Ford Probe 1989-1992 Repair Guide

Spark Plugs


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 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 do doing, ignites the air/fuel mixture in the combustion chamber.


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, 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.

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.1Cross section of a spark plug

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Fig. Fig. 2 Spark plug heat range


Remove the spark plugs one at a time. If you remove all the spark plug wires and spark plugs at the same time, the wires may get mixed up. Take a minute before you begin and number the wires with tape. The best location for numbering is near where the wires are attached to the cap. Refer to the Firing Order diagrams for the cylinder and corresponding distributor cap numbers.

  1. Grasp each wire by the rubber boot. Twist and pull the boot and wire from the spark plug. Never pull on the plug wire directly, or it may become separated from the connector inside the boot.
  3. Using a spark plug socket, loosen the plugs slightly by turning counterclockwise. Wipe or blow all dirt away from around the plug base.
  5. Unscrew and remove the spark plugs from the engine. Check the condition of the plugs using the Spark Plug Diagnosis chart.
  7. 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 the next size larger and next size smaller gauges. The smaller gauge should pass through easily while the larger gauge should not pass through at all. If the gap is incorrect, use the electrode bending tool on the gauge to adjust the gap. Only the side electrode is adjustable.

To install:

  1. Put a drop of penetrating oil on the base threads of the plug. Start the spark plug into the cylinder head by hand and turn until it is snug. Tighten the plug to 11-17 ft. lbs. (15-23 Nm) on the 2.2L engine or 7-15 ft. lbs. (9-20 Nm) on the 3.0L engine.
  3. Apply a small amount of silicone dielectric compound D7AZ-19A331-A or equivalent to the inside of the spark plug wire boot and connect the wire to the spark plug. If you did not number the wires, refer to the Firing Order diagrams to make sure the wires are connected properly.

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Fig. Fig. 3 Twist and pull on the rubber boot to remove the spark plug wire; never pull on the wire itself

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Fig. Fig. 4 Loosening the spark plugs using a ratchet, extension and spark plug socket

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Fig. Fig. 5 Removing the spark plug from the cylinder head

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Fig. Fig. 6 Always use a wire gauge to check the electrode gap; a flat feeler gauge may not give the proper reading Adjust the spark plug gap by bending the side electrode

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Fig. Fig. 7