A typical 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 sent through the spark plug wire to the spark plug. The current passes along the center electrode and jumps to the gap to the side electrode, and, in so doing, ignites the air/fuel mixture in the combustion chamber.
SPARK PLUG HEAT RANGE
See Figure 1
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, preignition 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 a pinging during low speeds and heavy loads.
The general rule of thumb for selecting the correct heat range 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 or short trips, use a hotter plug. Original equipment plugs are a compromise, but most people never have occasion to change their plugs from the factory-recommended heat range.
REPLACING SPARK PLUGS
See Figures 2, 3, 4 and 5
A set of spark plugs usually requires replacement after about 30,000 miles, depending on your style of driving. In normal operation, plug gap increases about 0.001 in. for every 1,000-2,500 miles of use. As the gap increases, the plug's voltage requirement also increases. It requires a greater voltage to jump the wider gap and about two or 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; 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.
- Grasp the boot firmly, twist to loosen it on the plug and remove the wire from the plug. Do not pull on the wire itself as this will ruin the wire.
- If possible, use a brush or rag to clean the area around the spark plug. Make sure that all of 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. Make sure the socket is firmly seated on the hex faces of the plug. Turn the socket counterclockwise to remove the plug. Be sure to hold the socket straight on the plug to avoid breaking the plug.
- 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 you're in doubt, try one size smaller and one larger. The smaller gauge should go through easily while the larger one shouldn't go through at all.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; don't oil the threads too heavily. Install it by hand, turning the plug in clockwise until it is snug.
- When the plug is finger tight, tighten it with a wrench to 22 ft. lbs. DO NOT overtighten the plug. The threads in aluminum heads are easily damaged or stripped.
- Install the plug boot firmly over the plug. Proceed to the next plug.