Dodge Omni/Horizon/Rampage 1978-1989 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 drivetrain.

The average life of a spark plug is dependent on a number of factors:

Mechanical condition of the engine
Type of engine
Type of fuel
Driving conditions
The driver

When you remove the spark plugs, check their condition. They are a good indicator of the condition of the engine. It isa good idea to remove the spark plugs at regular intervals, such as every 2000-3000 miles (3218-4827 km), just so you can keep an eye on the mechanical state of your 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.

The gap between the center electrode and the side or ground electrode can be expected to increase not more than 0.001 in. (.025mm) 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, regapped, and reinstalled in the engine without doing the engine any harm.

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

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 and 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 or 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.


A set of spark plugs usually requires replacement after about 10,000 miles (16,090 km) on cars with conventional ignition systems and after about 20,000-30,000 miles (32,180-48,270 km) on cars with electronic ignition, depending on your style of driving. In normal operation, plug gap increases about 0.001 in. (0.025mm) for every 1000-2500 miles (1609-4023 km). 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.

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Fig. Fig. 2 An extension is useful for reaching the spark plug

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Fig. Fig. 3 Once the plug is removed, inspect it

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.

  1. Twist the spark plug boot and remove the boot and wire from the plug. Do not pull on the wire itself as this will ruin the wire.
  3. If possible, use a brush or rag to clean the area around the spark plug. Make sure that all the dirt is removed so that none will enter the cylinder after the plug is removed.
  5. Remove the spark plug using the proper size socket. Turn the socket counterclockwise to remove the plug. Be sure to hold the socket straight on the plug to avoid breaking the plug, or rounding off the hex on the plug.

To install:

  1. Use a wire feeler gauge to check the plug gap. The correct size 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. If the gap is incorrect, use the electrode bending tool on the end of the gauge to adjust the gap. When adjusting the gap, always bend the side electrode. The center electrode is non-adjustable.

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Fig. Fig. 4 Checking the spark plug gap with a feeler gauge

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Fig. Fig. 5 Adjusting the spark plug gap

  1. Squirt a drop of penetrating oil on the threads of the new plug and install it. Don't oil the threads too heavily. Turn the plug in clockwise by hand until it is snug.
  3. Tighten the plug with a wrench.
  5. Install the plug boot firmly over the plug. Proceed to the next plug.


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.

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Fig. Fig. 6 Used spark plugs which show damage may indicate engine problems

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

Check spark plug gap before installation. The ground electrode must be aligned with the center electrode and the specified size wire gauge should pass through the gap with a slight drag. If the electrodes are worn, it is possible to file them level.