Spark plugs ignite the air and fuel mixture in the cylinder as the piston reaches the top of the compression stroke. The controlled explosion that results forces the piston down, turning the crankshaft and the rest of the drive train.
The average life of a spark plug is dependent on a number of factors: the mechanical condition of the engine, the type of fuel, the driving conditions and the driver. Although the standard factory plugs will last a considerable period of time, extended life may be gained in some engines by using platinum-tipped plugs. You must decide if the benefits outweigh the extra cost.
When you remove the spark plugs, check their condition. They are a good indicator of the condition of the engine. A small deposit of light tan or gray material (or rusty red with some fuels) on a spark plug that has been used for any period of time is to be considered normal. Any other color, or abnormal amounts of deposit, indicates that there is something amiss in the engine.
The gap between the center electrode and the side or ground electrode can be expected to increase very slightly under normal conditions.
When a spark plug is functioning normally or, more accurately, when the plug is installed in an engine that is functioning properly, the plugs can be taken out, cleaned, regapped, and reinstalled in the engine without doing the engine any harm. This is acceptable as an improvement or emergency measure, but new plugs are always recommended.
When, and if, a plug fouls and begins to misfire, you will have to investigate, correct the cause of the fouling, and either clean or replace the plug. Replacement is always recommended if possible.
Spark plugs suitable for use in your engine are offered in different heat ranges. The amount of heat which the plug absorbs is determined by the length of the lower insulator. The longer the insulator the hotter the plug will operate; the shorter the insulator, the cooler it will operate. A spark plug that absorbs little heat and remains too cool will accumulate deposits of lead, oil, and carbon, because it is not hot enough to burn them off. This leads to fouling and consequent misfiring.
A spark plug that absorbs too much heat will have no deposits, but the electrodes will burn away quickly and, in some cases, preignition may result. Preignition occurs when the spark plug tips get so hot, they ignite the air/fuel mixture before the actual spark fires. This premature ignition will usually cause a pinging sound under conditions of low speed and heavy load. In severe cases, the heat may become high enough to start the air/fuel mixture burning throughout the combustion chamber rather than just to the front of the plug. In this case, the resultant explosion will be strong enough to damage pistons, rings, and valves.
In most cases the factory recommended heat range is correct; it is chosen to perform well under a wide range of operating conditions. However, if most of your driving is long distance, high speed travel, you may want to install a spark plug one step colder than standard. If most of your driving is of the short trip variety, when the engine may not always reach operating temperature, a hotter plug may help burn off the deposits normally accumulated under those conditions.
See Figures 1, 2 and 3
Tag and number the spark plug wires so that you won't cross them during installation.
Remove the wire from the end of the spark plug by grasping the wire on the rubber boot. If the boot sticks to the plug, remove it by twisting and pulling at the same time. Do not pull wire itself or you will damage the core. On Dual Overhead Cam (DOHC) engines, the plugs are deeply recessed between the cams. The wires have long, solid tubes running through the tunnels to the plugs; use great care in removing the wires and tubes.
Fig. 1: Never pull on the wire; always grasp the boot of the spark plug wire
Use a spark plug socket to loosen all of the plugs about two turns. Depending on location of the plug, a short extension on the wrench can be very handy. Make certain to keep the line of the wrench exactly on the line of the plug; if the wrench is cocked to one side, the plug may break off.
Fig. 2: An extension is useful for reaching the spark plugs
The cylinder head is cast from aluminum alloy. Remove the spark plugs when the engine is cold, if possible, to prevent damage to the threads. If removal of the plugs is difficult, apply a few drops of penetrating oil or silicone spray to the area around the base of the plug, and allow it some time to work.
If compressed air is available, apply it to the area around the spark plug holes. Otherwise, use a rag or a brush to clean the area. Be careful not to allow any foreign material to drop into the spark plug holes.
Remove the plugs by unscrewing them the rest of the way from the engine. If the plug can be reached by hand, remove the plug manually. On DOHC engines, the plug must be unscrewed and lifted out with the plug wrench.
Fig. 3: Carefully withdraw the spark plug from the cylinder head after unscrewing it
See Figures 4, 5, 6 and 7
Check the plugs for deposits and wear. If they are not going to be replaced, clean the plugs thoroughly. This may be done with very fine sandpaper or a small flat file. Do not use a wire brush. 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. If the plugs are cleaned, the electrodes must be filed flat. Use an ignition point 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 percent.
Fig. 4: Used spark plugs which show damage may indicate engine problems
Check the 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 must fit in the gap with slight resistance. Correct gap for all Accords and Preludes covered by this information is 0.039-0.043 in. (1.0-1.1mm).
Fig. 5: Inspect the spark plug to determine engine running conditions
NEVER adjust the gap on a platinum-tipped spark plug.
Always check the gap on new plugs too. NEVER rely on so-called "pre-gapped plugs." Do not use a flat feeler gauge when measuring the gap; the reading will be inaccurate. Wire-type plug gapping tools usually have a bending tool attached. Use that to adjust the side electrode until the proper distance is obtained. Absolutely never bend the center electrode. Also, be careful not to bend the side electrode too far or too often. It may weaken and break off within the engine, requiring removal of the cylinder head to retrieve it.
Fig. 6: Check the spark plug gap with a feeler gauge
Fig. 7: Adjusting the plug gap
See Figure 8
On some 1990 and later models, a new type of plug with an extended tip and shorter terminal is used. When replacing these plugs, make certain the identical type of spark plug is used. Vehicles using this type of plug have small reminder emblems sealed onto the air cleaner cover.
Fig. 8: These marks indicate the use of a new design spark plug. Do not use conventional plugs in engines equipped with this type of spark plug
Lubricate the threads of the spark plugs with a drop of oil or anti-seize compound. Install the plugs by hand and tighten them just finger-tight. Never attempt to perform the initial installation with a wrench; take care not to cross-thread them. For DOHC engines, detach the ratchet driver; use your hand to turn the socket and extension very gently until the plug threads seat and the plug turns in.
Tighten the spark plugs with the socket (approximately a 1/2 turn). Do not apply the same amount of force you would use for a bolt; just snug them in. If a torque wrench is available, tighten them to 13 ft. lbs. (18 Nm).
Install the wires on their respective plugs. Make sure the wires are firmly connected. You will be able to feel them click into place. Additionally, make certain each wire is replaced into any clip or holder on its path. These wiring clips keep the spark plug wires from creating electrical interference in each other and also keep the engine looking neat.