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 15,000 miles (24,000 km), although manufacturers are now claiming spark plug lives of up to 60,000 miles (96,000 km) or more with the new platinum tipped plugs; in fact, your car may be equipped with just such plugs. This is, however, dependent on a number of factors: the mechanical condition of the engine, the type of fuel, the driving conditions and the driver.
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 7,500 miles (12,000 km) or so to keep an eye on the mechanical state of the engine. A small deposit of light tan or gray material (or rust red with unleaded fuel) 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 inches (0.025mm) every 1,000 miles (1600 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.
There are many spark plugs suitable for use in your Toyota's engine and are offered in a number of 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 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, pre-ignition may result. Pre-ignition occurs when the spark plug tips get so hot that 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.
- Number the spark plug wires so that you won't cross them when you reinstall them.
- Remove the wires one at a time from the ends of the spark plugs 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 on the wire itself or you will damage the core.
- Use compressed air to blow away any sand or dirt in the area around the spark plug holes. Otherwise, this foreign material may drop into the spark plug holes and cause severe damage over time.
- Use a 16mm spark plug socket to loosen the plugs.
- Remove the plugs by unscrewing them the rest of the way from the engine.
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%.
Make sure to 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 when checking the gap with the specified size wire gauge (see "Tune-Up Specifications").
Always check the gap on new plugs, too; they are not always set to the gap for your car 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. 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.
- Lubricate the threads of the spark plugs with some anti-seizing compound. Install the plugs and tighten them hand tight. 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. Torque them to 11-15 ft. lbs. (16-20 Nm)
- Install the wires on their respective plugs. Make sure the wires are firmly connected. You should be able to feel them click into place.