A typical spark plug consists of a metal shell surrounding a ceramic insulator. A copper, silver, or platinum electrode extends downward through the center of the insulator and protrudes a small distance. Located at the end of the plug and attached to the side of the outer metal shell is the side electrode. The side electrode bends in at a 90 angle so that its tip is even with, and parallel to, the tip of the center electrode. The distance between these two electrodes (measured in thousandths of an inch) is called the spark plug gap.
The spark plug in no way produces a spark but merely provides a gap across which the current can arc. The coil produces 20,000-40,000 volts or more, which travels from the coils, through the spark plug wires to the spark plugs. The current passes along the center electrode and jumps the gap to the side electrode, and, in so doing, ignites the air/fuel mixture in the combustion chamber.
SPARK PLUG HEAT RANGE
Spark plug heat range is the ability of the plug to dissipate heat. The longer the insulator (or the farther it extends into the engine), the hotter the plug will operate; the shorter the insulator the cooler it will operate. A plug that absorbs little heat and remains too cool will quickly accumulate deposits of oil and carbon since it is not hot enough to burn them off. This leads to plug fouling and consequently to misfiring. A plug that absorbs too much heat will have deposits also, but due to the excessive heat, the electrodes will burn away quickly and in some instances, pre-ignition may result. Preignition takes place when plug tips get so hot that they glow sufficiently to ignite the fuel/air mixture before the actual spark occurs. This early ignition will usually cause pinging during low speeds and heavy loads.
The general rule of thumb for choosing the correct heat range when picking a spark plug is, if most of your driving is long distance, high speed travel, use a colder plug; if most of your driving is stop and go, use a hotter plug. Original equipment plugs are compromise plugs, but most people never have occasion to change their plugs from the factory recommended heat range. The best rule of thumb is to use the factory recommended spark plug.
A set of copper-tipped spark plugs usually requires replacement after about 30,000 miles (50,000km) on cars with electronic ignition, depending on your style of driving. Platinum-tipped spark plugs may last as much as twice as long copper-tipped plugs. In normal operation, plug gap increases about 0.001 inch (0.0254mm) for every 1,000-2,500 miles (1600-4000km). As the gap increases, the plug's voltage requirement also increases. It requires a greater voltage to jump the wider gap and about two to three times as much voltage to fire a plug at high speeds than at idle.
REMOVAL & INSTALLATION
To avoid engine damage, do not remove spark plugs when the engine is warm; the spark plug threads may be stripped if removed on a hot engine. When you're removing spark plugs, you should work on one at a time. Don't start by removing the plug wires all at once, because unless you number them, they may become mixed up. Take a minute before you begin and number the wires with tape. The best location for numbering is near where the wires come out of the cap.
- Disconnect the negative battery cable.
- Use a small prying tool to remove the heat shields on the rear cylinders.
- Remove the first spark plug cable by twisting the boot half a turn, then pulling up. Then remove the spark plug.
- All engines used AC Type R45lts6 spark plugs until an updated non-ridged spark plug was introduced in February, 1992, designated 41-600 (this number may change at any time). Properly gap to 0.060 inches (1.5mm) prior to installation.
- Lubricate the threads lightly with penetrating oil and install the spark plug. Torque to 20 ft. lbs. (27 Nm). Install the cable to the plug and snap it in place.
- When installing the heat shields, make sure they are seated against the bump stop and the lower tabs extend over the spark plug's hex.
- Repeat for the remaining spark plugs.
- Connect the negative battery cable.
CHECKING AND REPLACING SPARK PLUG CABLES
Your vehicle is equipped with an electronic ignition system which utilizes 8mm wires to conduct the hotter spark produced. The boots on these wires are designed to cover the spark plug cavities on the cylinder head. The coil assembly is connected directly to the spark plug with rubber connectors.
Visually inspect the spark plug cables for burns, cuts, or breaks in the insulation. Check the spark plug boots and the nipples on the coil pack. Replace any damaged wiring. If no physical damage is obvious, the wires can be checked with an ohmmeter for excessive resistance or an open. The resistance specification is 30,000 ohms or less. Always coat the terminals of any wire removed or replaced with a thin layer of dielectric compound.
When installing a new set of spark plug cables, replace the cables one at a time so there will be no mix-up. Start by replacing the longest cable first. Install the boot firmly over the spark plug. Route the wire exactly the same as the original, through all convolute tubing and clamped in all holders. Make sure the ends snap into place. Repeat the process for each cable.