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 90º 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) 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 to 40,000 volts which travels to the distributor where it is distributed through the spark plug wires to the spark plugs. 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 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 engine; the type of fuel; driving conditions; and the driver.
Spark plugs should be checked frequently (approximately 5,000 miles) depending on use. All the recommendations are based on the ambient conditions as well as driving conditions. If you drive at high speeds constantly, the plugs will probably not need as much attention to those used for constant stop-and-start driving.
The electrode end of the plug (the end with the threads) is a good indicator of the internal condition of your engine. If a spark plug has fouled and caused the engine to misfire, the problem will have to be found and corrected. Often, reading the spark plugs will lead you to the cause of the problem. Spark plug conditions and probable causes are shown in the accompanying photos. It is a good idea to pull the plugs once in a while just to get an idea of the internal condition of your engine.
A small amount of light tan colored deposits on the electrode end of the spark plug is quite normal. These plugs need not be replaced unless they are severely worn.
The gap between the center electrode and the side or ground electrode can be expected to increase not more than 0.001 in. every 1,000 miles 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.
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 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 in some instances, preignition 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 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 compromise plugs, but most people never have occasion to change their plugs from the factory-recommended heat range.
See Figures 3, 4 and 5
- Raise the hood and locate all the spark plugs.
- If the spark plug wires are not numbered, mark each one with a small piece of masking tape. Print the number of the cylinder on the piece of tape.
- Disconnect the wire from the plug by grasping, twisting and pulling the molded cap from the plug. Do not simply yank the wire from the plug as the connection inside the cap can become damaged.
- Using a spark plug socket, loosen the plug a few turns.
- If compressed air is available, blow out the area around the base of the spark plug to remove foreign matter.
- Remove the plug the rest of the way and inspect them. It is a good idea to inspect the plugs whether or not they are going to be reused.
See Figures 6 through 15
- Compare the condition of the spark plugs to the plugs shown in the accompanying photos. It should be remembered that any type of deposit will decrease the efficiency of the plug. If the plugs are not to be replaced, they should be thoroughly cleaned before installation. If the electrode ends of the plugs are worn or damaged and if they are to be reused, wipe off the porcelain insulator on each plug and check for cracks or breaks. If either condition exists, the plug must be replaced.
- If the plugs are judged reusable, have them cleaned on a plug cleaning machine (found in most service stations) or remove the deposits with a stiff wire brush.
- Check the plug gap on both new and used plugs before installing them in the engine. The ground electrode must be parallel to the center electrode and the specified size wire gauge should pass through the opening with a slight drag. If the center or ground electrode has worn unevenly, level them off with a file. If the air gap between the two electrodes is not correct, open or close the ground electrode, with the proper tool, to bring it to specifications. Such a tool is usually provided with a gap gauge.
- Coat the threads of new plugs with an anti-seize compound. Insert the plugs into the engine and tighten them finger-tightly.
- Be sure that the plugs are not crossthreaded. If the plugs use metal gaskets, new gaskets should be installed each time the plugs are removed and installed.
- Tighten the spark plugs to 9-13 ft. lbs. (Rotary Pick-Up) or 11-15 ft. lbs. (piston engine).
- Install the spark plug wires on their respective plugs. Be sure that each wire is firmly connected.
- While you are checking the spark plugs, the spark plug wires should also be checked. Any wires that are cracked or brittle should be replaced.