Misfires can be caused by lack of combustion in a cylinder due to absence of spark, poor fuel metering, poor compression, or many other causes. Even a small number of misfires may result in excessive exhaust emissions due to the unburned mixture. Increased misfire rates cause damage to the catalytic converter. The PCM monitors the crankshaft speed variation to determine if any misfiring generated. The PCM identifies the specific cylinder in which the misfire has occurred and counts individual misfire events by monitoring changes in the crankshaft rotation for each cylinder. A random misfire indicates two or more cylinders are misfiring.
If there happen misfires over the threshold value at 2 cylinders or more, PCM sets DTC P0300.
Item | Detecting Condition | Possible Cause |
DTC Strategy | ● Engine roughness through crankshaft speed fluctuation | ● Poor connection ● Ignition system ● Fuel system ● Intake/exhaust air system ● Ignition timing ● Injector ● ECM |
Enable Conditions | ● Engine speed 520∼4520rpm ● Engine load > Zero torq. ● Eng. load change < Max. ● Eng. speed change < Max. ● Time after engine start > 4.5 sec ● Intake air temperature > -9.8℃ | |
Case1 | Threshold Value | ● Misfire rate for emission thresh > 4% |
Diagnostic Time | ● 1000 revs | |
MIL ON Condition | ● 3 driving cycle | |
Case2 | Threshold Value | ● 3% < Misfire rate for catalyst damage < 15% |
Diagnostic Time | ● 200 revs | |
MIL ON Condition | ● Immediately (Blink) |
Items | Resistance |
Primary Coil | 0.87 Ω ± 10% at 20 ℃ (68℉) |
Secondary Coil | 13.0 kΩ ± 10% at 20 ℃ (68℉) |