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. 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) 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 ignition coil produces anywhere from 20,000-40,000 volts or more, which travels to the distributor assembly 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 PLUG HEAT RANGE
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. (In some instances, pre-ignition may result.) Pre-ignition 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.
Original equipment plugs are compromise plugs, but most people never have occasion to change their plugs from the factory recommended heat range. As a general rule of thumb: if most of your driving is long distance, high speed travel, you may need a colder plug; if most of your driving is stop and go, you may need a hotter plug.
REMOVAL & INSTALLATION
See Figures 3, 4, 5, 6, 7 and 8
A set of spark plugs usually requires replacement every 20,000-30,000 miles (32,180-48,270 Km) for most cars with electronic ignition systems, or every 15,000 miles (24,135 Km) on turbocharged vehicles with electronic ignition, depending on your style of driving. In normal operation, plug gap increases about 0.001 in. (0.0254mm) for every 1,000-2,500 miles (1,609-4,023 Km). As the gap increases, the plug's voltage requirement also increases. It requires a greater voltage to jump the wider gap and about two to three times as much voltage to fire a plug at high speeds than at idle. Therefore, it is extremely important to check and/or regap each spark plug between replacements. While the need for such maintenance will be affected by your driving conditions, as well as the condition of your vehicle's ignition and fuel systems, checking the condition of your vehicle's spark plugs between tune-ups is a good practice. For vehicles regularly subjected to severe service, such as extensive idling or frequent short trips of 10 miles (16 Km) or less, it is recommended that the spark plugs be checked and regapped every 6,000 miles (9,654 Km).
When you are removing spark plugs, you should work on one at a time. Do not start by removing the plug wires (cables) all at once, because unless you number them, they may become mixed up. Take a minute before you begin and number the wires with tape. The best location for numbering is near the ends of the wires.
Apply a small amount of silicone dielectric compound (Ford part no. D7AZ-19A331-A or the equivalent) to the inside of the terminal boots whenever an ignition wire is disconnected from the plug, or coil/distributor cap connection.
- Twist the spark plug boot, then remove the boot and wire from the plug. Do not pull on the wire itself as this will eventually ruin the wire.
- If possible, use a brush or rag to clean the area around the spark plug. Make sure that all the dirt is removed so that none will enter the cylinder after the plug is removed.
- Remove the spark plug using either a 5 / 8 in. or 13 / 16 in. socket, depending on the engine. Turn the socket counterclockwise to remove the plug. Be sure to hold the socket straight on the plug to avoid breaking it, or rounding off its hexagonal wrenching surface.
- Once the plug is out, check it against the spark plug diagnostic chart in this section to determine engine condition. This is crucial since plug readings are vital indicators of engine condition.
- Use a round wire feeler gauge to check the plug gap. The correct size gauge should pass through the electrode gap with a slight drag. If you are in doubt, try one size smaller and one larger. The smaller gauge should go through easily while the larger one should not go through at all. If the gap is incorrect, use the electrode bending tool on the end of the gauge to adjust the gap. When adjusting, always bend the side electrode. The center electrode is non-adjustable.
- Squirt a drop of penetrating oil on the threads of the new plug and install it. Do not oil the threads too heavily. Turn the plug in clockwise by hand until it is snug.
- When the plug is finger-tight, tighten it with a socket. Torque to 5-10 ft. lbs. (7-14 Nm) for 4-cyl. engines or 10-15 ft. lbs. (14-20 Nm) for 6 and 8-cyl. engines. Take care not to overtighten.
- Install the wire boot firmly over the plug. Proceed to the next plug.