See Figure 1
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 just past and parallel to the tip of the center electrode. The distance between these two electrodes (measured in thousandths of an inch or hundredths of a millimeter) is called the spark plug gap.
The spark plug does not produce a spark but instead provides a gap across which the current can arc. The coil produces anywhere from 20,000 to 50,000 volts (depending on the type and application) which travels through the wires to the spark plugs. The current passes along the center electrode and jumps the gap to the side electrode, and in doing so, ignites the air/fuel mixture in the combustion chamber.
SPARK PLUG HEAT RANGE
See Figure 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 closer the electrode is to the block's cooling passages) 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 might possibly lead to preignition or other ignition problems. Preignition takes place when plug tips get so hot that they glow sufficiently to ignite the air/fuel 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 generally a good compromise between the 2 styles and most people never have the need to change their plugs from the factory-recommended heat range.
REMOVAL & INSTALLATION
See Figures 3, 4 and 5
Remove the spark plugs and wires one at a time to avoid confusion and miswiring during installation.
- Before removing the spark plugs, number the plug wires so that the correct wire goes on the plug when replaced. This can be done with pieces of adhesive tape.
- Next, clean the area around the plugs by blowing with compressed air. You can also loosen the plugs a few turns and crank the engine to blow the dirt away.
- Disconnect the plugs wires by twisting and pulling on the rubber cap, not on the wire.
- Remove each plug with a rubber insert spark plug socket. make sure that the socket is all the way down on the plug to prevent it from slipping and cracking the porcelain insulator.
- After removing each plug, evaluate its condition. A spark plug's useful life is approximately 30,000 miles (48,000 Km). Thus, it would make sense to replace a plug if it has been in service that long.
- If the plugs are to be reused, file the center and side electrodes flat with a fine, flat point file. Heavy or baked on deposits can be carefully scraped off with a small knife blade, or the scraper tool of a combination spark plug tool. However, it is suggested that plugs be test and cleaned on a service station sandblasting machine. Check the gap between the electrodes with a round wire spark plug gapping gauge. Do not use a flat feeler gauge; it will give an inaccurate reading. If the gap is not as specified, use the bending tool on the spark plug gap gauge to bend the outside electrode. Be careful not to bend the electrode tool far or too often, because excessive bending may cause the electrode to break off and fall into the combustion chamber. This would require removing the cylinder head to reach the broken piece, and could also result in cylinder wall, piston ring, or valve damage.
- Clean the threads of the old plugs with a wire brush. Lubricate the threads with a drop of oil.
- Screw the plugs in finger tight, and then tighten them with the spark plug socket to 20 ft. lbs. (27 Nm). Be very careful not to over tighten them.
- Reinstall the wires. If, by chance, you have forgotten to number the plug wires, refer to the Firing Order illustrations.
INSPECTION & GAPPING
See Figures 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16 and 17
Check the plugs for deposits and wear. If they are not going to be replaced, clean the plugs thoroughly. Remember that any kind of deposit will decrease the efficiency of the plug. Plugs can be cleaned on a spark plug cleaning machine, which can sometimes be found in service stations, or you can do an acceptable job of cleaning with a stiff brush. If the plugs are cleaned, the electrodes must be filed flat. Use an ignition points file, not an emery board or the like, which will leave deposits. The electrodes must be filed perfectly flat with sharp edges; rounded edges reduce the spark plug voltage by as much as 50%.
Check spark plug gap before installation. The ground electrode (the L-shaped one connected to the body of the plug) must be parallel to the center electrode and the specified size wire gauge (please refer to the Tune-Up Specifications chart for details) must pass between the electrodes with a slight drag.
NEVER adjust the gap on a used platinum type spark plug.
Always check the gap on new plugs as they are not always set correctly at the factory. Do not use a flat feeler gauge when measuring the gap on a used plug, because the reading may be inaccurate. A wire type gapping tool is the best way to check the gap. Wire gapping tools usually have a bending tool attached. Use that to adjust the side electrode until the proper distance is obtained. Absolutely never attempt to bend the center electrode. Also, be careful not to bend the side electrode too far or too often as it may weaken and break off within the engine, requiring removal of the cylinder head to retrieve it.