Spark Plug: Testing and Inspection

Spark Plug Diagnosis:

At idle speed worn or dirty spark plugs may provide satisfactory performance, but under more demanding operating conditions, they will frequently misfire. A number of symptoms can help detect misfiring spark plugs. These symptoms include poor fuel economy, power loss, loss of speed, hard starting and generally poor engine performance. Misfiring spark plugs may be caused by carbon fouling, an 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 electrodes. These black deposits are usually the result of slow-speed driving and short runs in which sufficient operating temperature is seldom achieved. Other causes of carbon deposits are worn pistons, rings, faulty ignition, over-rich air/fuel mixture and the incorrect spark plug heat range.

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 arc then ignites the air/fuel mixture later than normal which, in effect, retards ignition timing. Heavy carbon deposits may be conductive to the extent that the arc path now becomes a shunt path to 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-mileage spark plugs usually indicates that the engine is operating at speeds consistently higher than normal engine service 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 usually result from 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 short to ground or arc through the insulator to the spark plug shell.

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

Damaged spark plug wires and/or boots cause a similar condition to that of a cracked insulator. The high-voltage arc flashes through the wire or boot and grounds on the spark plug shell or the engine.

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 the flashover that causes engine misfiring.

Do not mistake corona discharge for flashover or a shorted insulator. Corona discharge is 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 system performance. This discharge may repel dust particles and leave a clear ring on the insulator just above the shell. This ring is sometimes mistaken as evidence that combustion gases have blown between the shell and the insulator.