GM Full-Size Trucks 1980-1987 Repair Guide

Spark Plugs


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 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 ignition 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.

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Fig. Fig. 1: Spark plug cutaway

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Fig. Fig. 2: The 5/8 spark plug does not require a gasket. Sealing is accomplished by the tapered seat on the plug.

Rough idle, hard starting, frequent engine miss at high speeds and physical deterioration are all indications that the plugs should be replaced.

The electrode end of a spark plug is a good indicator of the internal condition of your engine. If a spark plug is fouled, causing the engine to misfire, the problem will have to be found and corrected. Often, reading the plugs will lead you to the cause of the problem.

A small amount of light tan or rust red colored deposits at the electrode end of the plug is normal. These plugs need not be renewed unless they are severely worn.


See Figure 3

Spark plug heat range is the ability of the plug to dissipate heat. The longer the insulator 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 causes a plug to foul and consequently, to misfire. 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 to use the same type as the original plugs but 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.

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Fig. Fig. 3: Spark plug heat range showing different electrode lengths. Colder plug (shorter electrode) is on left


See Figures 4, 5, 6 and 7

A set of spark plugs usually requires replacement after about 20,000 to 30,000 miles, depending on your style of driving. In normal operation, plug gap increases about 0.001 inch for every 1,000-2,500 miles. 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 cap.

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Fig. Fig. 4: These tools are commonly required to replacement of the spark plugs.

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Fig. Fig. 5: Pull on the spark plug boot, not on the wire

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Fig. Fig. 6: Gap the plug with a wire gauge; a flat gauge will give and inaccurate reading

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Fig. Fig. 7: Bend the side electrode to adjust the gap. Never bend the center


  1. Before removing the spark plugs, number the plug wires so that the correct wire goes on the plug when replaced. This can be done with pieces of adhesive tape.
  3. Next, clean the area around the plugs by brushing or blowing with compressed air.
  5. Disconnect the plug wires by twisting and pulling on the rubber cap, not on the wire. On High Energy Ignition (HEI) systems, twist the plug caps 1 /2 turn in either direction to break the seal before removing the wire. Never remove the wires from HEI systems when the engine is running. Severe shock could result.
  7. Remove each plug with a rubber insert spark plug socket. Make sure that the socket is all the way down on the plug to prevent it from slipping and cracking the porcelain insulator. On some V8s the plugs are more accessible from under the truck.
  9. After removing each plug, evaluate its condition as outlined in Inspection and Gapping, located in this section. If necessary, replace the plug with the type recommended by the factory.

The letter codes on the General Motors original equipment type plugs are read as follows:

R resistor
S extended tip
T tapered seat
X wide gap

  1. If reinstalling the oil plugs, clean the plug threads with a wire brush. Lubricate the threads with a drop of oil.
  3. Screw the plugs in finger tight, and then tighten them with the spark plug socket. Be very careful not to overtighten them. If a torque wrench is available, torque them to 15 ft. lbs. for plug designations with a T, 25 ft. lbs. for all the rest.
  5. Reinstall the wires. If, by chance, you have forgotten to number the plug wires, refer to the Firing Order illustrations in this Section.


See Figures 8, 9, 10 and 11

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 on 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 gauge (please refer to the Tune-Up Specifications chart for details) must pass between the electrodes with a slight drag.

NEVER adjust the gap on a used platinum type spark plug.

Always check the gap on new plugs as they are not always set correctly at the factory. Do not use a flat feeler gauge when measuring the gap on a used plug, because the reading may be inaccurate. A round-wire type gapping tool is the best way to check the gap. The correct 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. Wire gapping tools usually have a bending tool attached. Use that to adjust the side electrode until the proper distance is obtained. Absolutely never attempt to bend the center electrode. Also, be careful not to bend the side electrode too far or too often as it may weaken and break off within the engine, requiring removal of the cylinder head to retrieve it.

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Fig. Fig. 8: Checking the spark plug gap with a feeler gauge

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Fig. Fig. 9: Adjusting the spark plug gap

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Fig. Fig. 10: If the standard plug is in good condition, the electrode may be filed flat-WARNING: do not file platinum plugs

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Fig. Fig. 11: Inspect the spark plug to determine engine running conditions