Reasons for Carbon-fouled Spark Plugs

Written by jen davis
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Reasons for Carbon-fouled Spark Plugs
It's important to replace spark plugs periodically to ensure that your vehicle's engine stays in its best running condition. (old spark plugs image by Rog999 from

A vehicle's spark plugs deliver the spark that ignites the fuel-air mixture and allows the engine to run. If your spark plugs have become fouled, your car or truck may not start or run properly. Carbon emissions from your engine may be the reason the plugs are fouled. And if they are fouled, a black, fluffy substance can be stuck to them.

Engine Temperature

Carbon fouling occurs when the engine does not heat up enough so the carbon created by the firing process can be burnt off. Spark plugs are designed so that when they reach a certain temperature, approximately 427 degrees Celsius, according to NGK Spark Plugs, the carbon will burn off. If the spark plugs do not reach this temperature, the carbon will form deposits on the plugs. This commonly occurs in vehicles that are driven short distances in a brief time without warming up properly.

Rich Fuel Mixture

A rich fuel-air mixture creates more carbon as it burns. In some cases, the rich mixture will burn improperly or incompletely and excess carbon will build up on the plugs.

Weak Spark

If a spark plug is not working properly and does not generate enough of a spark to heat up properly, carbon deposits will develop. In addition, a weak spark will cause the engine to misfire or fail to run properly, which will cause additional carbon deposits to form and compound the problem.

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