Spark Plug: Testing and Inspection

Spark Plug Diagnosis:

Worn or dirty spark plugs may provide satisfactory performance at idling speed, but under more demanding operating conditions, they will frequently misfire--poor performance symptoms can often be linked to misfiring spark plugs. These symptoms include poor fuel economy, power loss, loss of speed, hard starting, and generally poor engine performance. Spark plugs may misfire due to carbon fouling, excessive air gap, a broken insulator, bridged electrodes, or a damaged spark plug wire and/or boot.

Fouled spark plugs are indicated by BLACK carbon deposits on the electrodes. These BLACK deposits are usually the result of slow-speed driving and short runs in which sufficient engine operating temperature is seldom achieved. Worn pistons, rings, faulty ignition, over-rich air/fuel mixture, and the incorrect spark plug heat range will also result in carbon deposits.

Carbon deposits on the spark plug insulator tip may become conductive and cause the high voltage arc to track along the tip to some point where it arcs to join the spark plug shell. This condition prevents the high voltage from arcing and igniting the air/fuel mixture. Once arc tracking occurs, the spark plug may be permanently damaged and must be replaced.

Excessive electrode wear, on low-milage spark plugs, usually indicates that the engine is operating at speeds constantly higher than those for which the engine was designed or that the spark plug's heat range is too high. Electrode wear may also be the result of spark plug overheating caused by combustion gases leaking past the threads. Electrode wear can become excessive to the point that the high voltage no longer arcs across the electrodes.

Broken or cracked insulators are usually the result of improper installation. Broken upper insulators are usually the result of a poor fitting wrench or an outside blow. Although the cracked insulator may not be evident immediately, oil or moisture will soon penetrate the crack. The crack will then cause the high voltage to ground or arc through the insulator to the spark plug shell.

Broken lower insulators often result from improper gapping and are usually visable immediately. When gapping a spark plug, always make the gap adjustment by bending the ground side of the electrode. Spark plugs with broken insulators must always be replaced.

Damaged spark plugs are protected by an insulating boot comprised of heat resistant material which covers the spark plug terminal and extends downward over a portion of the spark plug insulator. These boots prevent flash-over that 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 a high-tension field, and has no effect on ignition performance. Usually it can be detected only in the darkness. This discharge may repel dust particles, leaving a clear ring on the insulator just above the shell. This ring is sometimes mistakenly regarded as evidence that combustion gases have blown out between the shell and the insulator.