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What Does the Term Misfire Mean?

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Generally, the term misfire refers to an incomplete combustion process inside the cylinder. When this becomes severe enough, the driver will feel a jerking action from the engine and/or power train. Often the owner will bring the vehicle into a shop complaining that "the timing is off." This is partially correct because a misfire does involve a mis-timed combustion event. However, the base ignition timing being out of adjustment is only one reason for a misfire to occur—and not the most likely. 

Here are some of the most common misfire scenarios.

Ignition Misfire
An ignition misfire is one of the most common reasons for an engine to misfire. As the spark plugs, ignition cables, distributor cap and rotor, and ignition coil wear over time, their ability to transfer the needed spark to ignite the air/fuel mixture inside the combustion chambers becomes compromised. In the early stages, the spark will only be weaker and the actual misfire will be subtle. As the ignition components continue to wear, the misfire will intensify and the combustion process can be interrupted completely. This will cause a severe jerk or shock in the operation of the engine (the engine can even backfire through the air intake system).

Any ignition misfire will cause raw, poisonous hydrocarbons to be released into the atmosphere. This is why all 1996 and later gasoline-powered vehicles have a dedicated "misfire monitoring system" built into their emission control systems. Raw hydrocarbons can also destroy a catalytic converter, so 1996 and later vehicles will flash the Check Engine Light to alert the driver of a misfire condition.

Lean Misfire
The lean misfire is another common reason for an engine "miss"—this is due to an imbalanced air/fuel ratio (too much air). Since an engine needs a richer (more fuel) mixture for a smooth idle, this problem is more noticeable when the vehicle is idling. The lean misfire may decrease or disappear as the engine speed increases because the efficiency of the volumetric flow into the combustion chambers increases dramatically. This is one reason why a vehicle gets better mileage on the freeway than in the city. An EGR valve that is stuck open, a leaking intake manifold gasket, a defective mass air flow sensor, a weak or failing fuel pump, or a plugged fuel filter are some of the many causes for a lean misfire.

Mechanical Misfire
Mechanical problems can also cause an engine to misfire. Common causes of a mechanical misfire are—worn piston rings, cylinder walls, or lobes on a camshaft, a leaking head gasket or intake manifold gasket, damaged or broken rocker arms, defective fuel injectors (and/or the electronics that control them), and a slipped or incorrectly-installed timing belt or timing chain. Generally, this type of misfire has more of a regular thumping-type of feel to it. It is usually noticeable regardless of engine speed; in fact, it may even intensify as the engine speed increases.

Sometimes a vehicle may misfire or run poorly and it has nothing to do with the operation of the engine. One common cause for "jerky" performance from the power train is a problem in the transmission and its ability to properly shift. If the misfire is at higher speeds, it could be a problem with the operation of the overdrive gear or a chattering clutch in the lockup torque converter. Make sure you have the vehicle properly inspected for the misfire. Entire engines have been replaced to solve a problem that was actually rooted in the transmission—much to the chagrin of the owner and the repair shop.

 

Daniel Dillon has 22 years of experience as a licensed Smog Technician in California. He helped write test questions for the California Smog Technician Exam, and has performed Consumer Assistance Program and gold shield diagnostic work for the state. He also was an instructor for SnapOn Tool Corporation.