Reasons Your Car’s Engine May Be Misfiring

By Lawrence Reaves

When one or more of the cylinders in your engine misfires, you’ll notice a hard idle, hard start, or hesitation during acceleration. In some cases, you’ll experience all three. While idling, your engine might shudder so violently that it shakes your entire steering column. While driving, you’ll notice a marked lag in performance. These problems will be more pronounced if you turn on your A/C or use any other accessories that add to the load placed upon your engine. The question is, what causes a misfire in the first place?

There are three primary reasons for a misfiring cylinder: insufficient spark, compression, or fuel. The challenge is to determine which factor is to blame.

Insufficient Spark For Ignition

Your engine undergoes a 4-stroke combustion process that results in the energy needed to turn your car’s crankshaft. Air and fuel travel into each cylinder. The intake and exhaust valves close, sealing the combustion chamber. A piston lies at the bottom, and begins to rise, compressing the air-fuel mixture. When the piston reaches the topmost point of its path, a spark plug ignites the mixture and causes a miniature explosion. Vapors expand within the chamber and push the piston downward. This rotates the crankshaft and powers your vehicle.

Note that the process described above requires the spark plug to ignite the compressed air-fuel blend. If this spark is unavailable, or too weak, the mixture cannot be ignited. This causes a misfire.

Insufficient spark can be traced to several possible root causes. For example, the plug may be worn, corroded, or damaged; the plug wire may also be worn, preventing voltage from reaching the spark plug; or the distributor cap may have become cracked or otherwise damaged. Each of these factors will prevent voltage from being delivered to the combustion chamber.

If you intend to fix the problem without the help of a mechanic, you’ll first need to identify which cylinder is misfiring. You can accomplish this by letting your car idle and unhooking the plug wires, one at a time. You’ll notice a change in the idle speed after disengaging each good cylinder. When you unhook a plug wire and notice no change, you’ll have found the misfiring cylinder. Check the plug and the plug wire. You’ll need an ohmmeter to check the plug wire’s resistance.

Compression Leaks In The Engine

Recall that during the 4-stroke combustion process, the air-fuel mixture is compressed by the rising piston. In order for this to occur properly, there should be very little compression leakage from the chamber (a small amount of leakage is normal). If a sizable leak has developed, the air-fuel mix cannot be sufficiently compressed for the ignition stroke.

Once you have identified the misfiring cylinder, perform a compression test (a manual tester costs less than $30 at most auto supply shops). If you discover a leak, the cause will likely be due to a bad exhaust valve. You’ll likely need to have the guide seals replaced. As a side note, if you detect a leak in two neighboring cylinders, the culprit is probably a failing head gasket.

Malfunctioning Fuel Injectors

If you have checked the spark plug, plug wire, distributor cap, and compression, and have been unable to find a problem, the misfire is likely due to insufficient fuel. The best place to begin troubleshooting is with your fuel injection system – specifically, the injector for the misfiring cylinder. Assuming enough fuel is reaching the injector, the most common issue is a blockage in the nozzle. Such blockages are often caused by deposits left behind when you turn off your engine.

Once you have ruled out ignition and compression as possible causes of the misfire, it’s best to have an experienced mechanic check the fuel system. If you can identify a clog in the injector, replace the unit. Otherwise, insufficient fuel could be caused by a failing oxygen sensor, malfunctioning powertrain control module (PCM), or a bad fuel pump. Your mechanic will have the necessary equipment for diagnosing which factor is causing the problem.

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