A common spark plug consists of a metal shell surrounding a ceramic insulator. A metal 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 anywhere from 20,000 to 40,000 volts which travels to the distributor, where it is distributed through the spark plug wires to the spark plugs. The current passes along the center electrode and jumps the gap to the side electrode; in so doing, it ignites the air/fuel mixture in the combustion chamber.
See Figures 1 and 2
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 no deposits, but, due to the excessive heat, the electrodes will burn away quickly and in some instances, pre-ignition may result. Pre-ignition 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 a pinging during low speeds and heavy loads.
The general rule of thumb for selecting the correct heat range when choosing 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.
REMOVAL & INSTALLATION
See Figures 3, 4, 5, 6, 7 and 8
Spark plugs do not foul by themselves! Check for what has caused the plug to foul. Installing new spark plugs will not correct a fouling condition.
A set of spark plugs usually requires replacement after about 20,000-30,000 miles (32,180-48,270 km), depending on your style of driving. In normal operation, plug gap increases about 0.001 in. (0.025mm) for every 1,000-2,500 miles (1,690-4,022 km). 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.
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.
- Twist the spark plug boot to remove the boot and wire from the plug. Do not pull on the wire itself, as this will ruin the wire.
- If possible, use a brush to clean the area around the spark plug. Make sure all the dirt is removed so that none will enter the cylinder after the plug is removed.
- Remove the spark plug using the proper size socket: either a 5 / 8 in. (16mm) or 13 / 16 in. (21mm) size socket depending on the engine. Fully seat the socket straight on the plug to avoid breakage or rounding off of the hexagonal wrenching portion, then turn the socket counterclockwise.
- Once the plug is out, check it against the plugs shown in the spark plug condition chart to determine whether the plug is good or not. (This is crucial since plug readings are vital signs of engine condition.) A plug that is in good condition may be filed and re-used.
- Use a round wire feeler gauge to check the plug gap. The correct size gauge should pass through the electrode gap with a slight drag. If the gap is incorrect, use the electrode bending tool on the end of the gauge to adjust the gap. When adjusting the gap, always bend the side electrode; the center electrode is non-adjustable.
- Squirt a drop of penetrating oil on the threads of the new plug and install it. Don't oil the threads too heavily. Turn the plug in clockwise by hand until it is snug.
- When the plug is finger-tight, tighten it with a wrench. If you don't have a torque wrench, tighten the plug firmly until the crush gasket seats, but be careful not to strip the threads of the plug or cylinder head.
On some models equipped with electronic ignition, a small amount of silicone dielectric compound should be applied to the inside of the terminal boots whenever an ignition wire is disconnected from the spark plug or coil/distributor cap terminal.
- Install the plug boot firmly over the plug. Proceed to the next plug.