Toyota Tercel 1984-1994 Repair Guide

Spark Plugs


See Figure 1

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 normal, spark plug (platinum plugs 60,000 mile change interval) is about 15,000-20,000 miles, although manufacturers are now claiming spark plug lives of up to 30,000 miles or more. This is, however, dependent on a number of factors: the mechanical condition of the engine, the type of fuel, the driving conditions and the driver.

Manufacturers are now required to certify that the spark plugs in their engines will meet emission specifications for 30,000 miles if all maintenance is performed properly. Certain types of plugs can be certified even beyond this point.

When you remove the spark plugs, check their condition. They are a good indicator of the condition of the engine. When a regular 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, gapped, and reinstalled without doing the engine any harm. When, and if, a spark plug fouls and being to misfire, you will have to investigate, correct the cause of the fouling, and either clean or replace the plug. There are several reasons why a spark plug will foul and you can learn which is at fault by just looking at the plug.

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Fig. Fig. 1: Spark plug heat range

Spark plugs suitable for use in your Toyota'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 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 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 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.


See Figures 2, 3, 4, 5, 6 and 7

  1. Number the spark plug wires (mark at the end of plug wire or boot) 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 the wire itself or you will damage the core.

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Fig. Fig. 2: Twist and pull the spark plug wire by its boot, not the wire

  1. Use the correct size spark plug socket (spark plug sockets come in two sizes 5 / 8 and 13 / 16 in. - use the correct size socket for the spark plug) to loosen all of the plugs about two turns.

On most engines the cylinder head is cast from aluminum. Remove the spark plugs when the engine is cold 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.

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Fig. Fig. 3: Removing the spark plug

Check the spark plugs for deposits and wear. If they are not going to be replaced, clean the plugs thoroughly (never clean platinum plugs - just replace the platinum plugs). Remember that any kind of deposit will decrease the efficiency of the plug. Regular spark 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 point 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: Regular spark plugs in good condition can be filed and reused

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Fig. Fig. 5: Adjust the spark plug gap by bending the side electrode

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Fig. Fig. 6: Always use a wire gauge to check the plug gap

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.

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Fig. Fig. 7: Be careful not to cross-thread when installing the spark plug

Always check the gap on new regular spark plugs, too; 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. Wire gapping tools usually have a bending tool attached, use that 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; it may weaken and break off within the engine, causing serious damage.

To install:
  1. Lubricate the threads of the spark plugs with a drop of oil. Install the plugs and tighten them hand tight (a long piece of vacuum hose attached to the top of the spark plug will help you start the spark plug by hand). Take care not to cross-thread them.
  3. Tighten the spark plugs with the correct size 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 13-15 ft. lbs. (18-20 Nm).
  5. Install the spark plug wires on their respective spark plugs. Make sure the spark plug wires are firmly connected. You will be able to feel them click into place.