Toyota Previa 1991-1997 Repair Information

Spark Plugs


See Figure 1

All Toyota Previas are equipped from the factory with platinum tipped plugs. Platinum tipped plugs should always be used on your vehicle, never be gapped or cleaned and always replaced at 60,000 mile (96,000km ) intervals.

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. Fig. 1: Cross-section of a spark plug


See Figure 2

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. Fig. 2: 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.


See Figures 3 through 7

A set of spark plugs on these models requires replacement at 100,000 miles (160,000 km). In normal operation plug gap increases about 0.001 in. (0.025mm) for every 2500 miles (4000 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 the plug at high speeds than at idle. The improved air/fuel ratio control of modern fuel injection combined with the higher voltage output of modern ignition systems will often allow an engine to run significantly longer on a set of standard spark plugs, but keep in mind that efficiency will drop as the gap widens (along with fuel economy and power).

When you're removing spark plugs, 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.

  1. If the vehicle has been run recently, allow the engine to thoroughly cool.
  3. Remove the RH seat and No. 2 cylinder head cover to access the spark plugs. To remove the seat perform the following:
    1. Remove the 3 screws and scuff plate.
    3. Remove the bolt and disconnect the RH seat belt from the front floor panel.
    5. Remove the 4 bolts and the RH front seat.
    7. Remove the 2 bolts and the RH front seat leg.
    9. Remove the 2 bolts and the jack holder.
    11. Remove the 9 bolts and the RH engine service cover.


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Fig. Fig. 3: Remove the 3 bolts retaining the No. 2 cylinder head cover

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Fig. Fig. 4: Pull the cover off and set aside

  1. Carefully twist the spark plug wire boot to loosen it, then pull upward and remove the boot from the plug. Be sure to pull on the boot and not on the wire, otherwise the connector located inside the boot may become separated.

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Fig. Fig. 5: Carefully twist and remove the spark plug wires

  1. Using compressed air, blow any water or debris from the spark plug well to assure that no harmful contaminants are allowed to enter the combustion chamber when the spark plug is removed. If compressed air is not available, use a rag or a brush to clean the area.

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. Using a spark plug socket (16mm) that is equipped with a rubber insert to properly hold the plug, turn the spark plug counterclockwise to loosen and remove the spark plug from the bore.

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Fig. Fig. 6: A deep socket is required to access these plugs

Be sure not to use a flexible extension on the socket. Use of a flexible extension may allow a shear force to be applied to the plug. A shear force could break the plug off in the cylinder head, leading to costly and frustrating repairs.

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Fig. Fig. 7: Check the condition of the spark plug. Replace as necessary

To install:
  1. Inspect the spark plug boot for tears or damage. If a damaged boot is found, the spark plug wire must be replaced.
  3. Carefully thread the plug into the bore by hand. If resistance is felt before the plug is almost completely threaded, back the plug out and begin threading again. In small, hard to reach areas, an old spark plug wire and boot could be used as a threading tool. The boot will hold the plug while you twist the end of the wire and the wire is supple enough to twist before it would allow the plug to crossthread.

Do not use the spark plug socket to thread the plugs. Always carefully thread the plug by hand or using an old plug wire to prevent the possibility of crossthreading and damaging the cylinder head bore.

  1. Carefully tighten the spark plug. the plug you are installing is equipped with a crush washer, seat the plug, then tighten about 1 / 4 turn to crush the washer. If you are installing a tapered seat plug, tighten the plug to 14 ft. lbs. (20 Nm).
  3. Apply a small amount of silicone dielectric compound to the end of the spark plug lead or inside the spark plug boot to prevent sticking, then install the boot to the spark plug and push until it clicks into place. The click may be felt or heard, then gently pull back on the boot to assure proper contact.
  5. Install and secure the No. 2 cylinder head cover and RH seat.
    1. Tighten the bolts to the following specifications:

      Service hole cover bolts-10 ft. lbs. (14 Nm)
      Jack holder-10 ft. lbs. (14 Nm)
      RH seat leg-29 ft. lbs. (39 Nm)
      RH seat-29 ft. lbs. (39 Nm)
      RH seat belt-to-floor pan-31 ft. lbs. (42 Nm)




See Figure 8

All Toyota Previas are equipped from the factory with platinum tipped plugs. Platinum tipped plugs should always be used on your vehicle and never be gapped or cleaned. Inspect the plugs for damage and deposits. It can give you a good idea as to how well your engine is running.

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