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Home >> Isuzu >> 1999 >> Trooper Part Time >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls - Self-Diagnostics >> Diagnostic Trouble Code Identification >> DTC P0300: Random Misfire Detected >> Diagnostic Aids

Diagnostic Aids

Scan tool message MISFIRE CUR. #1-6, is helpful for determining if only a single cylinder is affected by misfire. Check ignition coil for cracks or damage. Substitute a known-good ignition coil. Numerous short trip drives (under 3 miles), may fuel foul spark plugs and set DTC. If DTC cannot be duplicated, check scan tool FAILURE RECORDS to determine mileage since last DTC was set.

  1. Perform On-Board (OBD) System Check. Read and record FREEZE FRAME and/or FAIL RECORDS data for each DTC set. Go to next step.
  2. Start engine and run at idle. Using scan tool, read and record FREEZE FRAME data. Operate vehicle within FAILURE RECORDS conditions. Read MISFIRE CUR # display for each cylinder. If MISFIRE CUR # increases (indicating an occurring misfire) for any cylinder, go to next step. If MISFIRE CUR # does not increase, check for an intermittent problem. See DIAGNOSTIC AIDS .
  3. Check vacuum hoses for splits, kinks or improper connections. If a problem is found, repair as necessary. Retest system. If no problem is found, go to next step.
  4. Check intake manifold, injector "O" rings, EGR valve and EGR feed pipes for vacuum leaks. If a problem is found, repair as necessary. Retest system. If no problem is found, go to next step.
  5. Check PCV valve for improper installation or damaged hose. If a problem is found, repair as necessary. Retest system. If no problem is found, go to next step.
  6. Inspect MAF inlet screen for foreign objects or damage that may block airflow to MAF sensor. Repair as necessary. Retest system. If inlet screen is okay, go to next step.
  7. Remove EGR valve and inspect valve, seat and passages for damage. Inspect valve pintle to ensure it is not sticking partially open. If a problem is found, repair as necessary. Retest system. If no problem is found, go to next step.
  8. Install a spark tester on spark plug wire at spark plug end for cylinder indicating a misfire. Crank engine and check for a strong Blue spark. Repeat procedure for each affected cylinder. If spark is okay on all cylinders, go to step  13. If spark is not okay at all cylinders, go to next step.
  9. Remove ignition coil related to misfiring cylinder. Check ignition coil for cracks or carbon tracking. If a problem is found, replace damaged coil. Retest system. If no problem is found, go to next step.
  10. 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.
  11. Check spark plug insulators for cracks, carbon tracking or other damage. If a problem is found, replace faulty spark plug(s). Retest system. If no problem is found, go to next step.
  12. Disconnect MAF sensor harness connector. Operate vehicle in closed loop. while monitoring BANK 1 L.T. FUEL TRIM and BANK 1 S.T. FUEL TRIM parameters on scan tool. If L.T. FUEL TRIM decreases to less than 20 percent and ST FUEL TRIM decreases to less than 50 percent, replace HEX ignition coil control module (3.2L), or ignition coil (3.5L). On all models, if scan tool display is not as specified, replace ignition coil related to affected cylinder. Retest system.
  13. Ensure PCM injector ground, power ground and sensor ground connections are clean and tight. Repair as necessary. Retest system. If connections are okay, go to next step.
  14. See FUEL PUMP PRESSURE TEST in BASIC DIAGNOSTIC PROCEDURES article. If a problem was found, repair as necessary. Retest system. If no problem was found, go to next step.
  15. Check for fuel contamination. If fuel is contaminated, repair as necessary. Retest system. If fuel is okay, go to next step.
  16. Perform injector balance test. See FUEL SYSTEM in SYSTEM & COMPONENT TESTING article. If a problem is found, repair as necessary. Retest system. If no problem is found, go to next step.
  17. Diagnose mechanical engine problems such as faulty camshaft, leaking or sticking valves, weak valve springs, damaged piston rings, loose or worn rocker arms, incorrect valve timing, leaking head gasket or loose or broken motor mounts. If a problem is found, repair as necessary. Retest system. If no problems are found, go to next step.
  18. Check for a transmission Torque Converter Clutch (TCC) problem. See HYDRA-MATIC 4L30-E article in AUTOMATIC TRANSMISSIONS. Repair as necessary. If no problem is found, replace MAF sensor. Retest system.
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