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Home › Saturn › 2002 › L300 › Repair and Diagnosis › Engine Performance › System › Engine Controls - Self-Diagnostics › Diagnostic Tests › DTC P0300: Engine Misfire Detected › Diagnostic Procedures

Diagnostic Procedures

  1. Ensure fuel level is more than 10 percent. Operate engine for one minute. Using scan tool, monitor current misfire counters. If any current counters are incrementing, go to next step. If no current counters are incrementing, problem is intermittent. Review MISFIRE HISTORY to see which cylinders where misfiring. See DIAGNOSTIC AIDS  .
  2. If a particular counter is dominant, see DTC P0301-P0306: SPECIFIC CYLINDER MISFIRE DETECTED  . If a particular counter is not dominant, go to next step.
  3. If misfire counters from both banks are randomly filling up, go to next step. If misfire counters from one bank are dominate, go to step  5 . If only 2 cylinders on one bank are dominate, a mechanical failure may exist. Perform a compression test.
  4. Check the following for possible causes of random cylinder misfires:
    • Large vacuum leak.
    • Fuel pressure too low. Should be 39-49 psi.
    • EGR valve not closing. EGR SENSOR voltage on scan tool with ignition on, engine off should be 0.5-1.0 volt.
    • Fuel quality poor. Water will separate in test sample.
    • Clogged fuel injectors. Perform injector balance test. See appropriate SYSTEM & COMPONENT TESTING article.
    • Spark plug boots arcing beneath EI module.
    • Timing belt incorrectly timed or exhaust valves sticking.
  5. Turn ignition on, engine off. Using Scan tool, perform ignition coil cyl. #1 - 6 test on each individual cylinder that was misfiring, while listening for audible spark at each cylinder. If spark is not audible on at least one cylinder, go to next step. If spark is audible on at least one cylinder, a problem may exist in the HO2S sensor 1 circuit on the bank that was misfiring. A good indication of this would be if the HO2S sensor 1 lambda value on scan tool with engine running is at 3.99. See DTC P0130: HO2S CIRCUIT - BANK 1, SENSOR 1  or DTC P0150: HO2S CIRCUIT - BANK 2, SENSOR 1  . If this is not the case, the cam timing for the bank that was misfiring may be incorrectly timed or the pre-converter for the same bank may be restricted.
  6. Disconnect El module from the bank of cylinders that were misfiring. Connect test light between EI harness connector terminals No. 1 (Black wire) and No. 2 (Pink wire). If test light is on, check connector. If connector is okay, replace EI module. If test light is off, repair open in Pink wire or Black wire.
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