Diagnostic Aids
Scan tool provides information that can be useful in identifying misfiring cylinder. If DTC P0300 is currently stored as DTC status FAILED SINCE CODE CLEAR, misfire history counters (MISFIRE HIST #1-#6) will still contain a value that represents level of misfire detected on each cylinder.
A misfire DTC may set if components that affect Crankshaft Position (CKP) sensor have recently been replaced, and crankshaft position sensor variation learn procedure has not been performed. If diagnostic test does not identify a condition, perform CRANKSHAFT POSITION SENSOR VARIATION LEARN PROCEDURE under PROGRAMMING. Crankshaft position sensor variation learn procedure should be performed if any of the following conditions are true:
- PCM has been replaced.
- DTC P1336 is set.
- Engine has been replaced.
- Crankshaft has been replaced.
- Crankshaft harmonic balancer has been replaced.
- Crankshaft Position (CKP) sensor has been replaced.
Scan tool displayed misfire counter values (MISFIRE HIST. #1-#6) can be useful to determine whether misfire affects a single cylinder, a cylinder pair (cylinders that share an ignition coil), or random. Switch ignition coils and retest. If misfire follows coil, replace suspect ignition coil. If largest amount of activity is isolated to a cylinder pair, check for following conditions:
- Secondary Ignition Wires - Check secondary wires for with suspect cylinder pair for disconnected ignition wires or excessive resistance. Specification is 3000 ohms per ft, 250 ohms per INCH of spark plug wire.
- Damaged Or Malfunctioning Ignition Coil
Check for cracks, carbon tracking or other damage. Also check coil secondary resistance. Secondary resistance should be 5-8 K/ohms.
If misfire is random, check the following:
- System grounds - Ensure all ground connections are clean and properly tightened. See Fig 1.
- Mass Air Flow (MAF) sensor - MAF sensor output that causes PCM to sense a lower than normal air flow will cause a lean condition. Attempt to operate vehicle within failure records conditions with MAF sensor disconnected. If lean or misfiring condition is not present with MAF sensor disconnected, replace MAF sensor.
- Damaged accessory drive belt or driven accessory - A damaged serpentine belt or belt driven accessory can cause engine load variations sufficient to set a misfire DTC.
- Air induction system - Vacuum leaks that cause intake air to bypass MAF sensor will cause a lean condition. Check for disconnected or damaged vacuum hoses, incorrectly installed or malfunctioning crankcase ventilation valve or vacuum leaks at throttle body, EGR valve and intake manifold mounting surfaces.
- Fuel pressure - Perform a fuel system pressure test. See appropriate BASIC DIAGNOSTIC PROCEDURES article. A malfunctioning fuel pump, plugged filter or malfunctioning fuel pressure regulator will contribute to a lean condition.
- Fuel injectors - Perform fuel injector coil test. See appropriate SYSTEM & COMPONENT TESTING article.
- Water contamination in fuel system can cause a single cylinder to misfire as well as random misfire.
- Check for leaking EGR valve, adapter or feed pipes which will contribute to a lean condition or excessive EGR flow.
- Excessive Open Loop operation caused by extended idling or short trip driving may leave deposits on Heated Oxygen Sensors (HO2S). Deposits cause HO2S to respond slowly to exhaust oxygen content, affecting fuel control and causing a misfire to be indicated at idle. This condition is not permanent. To determine if this condition is causing DTC P0300 to set, review FREEZE FRAME/FAILURE RECORDS for DTC P0300. If DTC P0300 occurs at high engine speeds, deposits did not cause DTC to set. If DTC P0300 occurs at idle or very low engine speeds and at engine coolant temperatures below 176°F (80°C), deposits are very likely cause of DTC. Deposits on HO2S can be eliminated by running vehicle at operating temperature with MAF sensor above 15 g/s.