Follow the Scheduled Maintenance Service Recommendations, given in SECTION 0B, to assure satisfactory spark plug performance.

Normal spark plug operation will result in brown to grayish-tan deposits appearing on the portion of the plug that projects into the cylinder area. A small amount of red-brown, yellow, and white powdery material may also be present on the insulator tip around the center electrode. These deposits are normal combustion by-products of fuels and lubricating oils with additives. Some electrode wear will also occur.

Engines that are not running properly are often referred to as "misfiring." This means the ignition spark is not igniting the fuel/air mixture at the proper time. While other ignition and fuel system causes must also be considered, possible causes include spark plug conditions which allow the spark voltage to reach ground in some other manner than by jumping across the air gap at the tip of the plug, leaving the fuel/air mixture unburned. Misfiring may also occur when the tip of the spark plug becomes overheated and ignites the mixture before the spark jumps. This is referred to as "preignition." Refer to the appropriate DTC codes in Engine Controls.

Spark plug misfiring may be indicated in a number of ways : poor fuel economy, power loss, loss of speed, hard starting, and generally poor engine performance. Worn or dirty plugs may give satisfactory operation at idling speed, but will misfire under other operating conditions. Spark plugs may also misfire due to fouling, excessive gap, or a cracked or broken insulator. Should misfiring occur before the recommended replacement interval, locate and correct the cause.

Carbon fouling of the plug is indicated by dry, black carbon (soot) deposits on the portion of the plug in the cylinder. Excessive idling and slow speeds under light engine loads can keep the spark plug temperature so low that these deposits are not burned off. Over-rich fuel mixtures or poor ignition system output may also be the cause. Refer to diagnostic prosedures in Engine Controls.

Oil fouling of the plug is indicated by wet, oily deposits on the portion of the plug in the cylinder, usually with little electrode wear. This may be caused by oil getting past worn piston rings. This condition also may occur during break-in of new or newly overhauled engines.

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

Excessive gap means that the airspace between the center and side electrodes at the bottom of the plug is too wide for consistent plug firing. This may be due to improper gap adjustment or to excessive wear of the electrodes during use. A check of the gap size and comparison to the gap specified for the vehicle in SECTION 0B will tell if the gap is too wide. Excessive gap wear can be an indication of continuous operation at high speeds or with high engine loads, causing the plug to run too hot. Another possible cause is an excessively lean fuel mixture.

Low or high spark plug installation torque or improper seating of the plug can result in the plug running too hot and cause excessive gap wear. The plug and cylinder head seats must be in good contact for proper heat transfer and plug cooling. Dirty or damaged threads in the head or on the plug can keep the plug from seating even though the proper torque is applied. Once plugs are properly seated, tighten to torque shown under "Specifications" in this section. Low torque may result in poor contact of seats due to a loose plug. Over-tightening may cause the plug shell to be stretched and also may result in poor contact between the seats. In extreme cases, exhaust blow-by and damage beyond simple gap wear may occur.

Cracked or broken insulators may be the result of improper installation, damage during plug regapping, or heat shock to the insulator material. Upper insulators can be broken when a poorly fitting tool is used during installation or removal, or when the plug is hit from the outside. Cracks in the upper insulator may be inside the shell and not visible. Also, the breakage may not cause problems until oil or moisture penetrates the crack later.

A broken or cracked lower insulator tip (around the center electrode) may result from damage during regapping or from "heat shock" (plug suddenly operating too hot).

Damage during regapping can happen if the gapping tool is pushed against the center electrode or the insulator around it, causing the insulator to crack. When regapping a plug, make the adjustment by only bending the side electrode, keeping the tool clear of other parts.

"Heat shock" breakage in the lower insulator tip generally occurs during severe engine operating conditions (high speeds or heavy loading) and may be caused by over-advanced timing or low-grade fuels. Heat shock refers to a rapid increase in the tip temperature that causes the insulator material to crack.

Spark plugs with less than the recommended amount of service can sometimes be cleaned and regapped, the returned to service. However, if there is any doubt about the serviceability of a plug, replace it. Plugs with cracked or broken insulators should always be replaced.