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 degree angle so that its tip is even with, and parallel to, the tip of the center electrode. The distance between these two electrodes (measured in thousandths of an inch or millimeters) is called the spark plug gap. The spark plug in no way produces a spark but merely provides a gap across which the current can arc. The coil produces between 20,000 and 40,000 volts. This high voltage travels to the distributor where it is routed through the spark plug wires to the spark plugs. The current passes along the center electrode and jumps the gap to the side electrode; in so doing, it ignites the air/fuel mixture in the combustion chamber.
For the rotary engine used in the RX-7, each of the two rotors in the engine is equipped with both a leading and a trailing spark plug. The leading spark plug fires first, igniting the fuel/air mixture as in conventional engines; the trailing plug fires a moment later, igniting any unburned mixture. This aids in more complete combustion and helps to reduce exhaust emissions.
On 1986-89 models, the leading and trailing side spark plugs are different and must be installed in the proper position. Trailing side spark plugs are identified by blue lines on the top of the porcelain section of the plug.
The 1979-80 RX-7 uses a special three electrode spark plug for better ignition characteristics. The 1981-85 model uses a four electrode spark plug which, due to rotor housing modification, is the only type spark plug which can be used. On 1986-89 models, the spark plug incorporates a design that deletes the side electrodes and only uses the center electrode to produce a spark. Spark plug gap on these models is not adjustable. Use only the appropriate special type of spark plug for any RX-7 model.
See Figure 1
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 cold 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, pre-ignition may result. Pre-ignition 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, 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, and most people never have reason to change their plugs from the factory recommended heat range.
Some of the spark plugs listed in this section are especially designed and built for use in the Mazda rotary engine. Use only these plugs; do NOT substitute a different type of plug.
REMOVAL & INSTALLATION
A set of spark plugs usually requires replacement after about 10,000 miles (16,103 km) on cars with conventional ignition systems and after about 20,000-30,000 miles (32,206-48,309 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 1,000-2,500 miles (1,610-4,026 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.
Special platinum tipped spark plugs may be available for some of the vehicles covered by this information. These plugs wear at a greatly reduced rate when compared with conventional plugs. Refer to the plug manufacturers recommendations for service intervals, though keep in mind that periodic inspection is always a good idea.
On the RX-7, all of the ignition components for the leading spark plugs have darker colored caps on their wires than the trailing spark plug components. Even so, when replacing the spark plugs, it would be wise to remove and replace one plug at a time and reconnect its cable before moving on to the next plug. The spark plug holes can be identified by the letter and number codes adjacent to them. T1 means trailing spark plug, first rotor, L1 means leading spark plug, first rotor, etc.
Regardless of your type of vehicle and its ignition system, it is advisable to only work on one spark plug 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.Except RX-7
See Figures 2, 3, 4, 5, 6, 7, 8 and 9
- Grasp the spark plug wire by its boot and twist to remove the boot and wire from the plug. Do not pull on the wire itself, as this will ruin the wire.
- Remove the spark plug using the proper size deep-well socket. Turn the socket counterclockwise to remove the plug. Be sure the socket goes all the way onto the plug to avoid breaking the plug, or rounding off its hexagonal wrenching surface. If the spark plug is equipped with a crushable gasket, make sure that this gasket is removed with the plug.
- Once the plug is out, inspect it for abnormal wear and note any problems. Check the plug against the accompanying chart to determine engine condition. This is crucial since plug readings are vital signs of engine condition.
- Replace any plug that shows excessive wear or damage.
Unless any underlying problem is corrected, a replacement spark plug may also fail prematurely.
- Use a round 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, only bend the side electrode. The center electrode is non-adjustable.
- Squirt a drop of penetrating oil on the threads of the new plug and install it. Don't oil the threads too heavily. A suitable high temperature anti-seize compound may also be used. Turn the plug in clockwise by hand until it is snug, being careful not to cross-thread the plug. If you encounter resistance, remove the plug and try again.
- When the plug is finger-tight, use a deep-well socket to snug it. If the plug manufacturer provided a torque specification, use a torque wrench to tighten the plug. If no specification is available, tighten the plug carefully. An overtightened plug may distort the gap, or worse, may snap off in the cylinder head.
When tightening plugs which are equipped with crush washers, a good rule of thumb is to finger-tighten the plug until the washer comes in contact between the plug shoulder and the cylinder head. Then tighten the plug1/4turn to flatten the crush washer.
- Install the plug boot over the plug, so that the terminal inside the boot firmly engages the end of the plug. Proceed to the next spark plug.
See Figures 10, 11 and 12
- Remove the spark plug cable. Do not yank on the cable to remove it; firmly grasp the spark plug boot at the elbow and pull it straight off.
- 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.
If compressed air is available, use it to blow any dirt or debris away from the spark plug hole. Be sure to wear safety goggles when using compressed air.
- Using a deep-well socket wrench, remove the spark plug from the engine. If the spark plug is equipped with a crushable gasket, make sure that this gasket is removed with the plug.
- For 1979-85 models, clean the spark plugs with a wire brush or a spark plug cleaning machine and replace if any of the electrodes are burned away or badly eroded.
On 1986-89 RX-7 models, do not use a spark plug cleaning machine because sand particles may become trapped in the tip of the plug and damage the plug.
- Measure the electrode gap of each plug using a wire gauge (do not use a flat feeler gauge). On 1979-80 models, the gap for each of the three electrodes should be 0.037-0.041 in. (0.95-1.05mm). On 1981-85 models, the gap for each of the four electrodes should be 0.053-0.057 in. (1.35-1.45mm). If not, replace the plug. Do not attempt to adjust the electrode gap; you may crack and/or break the insulator or the electrodes. On 1986-89 models, measure the gap from the outer beveled edge of the plug to the edge of the center electrode. If the gap is not 0.08 in. (2.0mm), replace the plug.
- Apply a thread lubricant to the spark plug threads in order to prevent the plug from seizing in the rotor case.
- Position and start turning the spark plug by hand, being careful not to cross-thread the plug. If you encounter resistance, remove the plug and try again.
- When the plug is finger-tight, use a deep-well socket and tighten it to 9-13 ft. lbs. (12-17 Nm).
- Install the boot over the plug, so that the terminal inside the boot firmly engages the end of the plug. Proceed to the next spark plug.