Diagnostic Aids
Scan tool message MISFIRE CUR. #1-4 is helpful for determining if only a single cylinder is affected by misfire. If scan tool displays MISFIRE CUR. #1-4, check suspect ignition coil for cracks or damage. If problem is present, substitute a known-good ignition coil, or swap ignition coils cylinder-to-cylinder. If misfire follows coil, replace suspect ignition coil.
If misfire is random check following:
- Check PCM and system grounds. Ensure that all grounds are clean and secure. See WIRING DIAGRAMS article.
- Check MAF sensor. See SYSTEM & COMPONENT TESTING article.
- Check for disconnected or damaged vacuum hoses. Incorrectly installed or faulty PCV valve. Check for vacuum leaks at EGR valve, throttle body and intake manifold.
- Perform FUEL PUMP PRESSURE TEST under FUEL SYSTEM in BASIC DIAGNOSTIC PROCEDURES article. Check for faulty fuel pump, plugged fuel filter or suspect fuel pressure regulator.
- Check fuel injectors. Perform FUEL INJECTOR BALANCE TEST under FUEL SYSTEM in SYSTEM & COMPONENT TESTING article. Also check condition of fuel injector "O" ring seals.
- Check EGR valve. Check for leaking valve, adapter and feed pipes. See SYSTEM & COMPONENT TESTING article.
- Check fuel quality. Fuel with wrong octane rating may cause driveability issues.
- Excessive engine vibration may falsely set DTC P0301-P0304.