The term “engine misfire” is used to describe incomplete combustion within the cylinder. Most often, a driver will feel a sharp jerking action from the engine or power train when a misfire is occurring. There are a number of causes of engine misfire, including a miss-timed combustion event. Here are the four main causes of misfire along with an explanation of the problem.
Ignition misfire is probably the most common reason an engine misfires and occurs when the spark plugs, distributor cap and rotor, ignition cables and ignition coil wear over a period of time. As these parts wear, their ability to transfer the ignition spark to ignite the fuel and air mixture in the combustion chamber becomes affected and lessens. In the beginning, the spark will only begin to weaken and the misfiring event may not even be noticed. Slowly, however, the parts will wear further and the misfire will become worse until the combustion process is completely stopped. This is what causes the sudden jerk in the engine’s normal operation.
Mechanical problems may also be to blame for an engine misfire. In this case, worn piston rings, cylinder walls, valves, lobes on a camshaft, a leaky head gasket or intake manifold gasket, broken rocker arms, problems with the fuel injectors or problems with the timing belt/timing chain can cause the engine to misfire. While ignition misfire sounds like a jerking or even a pop, mechanical misfire will have a thump to it. It’s also usually noticeable at any speed and will get worse the faster the car goes.
Still another cause of engine misfire is what’s referred to as a lean misfire, and this arises when there is an unbalanced fuel to air ratio. Because an engine needs a higher fuel mixture to run smoothly while idle, this type of engine misfire is most noticeable while the car is idling. This problem can also lessen or go away completely as the speeds increase because the efficiency of the flow into the combustion chambers will increase. Lean misfire may be due to an EGR valve that’s stuck open, a defective mass air flow sensor, a weak fuel pump, a leak in the intake manifold gasket or even a plugged fuel filter.
Lastly, power train misfire can cause problems, although the problem here has nothing to do with a real misfire. Power train misfire will cause a jerking engine but the problem is with the transmission and arises when it has difficulty shifting. When the misfiring seems to happen at high speeds, the problem may be with the overdrive gear or the lockup torque converter. If the car seems to jerk during deceleration, it may warped rotors, the brake drums or sticking brake pads or the brake shoes.
Obviously, engine misfire is a simple term that’s used to cover a wide array of vehicle problems. It may be a simple fix or something very complicated. That’s why it’s important to have your vehicle inspected by a professional to find out the real cause of the misfiring. Be sure to explain when the engine misfires and what it sounds like to speed up the diagnosis.
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