Mitsubishi Pick-ups and Montero 1983-1995 Repair Guide

Spark Plugs


See Figures 1, 2, 3 and 4

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 drivetrain.

The average life of a spark plug is about 30,000 miles (48,000 km). This is, however, dependent on a number of factors: the mechanical condition of the engine, the type of fuel used, the driving conditions and the driver.

When you remove the spark plugs, check their condition. They are a good indicator of the condition of the engine. 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. Any other color, or abnormal amounts of deposit, indicates that there may be something wrong in the engine.

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.

Click image to see an enlarged view

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

Some later model vehicles, may use a platinum type spark plug. If the vehicle has platinum type plug installed, these plugs are usually marked with circular blue lines and are not to be cleaned and regapped.

There are several reasons why a spark plug will foul and you can learn which is at fault by just looking at the plug.

Click image to see an enlarged view

Fig. Fig. 2: Inspect the spark plug to determine engine running conditions

Click image to see an enlarged view

Fig. Fig. 3: Used spark plugs which show damage may indicate engine problems

There are many spark plugs suitable for use in your engine and 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 lead, 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 air/fuel 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 air/fuel mixture burning throughout the combustion chamber rather than just to the front of the plug. In this case, the resultant explosion will be strong enough to damage pistons, rings, and valves.

Click image to see an enlarged view

Fig. Fig. 4: Spark plug heat range

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.


See Figures 5, 6 and 7

On some later models engines, removal of engine air intake plenum and other components will be necessary.

  1. Always keep track of the spark plug cable routing and plug wire bracket locations. Number the spark wires so that you won't cross them when you replace them.
  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 wire itself or you will damage the core.
  5. Use a spark plug socket or equivalent to loosen all of the plugs about two turns.

Remove the spark plugs when the engine is cold, if possible, to prevent damage to the threads. 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 from the engine.

Click image to see an enlarged view

Fig. Fig. 5: Twist and pull on the rubber boot to remove the spark plug wires. Never pull on the wire itself

Click image to see an enlarged view

Fig. Fig. 6: An extension can be useful in reaching the spark plugs

Click image to see an enlarged view

Fig. Fig. 7: Extract the old spark plug from the cylinder head


See Figures 8 and 9

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 (see Tune-Up Specifications chart).

Click image to see an enlarged view

Fig. Fig. 8: Check the spark plugs with a wire feeler gauge

Click image to see an enlarged view

Fig. Fig. 9: Bend the side electrode to adjust the gap

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, because the reading will be inaccurate. A wire type gapping tool is the correct way to check the gap. Wire gapping tools usually have a bending tool attached. Use it 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 as 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 by hand first. Take care not to cross-thread them.
  3. Tighten the spark plugs with spark plug 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 15 ft. lbs. (25 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. Check the spark plug cable routing and always make sure the plug wires are in the correct plug wire bracket.