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 one or more cylinders are misfiring.
If a misfire exists that exceeds the threshold value, P0300 is set.
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 | |
Enable Conditions | ● Time after engine start > 25sec ● No error on CKPS ● Eng. load change ≤ 41.2∼200 %/cycle ● Eng. speed change ≤2105∼4839 rpm/s ● No fuel-cut condition ● Not in hard driving condition ( >25sec ) | ||
Case1 | Enable Conditions | ● Engine speed 400∼6000 rpm | |
Threshold Value | ● Misfire rate > 2.5% per 1000 rev. | ||
Diagnostic Time | ● Continuous | ||
MIL ON Condition | ● 2 driving cycle | ||
Case2 | Threshold Value | ● Misfire rate > 2.5~22.2% per 200 rev. (Depends on driving conditions) | |
Diagnostic Time | ● Continuous | ||
MIL ON Condition | ● Immediately (Blink) | ||
Ignition Coil Resistance (Primary) |
0.71 ± 0.11 Ω at 20 ℃ (68℉) |
