Theory Of Operation

The Powertrain Control Module (PCM) uses the Crankshaft (CKP) Sensor to detect and monitor the Crankshaft rotational speed. Normally, the Crankshaft rotational speed is fairly stable from cylinder to cylinder. When a misfire occurs in a cylinder, the Crankshaft speed changes abruptly for that cylinder. Therefore, anything that causes a sudden change in Crankshaft speed detected for a single cylinder is determined to be an engine misfire event. A misfire fault will usually occur due to poor fuel quality or metering, lack of spark, engine timing, low engine compression, engine mechanical or valvetrain issues, a vacuum or coolant leak entering the engine intake air system. Though not as common, other factors such as severe wheel balance or braking vibration, a worn serpentine belt or belt driven accessory binding should also be considered when diagnosing a misfire fault.

Depending on the engine involved, there can be several systems and components that can contribute to a misfire. In many cases, but not always,  there will be other DTCs set related to these systems. If that is the case, the focus should be on following the diagnostics for the system that has DTCs set against it. The different components or systems will affect misfire in different ways. Factors to consider when diagnosing a misfire are whether the misfire is a single cylinder or multiple cylinder misfire, and when does the misfire occur (idle, vehicle and engine speed, heavy engine load conditions, etc.). These factors can help in determining what components or systems should be focused on during diagnostics.