Jeep Wrangler/YJ 1987-1995 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 dependent on a number of factors; the mechanical condition of the engine; the type of fuel; driving conditions; and the driver.

When you remove the spark plugs, check their condition. They are a good indicator of engine condition.

A small deposit of light tan or gray material on a spark plug that has been used for any period of time is to be considered normal. Additives in unleaded fuels may give a number of unusual color indications; for instance, MMT (a manganese anti-knock compound) will cause rust red deposits.

The gap between the center electrode and the side or ground electrode can be expected to increase not more than 0.001 in. (0.025mm) every 1,000 miles 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, regapped, 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, then either clean or replace the plug.

There are several reasons why a spark plug will foul and you can learn which reason by just looking at the plug. The two most common problems are oil fouling and pre-ignition/detonation.

Oil fouling is easily noticed as dark, wet oily deposits on the plug's electrodes. Oil fouling is caused by internal engine problems, the most common of which are worn valve seals or guides and worn or damaged piston rings. These problems can be corrected only by engine repairs.

Pre-ignition or detonation problems are characterized by extensive burning and/or damage to the plug's electrodes. The problem is caused by incorrect ignition timing or faulty spark control. Check the timing and/or diagnose the spark control system.


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 or 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, preignition may result. Preignition 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.


See Figures 2, 3 and 4

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Fig. Fig. 2: Cross-section of a spark plug

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Fig. Fig. 3: When disconnecting the spark plug wires BE VERY careful not to touch the exhaust manifolds which are often still hot if the engine was run recently

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Fig. Fig. 4: Loosen, unthread and remove the spark plug using a socket and extension

  1. Remove the wires one at a time and number them so they won't become crossed during installation.
  3. Remove the wire from the end of the spark plug by grasping the wire by the rubber boot. If the boot sticks to the plug, remove it by twisting and pulling at the same time. Do not pull the wire itself or you will most certainly damage the core, or tear the connector.
  5. Use a spark plug socket to loosen all of the plugs about two turns.
  7. If compressed air is available, blow off 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.
  9. Remove the plugs by unscrewing them the rest of the way from the engine.


See Figures 5, 6, 7 and 8

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. 5: Checking the spark plug gap with a feeler gauge

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Fig. Fig. 6: Adjusting the spark plug gap

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Fig. Fig. 7: 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. 8: Inspect the spark plug to determine engine running conditions


  1. Insert the plugs in the spark plug hole and tighten them hand-tight. Take care not to crossthread them.
  3. Tighten the plugs to 26-30 ft. lbs. (35-41 Nm).
  5. Install the spark plug wires on their plugs. Make sure that each wire is firmly connected to each plug.


Visually inspect the spark plug cables for burns, cuts, or breaks in the insulation. Check the spark plug boots and nipples on the distributor cap and coil. Replace any damaged wiring. If no physical damage is obvious, the wires can be checked with an ohmmeter for excessive resistance. The resistance should be as follows:

Up to 15 inches-3000-10,000 ohms
15-25 inches-4000-15,000 ohms
25-35 inches-6000-20,000 ohms
Over 35 inches-8000-25,000 ohms

When installing a new set of spark plug cables, replace the cables one at a time so there will be no mix-up. Start by replacing the longest cable first. Install the boot firmly over the spark plug. Route the wire exactly the same as the original. Insert the nipple firmly into the tower on the distributor cap. Repeat the process for each cable.