GM Cutlass RWD 1970-1987 Repair Guide

Spark Plugs


Spark plugs ignite the air and fuel mixture in the cylinder as the piston reaches the top of the compression stroke. The controlled explosion that results forces the piston down, turning the crankshaft and the rest of the drive train.

The average life of a spark plug is 22,000-30,000 miles (35,400-48,300 km), on models equipped with HEI. Part of the reason for this extraordinarily long life is the exclusive use of unleaded fuel, which reduces the amount of deposits within the combustion chamber and on the spark plug electrodes themselves, compared with the deposits left by the leaded gasoline used in the past. An additional contribution to long life is made by the High Energy Ignition System (HEI), which fires the spark plugs with over 35,000 volts of electricity. The high voltage serves to keep the electrodes clean, and because it is a higher voltage blast of electricity than that produced by conventional breaker-point ignitions, the electrodes suffer less pitting and wear.

Nevertheless, the life of a spark plug is dependent on a number of factors, including the mechanical condition of the engine, driving conditions, and the driver's habits.

When you remove the plugs, check the condition of the electrodes; they are a good indicator of the internal state of the engine. Since the spark plug wires must be checked every 15,000 miles (24,100 km), the spark plugs can be removed and examined at the same time. This will allow you to keep an eye on the mechanical status of the engine.

A small deposit of light tan or rust-red material on a spark plug that has been used for any period of time is to be considered normal. Any other color, or abnormal amounts of wear or deposits, indicates that there is something amiss in the engine.

The gap between the center electrode and the side or ground electrode can be expected to increase not more than 0.001 in. (25.4 mm) every 1,000 miles (1609 km) under normal conditions.

When a spark plug is functioning normally or, more accurately, when the plug is installed in an engine that is functioning properly, the plugs can be taken out, cleaned, re-gapped, and reinstalled in the engine without doing the engine any harm.

When, and if, a plug fouls and begins to misfire, you will have to investigate, correct the cause of the fouling, and either clean or replace the plug.

Spark plugs suitable for use in you car's engine are offered in a number of different heat ranges. The amount of heat which the plug absorbs is determined by the length of the lower insulator. The longer the insulator, the hotter the plug will operate; the shorter the insulator, the cooler it will operate. A spark plug that absorbs (or retains) little heat and remains too cool will accumulate deposits of oil and carbon, because it is not hot enough to burn them off. This leads to fouling and consequent misfiring. A spark plug that absorbs too much heat will have no deposits, but the electrodes will burn away quickly and, in some cases pre- ignition may result. Pre-ignition occurs when the spark plug tips get so hot that they ignite the fuel/ air mixture before the actual spark fires. This premature ignition will usually cause a pinging sound under conditions of low speed and heavy load. In severe cases, the heat may become high enough to start the fuel/air mixture burning throughout the combustion chamber rather than just to the front of the plug. In this case, the resultant explosion (detonation) will be strong enough to damage pistons, rings, and valves.

In most cases the factory recommended heat range is correct; it is chosen to perform well under a wide range of operating conditions. However, if most of your driving is long distance, high speed travel, you may want to install a spark plug one step colder than standard. If most of your driving is of the short trip variety, when the engine may not always reach operating temperature, a hotter plug may help burn off the deposits normally accumulated under those conditions.

Spark plug coding using an AC-R45TSX as an example


See Figures 1, 2 and 3

  1. Number the wires with pieces of adhesive tape so that you won't cross them when you replace them.
  3. The spark plug boots have large grips to aid in removal. Grasp the wire by the rubber boot and twist the boot 1 / 2 turn in either direction to break the tight seal between the boot and the plug. Then twist and pull on the boot to remove the wire from the spark plug. Do not pull on the wire itself or you will damage the carbon cord conductor.
  5. Use a 5 / 8 in. spark plug socket to loosen all of the plugs about two turns. A universal joint installed at the socket end of the extension will ease the process.

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Fig. Fig. 1: A variety of tools and gauges are needed for spark plug service

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Fig. Fig. 2: Twist and pull on the rubber boot: never pull on the on the wire itself

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Fig. Fig. 3: Common spark plug removal

If removal of the plugs is difficult, apply a few drops of penetrating oil or silicone spray to the area around the base of the plug, and allow it a few minutes to work.

  1. If compressed air is available, apply it to the area around the spark plug holes. Otherwise, use a rag or a brush to clean the area. Be careful not to allow any foreign material to drop into the spark plug holes.
  3. Remove the plugs by unscrewing them the rest of the way.

INSPECTION See Figures 4, 5, 6 and 7

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%.

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Fig. Fig. 4: Stripped spark plug threads can be repaired using a threaded insert like the insert on the right

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Fig. Fig. 5: Always use a wire gauge to check the electrode gap

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Fig. Fig. 6: Adjust the electrode gap by bending the side electrode

Check the spark plug gap before installation. The ground electrode must be parallel to the center electrode and the specified size wire gauge should pass through the gap with a slight drag. Always check the gap on new plugs, too; they are not always correctly set 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. 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.

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Fig. Fig. 7: Used spark plugs can be inspected to help isolate malfunctions

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Fig. Fig. 8: Inspect the spark plug to help determine engine running condition


  1. Lubricate the threads of the spark plugs with a drop of oil or a shot of silicone spray. Install the plugs and tighten them hand tight. Take care not to cross-thread them.
  3. Tighten the spark plugs with a socket. Do not apply the same amount of force you would use for a bolt; just snug them in. These spark plugs do not use gaskets, and over-tightening will make future removal difficult. If a torque wrench is available, tighten to 7-15 ft.lbs. (9-20 Nm).

While over-tightening the spark plug is to be avoided, under-tightening is just as bad. If combustion gases leak past the threads, the spark plug will overheat and rapid electrode wear will result.

  1. Install the wires on their respective plugs. Make sure the wires are firmly connected. You will be able to feel them click into place. Spark plug wiring is shown in the firing order diagrams if you get into trouble.