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Spark Plug


Cold Fouling

This condition is the result of an excessively rich air/fuel mixture. It is characterized by a layer of dry, fluffy, black carbon deposits on the tip of the plug.

A cold- or carbon-fouled spark plug. Courtesy of Champion Spark Plug Company.

Cold fouling is caused by a rich air/fuel mixture or an ignition fault that causes the spark plug not to fire. If only one or two of the plugs show evidence of cold fouling, sticking valves are the likely cause. The plug can be used again, provided its electrodes are filed and the air gap is reset. Correct the cause of the problem before reinstalling or replacing the plugs.

If cold fouling is present on a vehicle that operates a great deal of the time at idle and low speeds, plug life can be lengthened by using hotter spark plugs.

Wet Fouling

When the tip of the plug is practically drowned in excess oil, this condition is known as wet fouling.

A wet- or oil-fouled spark plug. Courtesy of Champion Spark Plug Company.

In an overhead valve engine, the oil may be entering the combustion chamber by flowing past worn valve guides or valve guide seals. If the vehicle has an automatic transmission, a likely cause of wet-fouled plugs is a defective vacuum modulator that is allowing transmission fluid to enter the chamber. On high-mileage engines, check for worn rings or excessive cylinder wear. The best solution is to correct the problem and replace the plugs with the specified type.

Splash Fouling

This condition occurs immediately following an overdue tune-up. Deposits in the combustion chamber, accumulated over a period of time due to misfiring, suddenly loosen when the temperature in the chamber returns to normal. During high-speed driving, these deposits can stick to the hot insulator and electrode surfaces of the plug.

A splash-fouled spark plug. Courtesy of Champion Spark Plug Company.

These deposits can actually bridge across the gap, stopping the plug from sparking. Normally splash-fouled plugs can be cleaned and reused.

Gap Bridging

A plug with a bridged gap is not frequently seen in automobile engines.

There is no longer a gap on this spark plug. Courtesy of Champion Spark Plug Company.

It occurs when flying carbon deposits within the combustion chamber accumulate over a long period of stop-and-go driving. When the engine is suddenly placed under a hard load, the deposits melt and bridge the gap, causing misfire. This condition is best corrected by replacing the plug.


Under high-speed conditions, the combustion chamber deposits can form a shiny, yellow glaze over the insulator. When it gets hot enough, the glaze acts as an electrical conductor, causing the current to follow the deposits and short out the plug. Glazing can be prevented by avoiding sudden wide-open throttle acceleration after sustained periods of low-speed or idle operation. Because it is virtually impossible to remove glazed deposits, glazed plugs should be replaced.


This condition is characterized by white or light-gray blistering of the insulator. There may also be considerable electrode gap wear.

This spark plug shows signs of overheating. Courtesy of Champion Spark Plug Company.

Overheating can result from using too hot a plug, over-advanced ignition timing, detonation, a malfunction in the cooling system, an overly lean air/fuel mixture, using fuel too low in octane, an improperly installed plug, or a heat-riser valve that is stuck closed. Overheated plugs must be replaced.

Turbulance Burning

When turbulence burning occurs, the insulator on one side of the plugs wears away as the result of normal turbulence in the combustion chamber. As long as the plug life is normal, this condition is of little consequence. However, if the spark plug shows premature wear, overheating can be the problem.

Preignition Damage

Preignition damage is caused by excessive engine temperatures. Preignition damage is characterized by melting of the electrodes or chipping of the electrode tips.

A spark plug with preignition damage. Courtesy of Champion Spark Plug Company.

When this problem occurs, look for the general causes of engine overheating, including over-advanced ignition timing, a burned head gasket, and using fuel too low in octane. Other possibilities include loose plugs or using plugs of the improper heat range. Do not attempt to reuse plugs with preignition damage.