See Figures 1 and 2
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 at the bottom. 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 degree 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 or in millimeters) 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 20,000-40,000 volts. This electric charge passes to the distributor which then sends the charge through each spark plug wire to the plug itself. The current passes along the center electrode and jumps the gap to the side electrode, and in so doing, ignites the air/fuel mixture in the combustion chamber.
Spark plug life and efficiency depend upon the condition of the engine and the temperature to which the plug is exposed. Combustion chamber temperatures are affected by many factors such as compression ratio of the engine, air/fuel mixtures, exhaust emission equipment and the type of driving you do. Spark plugs are designed and classified by number according to the heat range at which they will operate most efficiently.
See Figure 3
While spark plug heat range has always seemed to be somewhat of a mystical subject for many people, in reality the subject is quite simple. Basically, it boils down to this: the amount of heat the plug absorbs is determined by the length of the lower insulator. The longer the insulator (or the farther it extends into the engine), the hotter the plug will operate. A shorter insulator operates cooler.
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 (pinging) 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 speed operation or under heavy loads such as accelerating up a hill. In severe cases, the heat may become high enough to start the fuel/air mixture burning throughout the cylinder, rather than just in front of the plug as is normal. The burning mass is compressed and a premature explosion results, forcing the piston back down in the cylinder while it is still trying to go up. Obviously, this is not a healthy situation for the piston-quite frequently pistons are severely damaged. For this reason, severe or continuous preignition should never be ignored.
The general rule of thumb for choosing the correct spark plug heat range is that if most of your driving is long distance or high speed travel, use a colder plug; if most of your driving is stop and go,or low speed travel, use a hotter plug. Factory-installed plugs are, of course, a compromise, since the factory has no way of knowing what sort of driving you do. It should be noted that most people never have occasion to change their plugs from the factory-recommended heat range. If you wish to use an alternate plug, consult the emissions label under the hood of cars from 1978 to the present; it will give you the recommended standard plug and the recommended alternate (generally colder) plug description.
REMOVAL & INSTALLATION
See Figures 4, 5, 6, 7 and 8
Every six months or 6,000 miles (9662 km), the spark plugs should be removed for inspection on older cars. At this time they should be cleaned and regapped. At 12 month/12,000 mile (19,324 km) intervals on 1973-74 models and at 12 month/15,000 mile (24,155 km) intervals on 1975 and later models, the plugs should be replaced. Although Volvo recommends replacing the plugs at 30,000 miles (48,309 km) on 1984 and later vehicles, it should be noted that this is a maximum interval. For some owners, 30,000 miles (48,309 km) represents more than three years of driving. A prudent owner will remove and check or replace the plugs more frequently. Even if they are not heavily worn, a new set will improve performance.
Remove and install spark plugs only when the engine is cold; this is particularly important on engines with aluminum heads.
Remove each spark plug wire by grasping its metal or rubber boot at the end and twisting slightly to free the wire from the plug. Using a spark plug socket, turn the plugs counterclockwise to remove them. Do not allow any foreign matter to enter the cylinders through the spark plugs holes.
The gap must be checked with a wire gauge before installing the plug in the engine. With the ground electrode positioned parallel to the center electrode, the specified wire gauge must pass through the opening with a slight drag. If the air gap between the two electrodes is not correct, the ground electrode must be bent to bring it to specifications.
After the plugs are gapped correctly, they may be inserted into their holes and hand-tightened. Be careful not to crossthread the plugs. After each plug is hand threaded several turns, it may be tightened with the wrench: DO NOT OVERTIGHTEN SPARK PLUGS. Correct torque is 15-18 ft. lbs (20-24 Nm) except B28 and B280 engines which require only 9 ft. lbs. (12 Nm). Overtightening a spark plug can become a very costly problem if you damage the cylinder head. Install each spark plug wire on its respective plug, making sure that each spark plug end is making good metal-to-metal contact in its wire socket.
CHECKING AND REPLACING SPARK PLUG WIRES
See Figures 9 and 10
Visually inspect the spark plug wires for burns, cuts, or breaks in the insulation. Check the spark plug boots and the nipples on the distributor cap and coil. Replace any damaged wiring. If no physical damage is obvious, the wires can be checked with an ohmmeter for excessive resistance. Remove the distributor cap and leave the wires connected to the cap. Connect one lead of the ohmmeter to the corresponding electrode inside the cap and the other lead to the spark plug terminal (remove it from the spark plug for the test). Replace any wire which shows over 50,000 ohms. Generally speaking, resistance should not run over 35,000 ohms and 50,000 ohms should be considered the outer limits of acceptability.
Test the coil wire by connecting the ohmmeter between the center contact in the cap and either of the primary terminals at the coil. If the total resistance of the coil and wire is more than 25,000 ohms, remove the wire from the coil and check the resistance of the wire alone. If the resistance is higher than 15,000 ohms, replace the wire. It should be remembered that wire resistance is a function of length, and that the longer the wire, the greater the resistance. Thus, if the wires on your car are longer than the factory originals, resistance will be higher and quite possibly outside of these limits.
When installing a new set of spark plug wires, replace the wires one at a time so there will be no mix-up. Start by replacing the longest wire first. Install the boot firmly over the spark plug. Route the wire exactly the same as the original. Insert the nipple firmly into the tower on the distributor cap. Repeat the process for each wire.