Spark Plug: Testing and Inspection

Spark Plug Diagnosis:

SPARK PLUG FAILURE MODES
Worn or dirty spark plug assemblies may give satisfactory operation at idling speed, but under operating conditions they frequently fail. Faulty spark plug assemblies are indicated in a number of ways: poor fuel economy, power loss, loss of speed, hard starting and generally poor engine performance. Spark plug assemblies may also fail due to carbon fouling, excessive gap, or a broken spark plug assembly insulator. Descriptions of these failure modes follow:

Fouled Spark Plug
Fouled spark plug assemblies are identified 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 piston assemblies, compression upper rings, compression lower rings, oil ring rails, ring rail spacers, a faulty ignition, an over-rich fuel mixture and spark plug assemblies which are too cold will also result in carbon deposits.

Excessive Gap
Excessive gap wear on spark plug assemblies of low mileage usually indicates the engine is operating at high speeds or loads that are consistently greater than normal or that a spark plug assembly which is too hot is being used. Spark plug assembly electrode wear may also be the result of spark plug assembly overheating caused by combustion gases leaking past the threads due to insufficient spark plug assembly torque. An excessively lean fuel mixture will also result in excessive spark plug assembly electrode wear.

Broken Spark Plug Insulators
Broken spark plug assembly insulators are usually the result of improper installation or carelessness when regapping the spark plug assembly. Broken upper spark plug assembly insulators usually result from a poor fitting wrench or an outside impact. 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 the shell and may not be visible.

Broken Lower Spark Plug
Broken lower spark plug assembly insulators usually result from carelessness when regapping and generally are visible. This type of break may also result from the spark plug assembly operating too hot, which may happen in periods of high-speed operation or under heavy loads. When regapping a spark plug assembly, always make the gap adjustment by bending the ground (side) spark plug assembly electrode. Spark plug assemblies with broken insulators should always be replaced.