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DTC P0300: Multiple Cylinder Misfire: Testing

  1. Turn ignition on. Using scan tool, read DTCs. If MISFIRE GOOD TRIP counter is displayed and equal to zero, go to next step. If MISFIRE GOOD TRIP counter is not displayed and equal to zero, go to step  8 .
  2. At this time, conditions required to set DTC are present. Using scan tool, select DTC's and RELATED FUNCTIONS. Record FREEZE FRAME DATA. Select OBD II MONITORS. Read and record MISFIRE SIMILAR CONDITIONS WINDOW DATA. Using these screens, attempt to duplicate condition(s) that set DTC. While in SIMILAR CONDITIONS screen, go to WHICH CYLINDER IS MISFIRING screen. If scan tool is counting misfires at this time, go to next step. If scan tool is not counting misfires at this time, go to step  8 .
  3. Using scan tool, read FREEZE FRAME DATA. Using this data, attempt to determine cause of misfire DTC. If ADAPTIVE FUEL PERCENTAGES in freeze frame are plus or minus 15 percent or less, go to next step. If ADAPTIVE FUEL PERCENTAGES in freeze frame are more than plus or minus 15 percent, check fuel delivery system. See FUEL SYSTEMS in appropriate SYSTEM & COMPONENT TESTING article.
  4. Using scan tool, read FREEZE FRAME DATA. Using this data, attempt to determine cause of misfire DTC. If LOAD VALUE in freeze frame is 50 percent or less with engine at normal operating temperature, go to next step. If LOAD VALUE in freeze frame is more than 50 percent with engine at normal operating temperature, check secondary ignition, compression and cylinder leakage.
  5. Using scan tool, read FREEZE FRAME DATA. Using this data, attempt to determine cause of misfire DTC. If ENGINE RPM in freeze frame is 3000 RPM or less with engine at normal operating temperature, go to next step. If ENGINE RPM in freeze frame is more than 3000 RPM with engine at normal operating temperature, test CMP and CKP signals with lab scope, check valve timing and perform running vacuum test.
  6. Check for any of the following conditions:
    • Ensure engine vacuum is at least 13 in. Hg with transmission in Neutral.
    • Ensure valve timing is within specification.
    • Ensure compression is within specification.
    • Ensure exhaust is free from restrictions or leaks.
    • Ensure PCV system flows freely.
    • Ensure torque converter stall speed is within specification.
    • Ensure power brake booster has no internal vacuum leaks.
    • Ensure fuel is not contaminated.
    • Ensure fuel injector is not plugged or restricted.
    • Ensure fuel injector harness connectors are connected to correct fuel injectors.

    If a problem is found, repair as necessary. If no problems are found, go to next step.

    NOTE: Anything that affects crankshaft speed can cause a misfire DTC.
  7. Check for these other possible causes of misfire: injector harness connectors, secondary ignition problem, mechanical engine problem, PCM power grounds, irregular CMP and CKP signal, plugged injectors, restricted exhaust, intake restriction, damaged trigger wheel, contaminated fuel, vacuum leak, weak valve springs, carbon deposits on valves or accessory drive belt. Check for technical service bulletins related to a misfire DTC. If a problem is found, repair as necessary. If no problems are found, test is complete.
    NOTE: An intermittent problem may have been caused by moisture in secondary ignition.
  8. At this time, misfire does not exist or is intermittent. Using scan tool, select DTC's and RELATED FUNCTIONS. Read and record FREEZE FRAME DATA. Select OBD II MONITORS. Read and record MISFIRE SIMILAR CONDITIONS WINDOW DATA. Using these screens, attempt to duplicate condition that set misfire DTC. While using FREEZE FRAME DATA, pay particular attention to DTC setting conditions, such as SPEED, TEMP, LOAD and MAP VACUUM. If misfire reoccurs, go to next step. If misfire does not reoccur, misfire DTC conditions no longer exist. Refer to any TSBs that are related to symptom. Operate vehicle for 2 misfire good trips and clear DTCs.
  9. Using scan tool, read FREEZE FRAME DATA. Using this data, attempt to determine cause of misfire DTC. If ADAPTIVE FUEL PERCENTAGES in freeze frame are plus or minus 15 percent or less, go to next step. If ADAPTIVE FUEL PERCENTAGES in freeze frame are more than plus or minus 15 percent, check fuel delivery system. See FUEL SYSTEMS in appropriate SYSTEM & COMPONENT TESTING article.
  10. Using scan tool, read FREEZE FRAME DATA. Using this data, attempt to determine cause of misfire DTC. If LOAD VALUE in freeze frame is 50 percent or less with engine at normal operating temperature, go to next step. If LOAD VALUE in freeze frame is more than 50 percent with engine at normal operating temperature, check secondary ignition, compression and cylinder leakage.
  11. Using scan tool, read FREEZE FRAME DATA. Using this data, attempt to determine cause of misfire DTC. If ENGINE RPM in freeze frame is 3000 RPM or less with engine at normal operating temperature, go to next step. If ENGINE RPM in freeze frame is more than 3000 RPM with engine at normal operating temperature, test CMP and CKP signals with lab scope, check valve timing and perform running vacuum test.
  12. Check for any of the following conditions:
    • Ensure engine vacuum is at least 13 in. Hg with transmission in Neutral.
    • Ensure valve timing is within specification.
    • Ensure compression is within specification.
    • Ensure exhaust is free from restrictions or leaks.
    • Ensure PCV system flows freely.
    • Ensure torque converter stall speed is within specification.
    • Ensure power brake booster has no internal vacuum leaks.
    • Ensure fuel is not contaminated.
    • Ensure fuel injector is not plugged or restricted.
    • Ensure fuel injector harness connectors are connected to correct fuel injectors.

    If a problem is found, repair as necessary. If no problems are found, test is complete.

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