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Home >> Cadillac >> 1996 >> Seville SLS >> Repair and Diagnosis >> Engine Performance >> System >> Engine Control System - Tests W/Codes >> Diagnostic Trouble Codes >> DTC P0300-P0308 - Engine Misfire Detected >> Diagnostic Procedures

Diagnostic Procedures

  1. Perform On-Board Diagnostic (OBD) System Check. After performing OBD system check, go to next step.
  2. Turn ignition on. Using scan tool, check for any other DTC(s) present. If other DTC(s) are present, diagnose affected DTC(s) first. If other DTC(s) are not present, go to next step.
  3. Start and idle engine. Using scan tool, observe MISFIRE CUR. counters in the misfire data list (there are 8 counters, one for each cylinder). If any of the counters are incrementing, go to next step. If counters are not incrementing, go to step 15).
  4. Turn ignition off. Connect Spark Tester (ST-125) to ignition wire(s) that indicated a misfire. Ground the companion cylinder(s) plug wire. Start engine. If spark is present and consistent at spark tester, go to next step. If spark is not present or is intermittent at spark tester, go to step 9).
  5. Turn ignition off. Remove and check spark plug from affected cylinder(s). If spark plug is okay (not oil or coolant fouled), diagnose fuel injector. See SYSTEM/COMPONENT TESTS article. If spark plug is fouled, go to next step.
  6. If spark plug is oil or coolant fouled, check engine for mechanical problems. If spark plug is not fouled, go to next step.
  7. If spark plug is cracked or improperly gapped, go to next step. If spark plug is okay, diagnose fuel injector(s). See SYSTEM/COMPONENT TESTS article.
  8. Replace or re-gap spark plug.
  9. Turn ignition off. Using DVOM, check resistance of each affected spark plug wire. If resistance is less than 15,000 ohms, go to next step. If resistance is greater than 15,000 ohms, go to step 13).
  10. Remove affected coil(s). Check for carbon tracking on coil. If carbon tracking is not present, go to next step. If carbon tracking is present, go to step 12).
  11. Switch position of coils at affected cylinder. Start and idle engine. If spark is present and consistent on spark tester, go to next step. If spark is not present or not consistent, go to step 14).
  12. Turn ignition off. Replace faulty ignition coil and spark plug wire.
  13. Replace spark plug wire.
  14. Replace ignition control module.
  15. Turn ignition on, with engine off. Review DTC P0300 snapshot data. Note conditions that the vehicle was operating when DTC was set and operate vehicle under the same condition. Using scan tool, observe MISFIRE CUR. counters in the misfire data list. If any of the counters are incrementing, repeat step 4). if counters are not incrementing, go to next step.
  16. Turn ignition on, with engine off. Using scan tool, observe MISFIRE HIST. counters in the misfire data list. If more than 3 cylinders show a high misfire rate, go to step 18). If less than 3 cylinders show a high misfire rate, go to next step.
  17. Note cylinder that shows a higher number of misfire. Check affected cylinder for damaged or high resistance spark plug wire, vacuum leak, damaged spark plug, etc. Repair as necessary. If no problems are found, see DIAGNOSTIC AIDS.
  18. Check engine for proper vent operation, vacuum leaks or intermittent rich or lean condition. Repair as necessary. If not problems are found, see DIAGNOSTIC AIDS.
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