The spark plugs are platinum tipped, tapered seat and resistor type. There are no gaskets on the spark plugs. When replacing the spark plugs, use only the type specified. Refer to General Information.

Idling, city, and highway driving are considered normal spark plug service. Occasional higher speed driving is needed for good spark plug performance. Driving at higher speeds increases combustion heat which burns away carbon or oxides which have built up from regular idling and stop-and-go city driving.

Insulated boots protect the spark plugs from heat. The boots cover the spark plug terminals and extend down over a portion of the spark plug insulators. The boots prevent spark plug flashover, which causes engine misfiring.

The corona is a steady blue light that appears around the insulator just above the shell crimp and can usually be detected only in darkness. The corona is sometimes mistaken for crossfire or a shorted insulator. The corona has no effect on engine performance.

The discharge may repel dust particles that leave a clear ring on the insulator just above the shell. This ring is not evidence that combustion gases have blown out between the shell and the insulator.

Worn or dirty spark plugs may operate well at idle speeds, but frequently fail at higher RPM. Bad spark plugs are often responsible for the following conditions:

1. Power loss
2. Poor fuel economy
3. Loss of speed
4. Hard starting
5. Poor engine performance

Normal spark plug operation results in brown to grayish tan deposits on the area of the spark plug that enters the cylinder. A small amount of reddish brown, yellow, and white powdery residue may also be present on the insulator tip around the center electrode. These deposits are normal combustion byproducts of fuels and lubricating oils which contain additives.

Misfiring is a general term that applies to a poor running engine. With misfiring, the ignition spark is not igniting the fuel/air mixture at the proper time. While other possible causes must be investigated, the spark plugs should be inspected first. Spark voltage should not reach ground before jumping across the gap at the tip of the spark plug. This leaves the fuel/air mixture unburned, causing misfiring. Preignition misfiring occurs when the spark plug tip overheats, igniting the mixture before the spark jumps.

Carbon fouling of the spark plug is indicated by dry carbon deposits on the portion of the spark plug inside of the cylinder. Excessive idling and driving at slower speeds under light engine loads can keep the spark plug temperatures so low that these deposits are not burned off. Rich fuels or poor ignition system output may also cause carbon fouling.

Oil fouling of the spark plug appears as wet oily deposits on the portion of the spark plug inside of the cylinder. This may be caused by the following conditions:

Deposit fouling of the spark plug occurs when the normal reddish brown, yellow, or white deposits of combustion by-products become sufficient enough to cause misfiring. In some cases, these deposits melt and form a shiny glaze on the insulator around the center electrode. If the fouling is found only in one or two of the cylinders, valve stem clearances or the intake valve seals may be allowing excess lubricating oil to enter the cylinder, particularly if the deposits are heavier on the intake valve side of the spark plug.

Excessive gap means that the air space between the center and side electrodes at the bottom of the spark plug is too wide for consistent firing. This may be due to improper gap adjustment or to excessive wear of the electrodes during use. Refer to General Information for proper gap. A gap that is too small may cause idling instability. Excessive gap wear might indicate vehicle operation at continual high speeds or with high engine loads. this causes the spark plugs to run too hot. Excessively lean fuel may also cause the wear.

Improper torque or seating can cause a spark plug to run hot, eventually leading to excessive gap wear. In extreme cases, an overtightened or undertightened spark plug can cause exhaust blowby. The cylinder head seats must make good contact for sufficient heat transfer and spark plug cooling. Dirty or damaged threads in the head or on the spark plug can keep the spark plug from seating even though the proper torque is applied. Once the spark plugs are properly seated, tighten the spark plugs properly.

Cracked or broken insulators and insulator tips may be the result of improper installation or heat shock. Heat shock is a rapid increase in the insulator tip temperature which causes the insulator material to crack. The upper insulators can be broken when a poorly-fitting tool is used during servicing, or when the spark plug is hit from the outside. Cracks in the upper insulator may be inside the shell or invisible. The breakage may not cause problems until oil or water penetrates the crack later. Heat shock breakage in the lower insulator tip generally occurs during severe engine operating conditions such as higher RPM or heavy loading. Overadvanced timing or low grade fuels may also cause heat shock breakage. Spark plugs with broken or cracked insulators should always be replaced.

Damage during gapping can occur when the tool is pushed against the center electrode or the surrounding insulator, causing the insulator to crack. When gapping a spark plug, bend only the outside electrode. Keep tools free of any other parts.

Spark plugs with less than the recommended amount of service can sometimes be cleaned and regapped, then returned to service. If there is any doubt about the serviceability of a spark plug, replace it.