Diagnostic Aids
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- Running the vehicle out of fuel causes sufficient misfire to set DTC P0300. A vehicle that is out of fuel may have fuel level DTCs also set.
- Water contamination in the fuel system can cause a single cylinder to misfire as well as cause a random misfire.
- A restricted fuel filter can cause sufficient misfire to set DTC P0300.
- Excessive vibration from sources other than the engine can cause a misfire DTC. The following are possible sources of vibration:
- Variable thickness brake rotor.
- Drive shaft not balanced.
- Certain rough road conditions.
- Observe, if more then one cylinder is misfiring, the scan tool may only display one cylinder misfiring. This will not be apparent until the repair is completed. Also, if an ignition coil ground circuit is open for one side of the engine, the scan tool may only display 2 or 3 cylinders misfiring. Inspect the ground circuit for the ignition coil on the cylinder bank of the engine that has more then one cylinder misfiring.
- Perform the following procedures to diagnosis conditions that may be causing the misfire counter to increment:
- Wetting down the secondary ignition system with water from a spray bottle may help locate damaged or deteriorated components. Look and listen for arcing or misfiring as you apply the water.
- If the Misfire Current counters are incrementing and there is no apparent misfire, an erratic CKP sensor signal could be the cause. Perform the diagnostic procedures for DTC P0335 first if this condition is suspected. See DTC P0335: CRANKSHAFT POSITION SENSOR CIRCUIT .
- If a misfire is present and you suspect a fuel control problem, force the fuel system into Open Loop using the scan tool and allow the engine to run for a few minutes. If this eliminates the misfire, refer to any fuel control related DTCs which are set. If no other DTCs are set, see DIAGNOSTIC TROUBLE CODE DEFINITIONS .
- A misfire may not be apparent at idle. The misfire may only occur above idle under a load. Road test the vehicle and monitor the misfire current counters.
- If more than one cylinder is misfiring, the misfire current counters may only increment for one cylinder. Example: Cylinders 1 and 8 are both misfiring, yet only cylinder 8 increments on the misfire current counter.
- If one of the injector fuses is open, only 2 or 3 misfire current counters may increment for the corresponding side of the engine.