Spark Plug: Testing and Inspection

WORN, CRACKED, OR DIRTY PLUGS
Worn, cracked or dirty plugs may give satisfactory operation at idling speed, but under operating conditions they frequently fail. Faulty plugs are indicated in a number of ways: poor fuel economy, power loss, loss of speed, hesitation, shudder, medium throttle intake manifold backfire, hard starting and general poor engine performance.

FOULED PLUGS
Fouled plugs may be indicated by black carbon deposits. The black deposits are usually the result of slow-speed driving and short runs where sufficient engine operating temperature is seldom reached. Worn pistons, rings, faulty ignition, over-rich fuel mixture or cold spark plugs may result in carbon deposits.

EXCESSIVE GAP
Excessive gap wear on plugs of low mileage, usually indicates the engine is operating at high speeds or loads that are consistently greater than normal or that a plug which is too hot is being used. Electrode wear may also be the result of plug overheating, caused by combustion gases leaking past the threads, due to insufficient torqueing of the spark plug. Excessively lean fuel mixture will also result in excessive electrode wear.

BROKEN INSULATORS
Broken insulators are usually the result of improper installation or carelessness when re-gapping the plug.

Lower Insulators
Broken upper insulators usually result from a poor fitting wrench or an outside blow. The cracked insulator may not show up right away, but will as soon as oil or moisture penetrates the crack. The crack is usually just below the crimped part of shell and may not be visible.

Upper Insulators
Broken lower insulators usually result from carelessness when re-gapping and generally are visible. This type of break may result from the plug operating too "Hot", which may happen in periods of high-speed operation or under heavy loads. When re-gapping a spark plug, always make the gap adjustment by bending the ground (side) electrode. Spark plugs with broken insulators should always be replaced.

CORONA DISCHARGE
Each spark plug boot covers the spark plug terminal and a portion of the plug insulator. These boots prevent flash-over which causes engine misfiring. Do not mistake corona discharge for flash-over or a shorted insulator. Corona is a steady blue light appearing around the insulator, just above the shell crimp. It is the visible evidence of high-tension field and has no effect on ignition performance. Usually it can be dust particles leaving a brown ring on the insulator just above the shell. This ring is sometimes mistakenly regarded as evidence that combustion gases have blown out between shell and insulator.