GM Cadillac 1967-1989 Repair Guide

Spark Plugs



See Figures 1, 2 and 3

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 short distance. Located at the end of the plug and attached to the side of the outer metal shell is the side electrode. This side electrode bends in at a 90 degree angle so its tip is even with, and parallel to, the tip of the center electrode. This distance between these two electrodes (measured in thousandths of an inch) is called spark plug gap. The spark plug in no way produces a spark but merely provides a gap across which the current can arc. The electronic ignition system produces approximately 50,000 volts, which travels to the distributor where it is distributed through the spark plug wires to the 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. All plugs used since 1969 have a resistor built into the center electrode to reduce interference to any nearby radio and television receivers. The resistor also cuts down on erosion of plug electrodes caused by excessively long sparking. Resistor spark plug wiring is original equipment on all models.

Spark plug life and efficiency depend upon the condition of the engine and the temperatures to which the plug is exposed. Combustion chamber temperatures are affected by many factors such as compression ratio of the engine, fuel/air mixtures, exhaust emission equipment, and the type of driving you do. Spark plugs are designed and classified by number according to the heat range at which they will operate most efficiently. The amount of heat that the plug absorbs is determined by the length of the lower insulator. The longer the insulator (it extends farther into the engine), the hotter the plug will operate; the shorter it is, the cooler it will operate. A plug that has a short path for heat transfer 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 has a long path for heat transfer 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 spark does. This early ignition will usually cause a pinging (sounding much like castanets) during low speeds and heavy loads. In severe cases, the heat may become enough to start the fuel/air mixture burning throughout the combustion chamber rather than just to the front of the plug as in normal operation. At this time, the piston is rising in the cylinder making its compression stroke. The burning mass is compressed and an explosion results, producing tremendous pressure. Something has to give, and it does - pistons are often damaged. Obviously, this detonation (explosion) is a destructive condition that can be avoided by installing a spark plug designed and specified for your particular engine.

A set of spark plugs usually requires replacement after 22,000 miles depending on the type of driving you do; this interval has been increased to 30,000 miles for 1977 and later models. The electrode on a new spark plug has a sharp edge but, with use, this edge becomes rounded by erosion causing the plug gap to increase. In normal operation, plug gap increases about 0.001 in. (0.025mm) in every 1,000-2,000 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 and acceleration than at idle. Remember that gapping a plug too close will produce a rough idle while gapping it too wide will increase its voltage requirement and cause missing at high speeds and during acceleration.

The higher voltage produced by the HEI (High Energy Ignition) coil is one of the primary reasons for the prolonged replacement interval for spark plugs in some 1974 models, and all 1975 and later cars covered in this guide. A consistently hotter spark prevents the fouling of plugs for much longer than could normally be expected; this spark is also able to jump across a larger gap more efficiently than a spark from a conventional system. However, even plugs used with HEI system wear after time in the engine.

Worn plugs become obvious during acceleration. Voltage requirement is greatest during acceleration and a plug with an enlarged gap may require more voltage than the coil is able to produce. As a result, the engine misses and sputters until acceleration is reduced. Reducing acceleration reduces the plug's voltage requirement and the engine runs smoother. Slow, city driving is hard on plugs. The long periods of idle experienced in traffic creates an overly rich gas mixture. The engine isn't running fast enough to completely burn the gas and, consequently, the plugs are fouled with gas deposits and engine idle becomes rough. In many cases, driving under the right conditions can effectively clean these fouled plugs.

Normal driving is assumed to be a mixture of idling, slow speed and high speed operation, with some of each making up the daily total driving. Occasional high speed driving is essential to good spark plug performance as the increased combustion heat burns away excess deposits of carbon and oxides that build up from frequent idling or stop-and-go driving.

Accelerate your car to the speed where the engine begins to miss and then slow down to the point where the engine smooths out. Run at this speed for a few minutes and then accelerate again to the point of engine miss. With each repetition this engine miss should occur at increasingly higher speeds and then disappear altogether. Do not attempt to short cut this procedure by hard acceleration; this will compound problems by fusing deposits into a hard permanent glaze.

Fouled spark plugs can also be caused by oil getting past the piston rings into the combustion chamber. A hotter plug may temporarily solve the problem, but in this case engine repair is necessary.

The type of driving you do may require a change in spark plug heat range. If the majority of your driving is done in the city and rarely at high speeds, plug fouling may necessitate changing to a plug with a heat range one number higher than that specified by the car manufacturer. For example, a 1980 Cadillac with 252 V6 engine requires an R45TSX plug. Frequent city driving may foul these plugs making engine operation rough. An R46TSX is the next hottest plug in the AC heat range (the higher the AC number, the hotter the plug) and its insulator is longer than the R45TSX so that it can absorb and retain more heat than the shorter plug. This hotter R46TSX burns off deposits even at low city speeds but would be too hot for prolonged turnpike driving. Using this plug at high speeds would create dangerous pre-ignition. On the other hand, if the aforementioned Cadillac were used almost exclusively for long distance, high speed driving, the specified R45TS might be too hot resulting in rapid electrode wear and dangerous pre-ignition. In this case, it might be wise to change to a colder R44TSX. If the car is used for abnormal driving (as in the examples above), or the engine has been modified for higher performance, then a change to a plug of a different heat range may be necessary. For a modified car it is always wise to go to a colder plug as a protection against pre-ignition. It will require more frequent plug cleaning, but destructive detonation during acceleration will be avoided.

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Fig. Fig. 1: Cross section of a spark plug

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Fig. Fig. 2: Spark plug heat range

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Fig. Fig. 3: Spark plug type number chart - using the R45TSX as an example


See Figures 4 and 5

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 on.

  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. Most of the spark plugs on the engines covered in this guide take a 5 / 8 in. plug socket, but some may take a 13 / 16 in. socket.
  5. If removing the plug is difficult, drip some penetrating oil on the plug threads, allow it to work, then remove the plug. Also, be sure that the socket is straight or square on the plug, especially on those hard to reach rear cylinder plugs.

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Fig. Fig. 4: Twist and pull on the rubber boot to remove the spark plug wires - never pull on the wire itself

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Fig. Fig. 5: Plugs still in good condition can be filed and reused


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 as 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 (see Tune-Up Specifications) should pass through the gap with a slight drag.

Never adjust the gap on a used platinum tipped spark plug.

Always check the gap on new plugs, too; they are not always set correctly at the factory. Do not use a flat feeler gauge when measuring the gap on used plugs, 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 attempt to 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.


See Figures 6, 7 and 8

  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 11-15 ft. lbs. (15-20 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.

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Fig. Fig. 6: A variety of tools and gauges are needed for spark plug service

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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