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 12,000 miles (19,000 km); 26,000-30,000 miles (42,000-48,000 km) in models with electronic ignition. This is, however, dependent on a number of factors:
When you remove the spark plugs, check their condition. They are a good indicator of the condition of the engine. It is a good idea to remove the spark plugs every 6000 miles (10,000 km) to keep an eye on the mechanical state of the engine.
A small deposit of light tan, gray or rust red material 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 not more than 0.001 in. (0.025mm) every 1000 miles (1100 km) 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.
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.
There are several reasons why a spark plug will foul and you can learn which is at fault by just looking at the plug.
Spark plugs suitable for use in your Subaru's 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 (or retains) little heat and remains to cool will accumulate deposits of 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 that they ignite the fuel/air 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 fuel/air 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.
REMOVAL & INSTALLATION
- Number the wires so that you won't cross them when you reinstall them.
- Remove the wire from the end of the spark plug by grasping the wire by the rubber boot. If the boot sticks to the plug, remove it by twisting and pulling at the same time. Do not pull the wire itself or you will damage the core.
- Use a 13 / 16 in. spark plug socket to loosen all of the plugs about two turns.
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 a few minutes 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.
- Lubricate the threads of the spark plugs with a drop of oil. Install the plugs and hand-tighten them. Take care not to cross-thread them.
- Tighten the spark plugs with the socket. 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 to 11-15 ft. lbs. (14-20 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.
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 reduced the spark plug voltage by as much as 50%.
Check spark plug gap before installation. The ground electrode must be parallel to the center electrode and the specified size wire gauge should pass through the gap with a slight drag. Always check the gap on new plugs, too. They are not always correctly set at the factory. Do not use a flat feeler gauge when measuring the gap, because the reading will be inaccurate. Wire 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.