Primary causes of engine cylinder misfires include loss of compression, an unbalanced air or fuel ratio, a loss of spark and a malfunctioning fuel injector. The misfire occurs as a result of a partial combustion process occurring inside one of the cylinders. When incomplete combustion occurs, the driver experiences a jerking action within the engine.
The ignition system is where most engine misfires develop. This system consists of the spark plugs, distributor cap, ignition cables, rotor and ignition coil. As the engine components experience wear over time, they lose the ability to transfer the spark required for combustion. As the wear increases, the engine misfires intensify. A lean misfire occurs as a result of an imbalanced air or fuel ratio. This misfire occurs when there is too much air or too little fuel. Causes of a lean misfire include an open EGR valve, a weak fuel pump, a leaking intake manifold gasket, a clogged fuel filter, a failing fuel pump and a malfunctioning mass airflow sensor. Causes of engine misfires can also be mechanical, and these types of misfires occur as a result of worn piston rings, defective fuel injectors, leaking head gaskets, broken rocker arms and worn valves and cylinder walls. A mechanical misfire is identified by a "thumping" feel inside the engine that is present regardless of engine speed.