GM Caprice 1990-1993 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) 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 or more, 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.


See Figure 1

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Fig. Fig. 1: 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 deposits to, but due to the excessive heat the electrodes will burn them away quickly and in some instances, pre-ignition may result. Pre-ignition takes place when the combustion chamber temperature is to high and the air/fuel mixture explodes 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 your best choice, but most people never have occasion to change their plugs from the factory recommended heat range. It is first recommended that the cause of the fouled plug be diagnosed instead of temporarily fixing the problem by increasing or decreasing the spark plug heat range.


See Figures 2 and 3

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Fig. Fig. 2: Twist the boot and pull the wire off the spark plug; never pull on the wire itself

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Fig. Fig. 3: Spark plug removal

Spark plug replacement is recommended every 30,000 miles. 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're going to get mixed up. On some models though, it will be more convenient for you to remove all the wires before you start to work on the plugs. If this is necessary, take a minute before you begin and number the wires with tape before you take them off. The time you spend here will pay off later.

  1. Twist the spark plug boot and remove the boot from the plug. You may also use a plug wire removal tool designed especially for this purpose. Do not pull on the wire itself. When the wire has been removed, take a wire brush and clean the area around the plug. Make sure that all the grime is removed so that none will enter the cylinder after the plug has been removed.
  3. Remove the plug using the proper size socket, extensions, and universals as necessary.
  5. If removing the plug is difficult, apply some penetrating oil near the plug threads, allow it to work in, then remove the plug. Also, be sure that the socket is straight on the plug, especially on those hard to reach plugs.


See Figures 4, 5, 6 and 7

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Fig. Fig. 4: Used spark plugs in good condition may befiled and re-used

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Fig. Fig. 5: Adjust the plug gap by bending the side electrode

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Fig. Fig. 6: Spark plug gap may also be set using a wire gauge

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Fig. Fig. 7: Cross section of a spark plug

Check the plugs for deposits and wear. If they are not going to be replaced, clean the plugs thoroughly. Remember that any kind of deposit will decrease the efficiency of the plug. Plugs can be cleaned in a spark plug cleaning machine, which can sometimes be found in service stations, or you can do an acceptable job of cleaning with a stiff brush. If the plugs are cleaned, the electrodes must be filed flat. Use an ignition points file, not an emery board or the like, which will leave deposits. The electrodes must be filed perfectly flat with sharp edges; rounded edges reduce the spark plug voltage by as much as 50%.

Check spark plug gap before installation. The ground electrode (the L-shaped one connected to the body of the plug) must be parallel to the center electrode and the specified size wire/blade gauge (see Tune-Up Specifications) should pass through the gap with a slight drag. Always check the gap on new plugs, they are not always set correctly at the factory. Do not use a flat feeler gauge when measuring the gap, because the reading will be inaccurate.

Wire gapping tools usually have a bending tool attached. Use that to adjust the side electrode until the proper distance is obtained. Absolutely never bend the center electrode. Also, be careful not to bend the side electrode too far or too often; it may weaken and break off within the engine, requiring removal of the cylinder head to retrieve it.


  1. Lubricate the threads of the spark plugs with a drop of oil. Install the plugs and tighten them hand-tight. Take care not to cross-thread them.
  3. Tighten the spark plugs with the socket. Do not apply the same amount of force you would use for a bolt; just snug them in. If a torque wrench is available, tighten to 11 ft. lbs. (15 Nm).
  5. Install the wires on their respective plugs. Make sure the wires are firmly connected. You will be able to feel them click into place.