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 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, and, in do 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 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 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.
REPLACING SPARK PLUGS
See Figures 1 through 8
A set of spark plugs usually requires replacement after about 20,000 to 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. 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.
REMOVAL & INSTALLATION
- If the spark plug wires are not numbered (by cylinder) place a piece of masking tape on each wire and number it.
- Grasp each wire by the rubber boot. Twist and pull the boot and wire from the spark plug. Never pull on the plug wire alone--you may damage the conductor inside.
- Use a spark plug socket, loosen the plugs slightly and wipe or blow all dirt away from around the plug base.
- Unscrew and remove the spark plugs from the engine. Clean, regap or replace as necessary. If you do not number the plug wires and get mixed up on their correct location, refer to the firing order illustrations.
- If, after removing and examining the spark plugs you feel that cleaning and regapping them is all that is necessary; use a stiff wire brush and clean all the carbon deposits from the electrodes and insulator or take the plugs to a service center and have them cleaned in a plug cleaning machine. New spark plugs come pre-gapped, however different models require different size gaps. Always check the gap and reset if necessary.
- To set the gap on new or cleaned spark plug, use a spark plug wire feeler gauge. The wire gauge should pass through the electrodes with just a slight drag. Use the electrode bending tool on the end of the gauge to adjust the gap. Never attempt to adjust the center electrode.
- Put a drop of oil on the base threads of the plug. Start the spark plug into the cylinder head by hand. Use a socket and tighten to no more than 15 ft. lbs.