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Signs & Symptoms of Misfiring

by TJ Hinton

An engine misfire condition can be caused by a number of things, and manifest itself through a number of symptoms. Though misfires can be caused by simple problems that are easily fixed, they can also indicate of a serious problem in your engine. Address misfires in a prompt fashion, as the symptoms themselves can cause damage over time.

Sound

Depending on how good your muffler is, you may be able to hear a misfire in one or more cylinders. As the affected cylinders come up in the rotation, the decibel level of the exhaust note on the misfiring cylinder will drop, followed by a full-volume report as the next cylinder fires. This dip-and-spike in the exhaust note will be present regardless of the cause of the misfire. Popping and sneezing through the intake, especially when the engine's cold, means that you have a lean-running condition causing a lean misfire. Backfiring with a loud report is caused when an air-fuel charge passes through the combustion chamber and enters the exhaust system unburned. When the next cylinder fires, it detonates the charge in the system, making the loud report.

Smell

When a cylinder misfires, it will discharge an unburned, or partially burned, fuel-air charge into the exhaust stream. This will manifest itself as a heavy gas smell in the exhaust. The gas smell by itself will be present with most misfire conditions, but when accompanied by the smells of engine oil, or coolant and steam, then you likely have a serious problem such as a blown head gasket, cracked head or damaged rings and cylinder walls.

Power Pulses

The loss of power caused by a miss in one or more cylinders will be felt in the chassis. Much like the exhaust note, there is a loss of power on the dead cylinder, and a surge of power when the next good cylinder fires. This causes a vibration in the vehicle that varies with engine rpm. This particular symptom causes damage of its own. The power waxes and wanes as the missing cylinders come up in the rotation, causing the internal components in the engine and drivetrain to accelerate and decelerate, and increasing the wear on them.

Sight

Misses can display certain symptoms in the exhaust stream, depending on the cause of the miss. Misses being caused by carbon fouling due to a rich-running condition will have a puffy, black appearance. Diesel engines can display a puff of grey smoke from cylinders with leaky injectors or low compression. As with the symptoms you can smell, exhaust displaying a blue tinge or a steamy quality are indicative of a potentially serious internal problem.

About the Author

TJ Hinton trained as an auto mechanic at Mississippi Gulf Coast Community College and then later graduated from MMI as a certified motorcycle mechanic . He's also worked for 20+ years in home construction, remodeling and repair. His articles appear on InternetAutoGuide.com and TopSpeed.com.

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