Ford Mid-Size Cars 1971-1985 Repair Guide

Spark Plugs


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 90° 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 hundredths of a millimeter) is called the spark plug gap. The spark plug in no way produce a spark but merely provides a gap across which the current can arc. The coil produces 20,000-40,000 volts which travels to the distributor then through the spark plug wires to each individual spark plug. The current passes along the center electrode and jumps the gap to the side electrode, and, in do doing, ignites the air/fuel mixture in the combustion chamber.

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Fig. Fig. 1: A cross-section of a spark plug


See Figure 2

Spark plug heat range is a measure of a plug's ability 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, pre-ignition 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.

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Fig. Fig. 2: Spark plug heat range


See Figures 3 through 10

A set of spark plugs usually requires replacement after about 20,000-30,000 miles (32,000-48,000 km), depending on your style of driving (for more information, please refer to the Maintenance Interval Chart in Routine Maintenance ). In normal operation, plug gap increases about 0.001 in. (0.025mm) for every 1000-2500 miles (1600-4000 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.

When you're removing spark plugs, you should work on one at a time. Don't start by removing the plug wires 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 where the wires come out of the distributor cap. Refer to the engine "Firing Order'' illustrations if necessary.

On models equipped with electronic ignition, apply a small amount of silicone dielectric compound (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.

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Fig. Fig. 3: A variety of tools and gauges are needed for spark plug service

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Fig. Fig. 4: Remove the spark plug wire boot using a twisting motion

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Fig. Fig. 5: Using a suitable socket and ratchet, loosen the spark plug

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Fig. Fig. 6: Remove the spark plug from the engine

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Fig. Fig. 7: Checking the spark plug gap with a feeler gauge

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Fig. Fig. 8: Adjusting the spark plug gap

  1. Twist the spark plug boot and remove the boot and wire from the plug. Do not pull on the wire itself as this may ruin the wire. A special tool is available for this job if necessary. 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 no debris can enter the cylinder after the plug is removed. Remove the spark plug using the proper sized socket and rachet. Most vehicles use either a 5 / 8 in. or 13 / 16 in. size socket depending on the engine. Turn the socket counterclockwise to remove. Be sure to hold the socket straight on the plug to avoid breakage, or rounding off the hex. Once the plug is out, check it against the plugs shown in the section on spark plug condition to determine overall engine fitness. This is crucial since spark plug condition is a vital sign of engine condition.

Set the spark plug gap to the specification as shown on the Vehicle Emission Control Information Decal in the engine compartment.

  1. 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're in doubt, try one size smaller and one larger. The smaller gauge should go through easily while the larger one shouldn't 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 the gap, always bend the side electrode. The center electrode is non-adjustable. Apply a thin coat of anti-sieze® to the threads of the new plug and install it. Turn the plug in clockwise by hand until it is snug. Always start the spark plug threads by hand. Never use a wrench to start the spark plug in the cylinder head. When the plug is finger-tight, tighten it with a correct sized socket and rachet. Torque specifications should be 7-15 ft. lbs. (9-19 Nm) on 14mm thread spark plugs and 15-20 (19-26 Nm) on 18mm thread spark plugs. Note that the torque specification is critical on all spark plug applications. Install the plug wire firmly over the spark plug. Then, proceed to the next spark plug.

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Fig. Fig. 9: Spark plug condition chart

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Fig. Fig. 10: Spark plug electrode/insulator condition chart