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Home >> Oldsmobile >> 1998 >> Regency >> Repair and Diagnosis >> Engine Performance >> System >> Engine Control Systems Tests W/Codes >> Diagnostic Trouble Codes >> DTC P0300-P0306 - Engine Misfire Detected >> Diagnostic Procedures
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Diagnostic Procedures

  1. Perform POWERTRAIN ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK . Read and record FREEZE FRAME and/or FAIL RECORDS data for each DTC set. After performing OBD system check, go to next step.
  2. If DTC P1200 is also set, go to DTC P1200. If DTC P1200 is not set, go to next step.
  3. Start engine and allow it to idle. Using scan tool, read and record FREEZE FRAME data. Operate vehicle within conditions noted in FREEZE FRAME data. Using scan tool, read MISFIRE CUR # display for each cylinder. If MISFIRE CUR # is increasing (indicating a misfire currently occurring) for any cylinder, go to next step. If MISFIRE CUR # is not increasing, see DIAGNOSTIC AIDS.
  4. If MISFIRE HIST # displays a very large number for more than one cylinder, go to next step. If MISFIRE HIST # does not display a very large number for more than one cylinder, go to step 14).
  5. Check vacuum hoses for splits, kinks, or improper connections. Repair as necessary. After repairs, go to step 27). If no problem is found, go to next step.
  6. Check PCV valve for improper installation or damaged "O" rings. If a problem is found, go to step 27). If no problem is found, go to next step.
  7. Check throttle body inlet screen for blockage or foreign objects which may partially block airflow sample through MAF sensor. Repair as necessary. After repairs, go to step 27). If no problem is found, go to next step.
  8. Perform injector balance test. See SYSTEM/COMPONENT TESTS article. Repair as necessary. After repairs, go to step 27). If no problem is found, go to next step.
  9. Check for fuel contamination. If fuel is contaminated, repair condition causing contamination and go to step 27). If fuel is okay, go to next step.
  10. Check PCM injector, power and sensor grounds for clean and tight connection, and for proper location. Repair as necessary. After repairs, go to step 27). If no problem is found, go to next step.
  11. Disconnect MAF sensor harness connector. Operate vehicle within conditions noted in FREEZE FRAME data. Using scan tool, read MISFIRE CUR # display for each cylinder. If MISFIRE CUR # is increasing (indicating a misfire currently occurring) for any cylinder, go to next step. If MISFIRE CUR # is not increasing, go to step 25).
  12. Check intake manifold, injector "O" rings, EGR adapter, EGR valve, and EGR feed pipes for vacuum leaks. Repair as necessary. After repairs, go to step 27). If no problem is found, go to next step.
  13. Remove EGR valve and inspect valve, seat and passages for damage. Inspect valve pintle to ensure it is not sticking partially open. Repair as necessary. After repairs, go to step 27). If no problem is found, go to next step.
  14. Perform injector circuit test. See BASIC TESTING article. Repair as necessary and go to step 27). If no problem is found, go to next step.
  15. Check ignition wire(s) associated with cylinder(s) that is misfiring. Repair as necessary. After repairs, go to step 27). If no problem is found, go to next step.
  16. Install a spark tester on spark plug wire at spark plug end for cylinder indicating most severe misfire. Connect a jumper wire between engine ground and companion cylinder spark plug wire at spark plug end. Companion cylinders are 1 and 4, 2 and 5, and 3 and 6. Crank engine and check for crisp, Blue spark at tester. Repeat procedure for each affected cylinder. If spark is okay on all cylinders, go to step 21). If spark is not okay on any cylinder, go to next step.
  17. Remove ignition coil(s) associated with misfiring cylinder(s). Check ignition coils for cracks or carbon tracking. If a problem is found, replace damaged coil(s) and go to step 27). If no problem is found, go to next step.
  18. Check ignition coil secondary resistance. If resistance is not 7000 ohms on VIN 1 or 10,000 ohms on VIN K, replace ignition coil. After replacing coil, go to step 27). If resistance is as specified, go to next step.
  19. Remove ignition wire(s) associated with misfiring cylinder(s). Check ignition wire(s) boot for carbon tracking or damaged insulation. If a problem is found, replace faulty ignition wire(s). After replacing ignition wire(s), go to step 27). If no problem is found, go to next step.
  20. Check resistance of ignition wire(s) associated with misfiring cylinder(s). If resistance is not 5000-8000 ohms, replace ignition wire(s). After replacing ignition wire(s), go to step 27). If resistance is 5000-8000 ohms, go to step 26).
  21. Remove spark plug(s) associated with misfiring cylinder(s). Check spark plugs for excessive fouling. If spark plugs are excessively fouled, diagnose and repair mechanical engine problem. If spark plugs are not excessively fouled, go to next step.
  22. Check spark plug insulation for cracks, carbon tracking or other damage. If a problem is found, replace faulty spark plug(s). After replacing spark plug(s), go to step 27). If no problem is found, go to next step.
  23. Diagnose mechanical engine problem. Repair as necessary. After repairs, go to step 27). If no problem is found, go to next step.
  24. Check for transmission DTCs. See the AUTO TRANS DIAGNOSIS article in the AUTO TRANS DIAGNOSIS section. Repair as necessary. After repairs, go to step 27). If no problem is found, see DIAGNOSTIC AIDS.
  25. Replace MAF sensor. After replacing sensor, go to step 27).
  26. Replace ignition control module. After replacing module, go to next step.
  27. Using scan tool, read and record FREEZE FRAME data. Clear DTCs. Start engine and allow it to idle. Operate vehicle within conditions noted in FREEZE FRAME data. Using scan tool, read MISFIRE CUR # display for each cylinder. If MISFIRE CUR # is increasing (indicating a misfire currently occurring) for any cylinder, return to step 2). If MISFIRE CUR # is not increasing, repair is complete.
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