Toyota Celica, Corolla, ECHO & MR2 1999-05

Spark Plugs


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 small distance. Located at the end of the plug and attached to the side of the outer metal shell is the side electrode. The side electrode bends in at a 90° angle so that its tip is just past and parallel to the tip of the center electrode. The distance between these two electrodes (measured in thousandths of an inch or hundredths of a millimeter) is called the spark plug gap.

The spark plug does not produce a spark but instead provides a gap across which the current can arc. The coil produces anywhere from 20,000 to 50,000 volts (depending on the type and application) which travels through the wires to the spark plugs. The current passes along the center electrode and jumps the gap to the side electrode, and in doing so, ignites the air/fuel mixture in the combustion chamber.

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Fig. Cross-section of a spark plug


  1. Inspect the terminal post for damage
  3. Inspect for a bent or broken terminal post
  5. Test for a loose terminal post by twisting and pulling the post. The terminal post should NOT move.
  7. Inspect the insulator for flashover or carbon tracking, soot. This is caused by the electrical charge traveling across the insulator between the terminal post and ground.
  9. Inspect for the following conditions:
  11. Inspect the spark plug boot for damage
  13. Inspect the spark plug recess area of the cylinder head for moisture, such as oil, coolant, or water. A spark plug boot that is saturated causes arcing to ground.
  15. Inspect the insulator for cracks. All or part of the electrical charge may arc through the crack instead of the electrodes
  17. Inspect for evidence of improper arcing
  19. Measure the gap between the center electrode and the side electrode terminals. An excessively wide electrode gap can prevent correct spark plug operation.
  21. Inspect for the correct spark plug torque. Insufficient torque can prevent correct spark plug operation. An over-torqued spark plug causes the insulator to crack.
  23. Inspect for signs of tracking that occurred near the insulator tip instead of the center electrode
  25. Inspect for a broken or worn side electrode
  27. Inspect for a broken, worn, or loose center electrode by shaking the spark plug.
  29. A rattling sound indicates internal damage
  31. A loose center electrode reduces the spark intensity.
  33. Inspect for bridged electrodes. Deposits on the electrodes reduce or eliminates the gap.
  35. Inspect for worn or missing platinum pads on the electrodes if equipped
  37. Inspect for excess fouling
  39. Inspect the spark plug recess area of the cylinder head for debris. Dirty or damaged threads can cause the spark plug not to seat correctly during installation
  41. Normal operation--Brown to grayish-tan with small amounts of white powdery deposits are normal combustion by-products from fuels with additives.
  43. Carbon fouled--Dry, fluffy, black carbon, or soot caused by the following conditions:

    Rich fuel mixtures
    Leaking fuel injectors
    Excessive fuel pressure
    Restricted air filter element
    Incorrect combustion
    Reduced ignition system voltage output
    Weak coils
    Worn ignition wires
    Incorrect spark plug gap

  45. Excessive idling or slow speeds under light loads can keep spark plug temperatures so low that normal combustion deposits may not burn off.
  47. Deposit fouling: Oil, coolant, or additives that include substances such as silicone, very white coating, reduces the spark intensity. Most powdery deposits will not effect spark intensity unless they form into a glazing over the electrode.

Inspection & Gapping

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Fig. Inspect the spark plug to determine engine running conditions

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

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Fig. Checking the spark plug gap with a feeler gauge

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Fig. Adjusting the spark plug gap

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Fig. If the standard plug is in good condition, the electrode may be filed flat-WARNING: do not file platinum plugs

  1. 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%.

  1. Check spark plug gap before installation. All Toyota spark plugs from the manufacture are already pregapped, but do NOT assume that the gap is correct, check them anyway prior to installation.
  3. 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.

  1. 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.
  3. 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.

Removal & Installation

Observe the following service precautions:

Allow the engine to cool before removing the spark plugs. Attempting to remove spark plugs from a hot engine can cause the spark plugs to seize. This can damage the cylinder head threads.
Clean the spark plug recess area before removing the spark plug. Failure to do so can result in engine damage due to dirt or foreign material entering the cylinder head, or in contamination of the cylinder head threads. Contaminated threads may prevent proper seating of the new spark plug.
Use only the spark plugs specified for use in the vehicle. Do not install spark plugs that are either hotter or colder than those specified for the vehicle. Installing spark plugs of another type can severely damage the engine.

  1. Turn the ignition OFF .
  3. If you are replacing the engine right bank (rear) spark plugs, rotate the engine for service access
  5. Remove the spark plug wires from the spark plugs.
  7. Remove the spark plugs from the engine.

To install:

It is important to check the gap of all new and reconditioned spark plugs before installation. Pre-set gaps may have changed during handling. Use a round wire feeler gauge to be sure of an accurate check, particularly on used plugs. Installing plugs with the wrong gap can cause poor engine performance and may even damage the engine. Gap the spark plugs to the specifications.

Be sure plug threads smoothly into cylinder head and is fully seated. Use a thread chaser if necessary to clean threads in cylinder head. Cross-threading or failing to fully seat spark plug can cause overheating of plug, exhaust blow-by, or thread damage. Follow the recommended torque specifications carefully. Over or under-tightening can also cause severe damage to engine or spark plug.

Use the correct fastener in the correct location. Replacement fasteners must be the correct part number for that application. Fasteners requiring replacement or fasteners requiring the use of thread locking compound or sealant are identified in the service procedure. Do not use paints, lubricants, or corrosion inhibitors on fasteners or fastener joint surfaces unless specified. These coatings affect fastener torque and joint clamping force and may damage the fastener. Use the correct tightening sequence and specifications when installing fasteners in order to avoid damage to parts and systems.

  1. Install the spark plugs.
  3. If the spark plugs are installed into a new cylinder head, tighten the spark plugs to 15 lb-ft (20 Nm). If the spark plugs are installed into an existing cylinder head, tighten the spark plugs to 11 lb-ft (15 Nm).
  5. Install the spark plug wires to the spark plugs.

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 closer the electrode is to the block's cooling passages) 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 might possibly lead to preignition or other ignition problems. Preignition takes place when plug tips get so hot that they glow sufficiently to ignite the air/fuel mixture before the actual spark occurs. This early ignition will usually cause a pinging during low speeds and heavy loads.

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Fig. Spark plug heat range

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 generally a good compromise between the 2 styles and most people never have the need to change their plugs from the factory-recommended heat range.