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Home >> GMC >> 2003 >> Safari Van Cargo, AWD >> Repair and Diagnosis >> External Pages >> Different car >> Section 85 (Engine Controls - Self-Diagnostics - 8.1L) >> Diagnostic Tests >> DTC P0300-P0308: Engine Misfire Detected >> Diagnostic Procedures

Diagnostic Procedures

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  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: You must perform the CKP system variation learn procedure before proceeding with this diagnostic table. See CRANKSHAFT POSITION SENSOR  under PROGRAMMING.

    If any DTCs other than P0135 and P0155 are set, refer to those DTCs before proceeding with this diagnosis. See DIAGNOSTIC TROUBLE CODE DEFINITIONS .

    If any abnormal engine mechanical noise can be heard, see appropriate article in ENGINES.

  2. Connect a scan tool. Idle the engine. Monitor all the Misfire Current counters on the misfire data list using a scan tool. There are a total of 8 counters, one counter per cylinder. Are any of the Misfire Current counters incrementing? If yes, go to step  4. If no, go to next step.
  3. Turn ON the ignition, with the engine OFF. Review the Freeze Frame/Failure Records data for this DTC. Turn OFF the ignition for 15 seconds. Start the engine. Operate the vehicle within the Conditions for Running DTC, and as closely as possible to the Freeze Frame/Failure Records conditions. Select the Diagnostic Trouble Code (DTC) option, the Specific DTC option, and then enter the DTC number using the scan tool. Does the scan tool indicate that this diagnostic failed this ignition cycle? If yes, go to next step. If no, see DIAGNOSTIC AIDS .
  4. Is only one misfire counter incrementing? If yes, go to step  6. If no, go to next step.
  5. Inspect the following items:
    • Inspect the vacuum hoses for splits, kinks, and the correct connections.
    • Inspect throttle body and the intake manifold for vacuum leaks.
    • Inspect the crankcase ventilation system for any vacuum leaks.
    • Verify the PCM grounds are clean and secure.
    • Inspect the ignition coil ground circuit for an open on the cylinders that are showing misfires. An open ground causes all 4 ignition coils to be inoperative. The Misfire Current counters only display 2-3 cylinders misfiring, not all four cylinders.
    • Verify that all of the injector harness connectors are connected to the proper injector.
    • Inspect the injector fuses and the ignition control fuses. If you find an open fuse, locate and repair the short circuit.
    • Test for incorrect fuel pressure and for a restricted fuel flow. See appropriate BASIC DIAGNOSTIC PROCEDURES article.
    • Inspect for a restricted exhaust.
    • Test for fuel contamination.
    • If an abnormal engine mechanical noise is heard, see appropriate article in ENGINES.

    Did any of the above checks isolate a condition requiring a repair? If yes, go to step  17. If no, go to next step.

    6. NOTE: Before disconnecting the injector harness, see appropriate REMOVAL, OVERHAUL & INSTALLATION article. There is a special procedure for disconnecting the fuel injector harness connectors.
  6. Turn OFF the ignition. Disconnect the injector that corresponds to the Misfire Current counters that were incrementing. Connect the injector test lamp to the injector electrical connector. Idle the engine. Does the injector test lamp flash? If yes, go to next step. If no, see DTC P0200: FUEL INJECTOR CONTROL CIRCUIT .
  7. Turn OFF the ignition. Connect the injector harness electrical connector. Disconnect the ignition wires from the spark plug that corresponds to the Misfire Current counters that were incrementing. Install the J 26792 Spark Tester to a ground. Start the engine. Does the spark jump the tester gap and is the spark consistent? If yes, go to step  9. If no, go to next step.
  8. Remove the ignition wire for the cylinder that is misfiring. Measure the resistance of the ignition wire using the Digital Multimeter (DMM). Is the resistance less than 700 ohms? If yes, diagnose ignition system. See appropriate BASIC DIAGNOSTIC PROCEDURES article. If no, go to step  16.
  9. Remove the spark plugs from the cylinder that indicated a misfire. Does the spark plug appear to be okay? If yes, go to next step. If no, go to step  11.
    10. NOTE: If the Injector Coil Test Procedure is okay, repair engine mechanical problem. See appropriate article in ENGINES.
  10. Switch the suspected spark plug with the spark plug from 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, perform fuel injector coil test. See appropriate SYSTEM & COMPONENT TESTING article.
  11. Are the spark plugs oil fouled or coolant fouled? If yes, see appropriate article in ENGINES. If no, go to next step.
    12. NOTE: If the fuel system is okay, diagnose engine mechanical problem. See appropriate article in ENGINES.
  12. Are the spark plugs gas fouled? If yes, diagnose fuel system. See appropriate BASIC DIAGNOSTIC PROCEDURES article. If no, go to next step.
    13. NOTE: If the Injector Coil Test Procedure is okay, repair engine mechanical problem. See appropriate article in ENGINES.
  13. Do the spark plugs show any signs of being cracked, worn, or improperly gapped? If yes, go to next step. If no, perform fuel injector coil test. See appropriate SYSTEM & COMPONENT TESTING article.
  14. Replace or re-gap the spark plugs. If an improper gap is found, use a wire-type gauge to re-gap the spark plugs. After repairs, go to step  17.
  15. Replace the faulty spark plugs. After repairs, go to step  17.
  16. Replace the faulty ignition wires. After repairs, go to next step.
  17. If the customer's concern is that the MIL was flashing, are any catalyst DTCs set? If yes, see DTC P0420 & P0430: TWC SYSTEM - LOW EFFICIENCY - BANK 1 & 2 . If no, go to next step.
  18. Clear the DTCs with a scan tool. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions for Running DTC. Does the DTC run and pass? If yes, go to next step. If no, go to step  2.
  19. Observe the stored information, Capture Info with a scan tool. 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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