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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 181 (ENGINE CONTROLS - SELF-DIAGNOSTICS - 2.2L)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 Saturn Vue. 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: You must perform the Crankshaft Position (CKP) system variation learn procedure before proceeding with this diagnostic test. Refer to CRANKSHAFT POSITION VARIATION LEARN PROCEDURE  under PROGRAMMING.
  2. Start engine. Allow the engine to idle or operate within the conditions listed in the Freeze Frame/Failure Records. Monitor all of the Misfire counters with the scan tool. Are any of the Misfire Current counters incrementing? If yes, go to next step. If no, problem is intermittent. See INTERMITTENTS in TROUBLE SHOOTING - NO CODES - 2.2L & 3.0L VUE article.
  3. Are any other DTCs set? If yes, diagnose affected DTC(s). See DIAGNOSTIC TROUBLE CODE DEFINITIONS . If no, go to next step.
  4. Can any abnormal engine noise be heard? If yes, diagnose engine mechanical problem. If no, go to next step.
  5. Inspect vacuum hoses and seals for splits, restrictions, and improper connections. Check crankcase ventilation system for vacuum leaks. Check PCM grounds for corrosion and loose connections. Check exhaust system for restrictions or fuel for contamination. Did you find and correct the condition? If yes, go to step  19. If no, go to next step.
  6. Turn ignition off. Remove the fuel pump relay. Remove the ignition coil housing assembly, keeping the IC module assembly connected to the harness connector. Connect a jumper wire between the top of the IC module and a good ground. Install the Ignition System Diagnosis Harness (J-36012-A). Install Spark Tester (J-26792) on the No. 1 spark plug jumper wire. Ground the No. 4 spark plug jumper wire. The No. 4 wire is the companion to No. 1. Crank engine with the remaining spark plug wires connected. Repeat the steps by installing the spark tester on cylinders No. 4 and grounding No. 1. Do the same for cylinders No. 2 and 3 spark plugs. Ensure the companion wire is grounded. Does the spark tester spark on all cylinders? If yes, go to next step. If no, diagnose electronic ignition system. See ELECTRONIC IGNITION SYSTEM (2.2L) in BASIC DIAGNOSTIC PROCEDURES - 2.2L & 3.0L VUE article.
  7. Remove the spark plug from the cylinder that indicated a misfire. Inspect spark plug. Does the spark plug appear to be okay? If yes, go to next step. If no, go to step  9.
  8. Exchange the suspected spark plug with another cylinder that is operating properly. Operate the vehicle under the same conditions that the misfire occurred. Did the misfire move with the spark plug? If yes, go to step  15. If no, go to step  12.
  9. Is the spark plug oil or coolant fouled? If yes, diagnose for an engine mechanical problem. If no, go to next step.
  10. Is the spark plug gas fouled? If yes, go to step  13. If no, go to next step.
  11. Did the spark plug show any signs of being cracked, worn, or improperly gapped? If yes, go to step  14. If no, go to next step.
  12. Perform the fuel injector coil test. Refer to FUEL INJECTOR COIL TEST in SYSTEM & COMPONENT TESTING - 2.2L & 3.0L VUE article. Did you find and correct the condition? If yes, go to step  19. If no, diagnose for an engine mechanical problem.
  13. Perform fuel system diagnosis. Refer to FUEL SYSTEM DIAGNOSIS in BASIC DIAGNOSTIC PROCEDURES - 2.2L & 3.0L VUE article. Did you find and correct the condition? If yes, go to step  16. If no, diagnose for an engine mechanical problem.
  14. Replace or gap spark plug. Did you complete the action? If yes, go to step  16.
  15. Replace faulty spark plug. After replacing spark plug, go to next step.
  16. Was the customer concern the MIL flashing? If yes, go to next step. If no, go to step  18.
  17. Operate engine at 2500 RPM for 4 minutes. Operate the vehicle within the Conditions for Running the DTC. See CONDITIONS FOR RUNNING DTC . Does the DTC run and pass? If yes, go to next step. If no, go to DTC P0420: CATALYST SYSTEM EFFICIENCY BELOW THRESHOLD .
  18. Using scan tool, clear DTCs. Turn ignition off for 30 seconds. Operate vehicle within the conditions for running the DTC. See CONDITIONS FOR RUNNING DTC . You may also operate vehicle within the conditions observed from the Freeze Frame/Failure Records. Did the DTC fail this ignition? If yes, go to step  2. If no, go to next step.
  19. With a scan tool, observe the stored information, Capture Info. Does the scan tool display any DTCs that you have not 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.