . If no, go to DIAGNOSTIC AIDS .
  5. Disconnect all fuel injector harness connectors. Remove ignition coil 1. Connect ignition coil harness connector. Connect Spark Tester (J 26792), or equivalent, to ignition coil 1 boot. Crank engine and observe spark tester. A crisp, blue spark should be observed. Install ignition coil. Repeat steps 2)  through 5)  for each remaining ignition coils. Is a crisp blue spark present for each ignition coil? If yes, go to step  7. If no, go to next step.
  6. Turn ignition OFF. Exchange ignition coils. Replace suspected faulty ignition coil with a coil that produced good spark. Connect Spark Tester to the ignition coil boot. Crank engine while observing spark tester. Was a crisp blue spark present? If yes, go to step  14. If no, go to step  15.
  7. Perform a visual and physical inspection of the ignition coil boots and spark plugs for the following conditions:
    • Any water intrusion.
    • Any corroded or poor connections to the spark plugs.
    • Any carbon tracking.
    Repair condition or replace faulty coil or spark plug as necessary. See IGNITION COIL OR SPARK PLUG under IGNITION SYSTEMS in REMOVAL & INSTALLATION article. Was a repair necessary? If yes, go to step  17. If no, go to next step.
  8. Remove spark plugs. Perform a visual and physical inspection of the spark plugs. Repair or replace as necessary. Was a faulty spark plug condition found and repaired? Go to step  17. If no, go to next step.
  9. Turn ignition OFF. Disconnect all 4 fuel injector harness connectors. Install a Fuel Injector Test Light (J 34730-2B), or equivalent, into each fuel injector harness connector. Observe injector test light for each fuel injector while cranking engine. Does fuel injector test light flash for each injector while cranking the engine? If yes, go to next step. If no, go to FUEL INJECTOR CIRCUIT DIAGNOSIS under FUEL SYSTEMS in SYSTEM & COMPONENT TESTING - 1.8L VIBE article.
  10. Perform FUEL INJECTOR COIL TEST - ENGINE COOLANT TEMPERATURE BETWEEN 50-95°F (10-35°C) and FUEL INJECTOR BALANCE TEST WITH SPECIAL TOOL procedures under FUEL SYSTEMS in SYSTEM & COMPONENT TESTING - 1.8L VIBE article. Did any fuel injectors require replacement? If yes, go to step  17. If no, go to next step.
  11. Turn ignition OFF. Relieve fuel pressure. See FUEL SYSTEM PRESSURE RELEASE under FUEL SYSTEMS in SYSTEM & COMPONENT TESTING - 1.8L VIBE article. Install fuel pressure gauge. See FUEL GAUGE under FUEL SYSTEMS in REMOVAL & INSTALLATION - 1.8L VIBE article. Install scan tool. Turn ignition switch to RUN position, engine OFF. Select FUEL PUMP in ENGINE OUTPUT CONTROLS menu of scan tool. Turn fuel pump ON, using scan tool. Observe fuel pressure indicated by fuel pressure gauge with fuel pump running. Is fuel pressure 44-50 psi (301-347 kPa)? If yes, go to next step. If no, diagnose fuel system. See FUEL SYSTEMS in SYSTEM & COMPONENT TESTING - 1.8L VIBE article.
  12. Check a fuel sample for any alcohol or other contamination and fuel type and quality. See ALCOHOL/CONTAMINANTS-IN-FUEL DIAGNOSIS under FUEL SYSTEMS in SYSTEM & COMPONENT TESTING - 1.8L VIBE article. Replace fuel as necessary. Was a repair necessary? If yes, go to step  17. If no, go to next step.
  13. Visually and physically check PCV valve, intake manifold gasket, vacuum lines, vacuum hoses, and fittings for vacuum leaks or for a misrouted vacuum line or hose. Repair as necessary. Did you find and correct a condition? If yes, go to step  17. If no, go to step  16.
  14. Replace faulty ignition coil. After repair, go to step  17.
  15. Check electrical circuits of faulty ignition coil for the following conditions:
    • An open in ignition coil ground circuit.
    • An open in positive voltage supply circuit.
    • An open in trigger signal circuit.
    No trigger signal generated by PCM. Repair as necessary. After repair, go to step  17.
  16. Check for any of the following engine mechanical concerns:
    • A worn or faulty camshaft.
    • Any leaky or sticky valves.
    • Any excessive valve deposits.
    • Any weak valve springs.
    • Incorrect valve timing.
    • Faulty rocker arm operation.
    • Any intake manifold runner leaks or obstructions.
    • Any broken or worn piston rings.
    • A leaking head gasket.
    Repair as necessary. See appropriate article in ENGINES. Was a basic engine mechanical concern found and repaired? If yes, go to step  17. If no, go to DIAGNOSTIC AIDS .
  17. Use scan tool to clear DTCs. Operate vehicle within freeze frame conditions that set the DTC. Monitor TOTAL MISFIRE CURRENT COUNT parameter on scan tool. Did a DTC set or did scan tool indicate any misfire counts? If yes, go to DIAGNOSTIC TROUBLE CODE DEFINITIONS . If no DTCs are displayed, system is okay at this time.
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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.