See Figures 1 through 5
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 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 anywhere from 20,000 to 40,000 volts which travels to the distributor where it is distributed through the spark plug wires to the spark plugs. The current passes along the center electrode and jumps the gap to the side electrode, and, in do doing, ignites the air/fuel mixture in the combustion chamber. Resistor spark plugs are used in all engines and have resistance values of 6,000-20,000 ohms when checked with at least a 1000 volt tester.
Remove the spark plugs and examine them for burned electrodes and fouled, cracked or broken porcelain insulators. Keep the plugs arranged in the order in which they were removed from the engine. An isolated plug displaying an abnormal condition indicates that a problem exists in the corresponding cylinder. Replace the spark plugs with new ones every 30,000 miles (48,000 km).
Undamaged low mileage spark plugs can be cleaned and reused. After cleaning, file the center electrode flat with a small point file or a jewelers file. Adjust the gap between the electrodes to the following values based on engines:
Always tighten spark plugs to the specified torque. Over-tightening can cause distortion and change spark plug gap. Tighten the spark plugs to 20 ft. lbs. (28 Nm) for all engines except for the 3.0L in Premier and Monaco. Tighten the spark plugs in the Premier and Monaco models to 11 ft. lbs. (15 Nm).
On spark plugs with normal wear, the few deposits present will be probably light tan or slightly gray in color with most grades of commercial gasoline. There will not be evidence of electrode burning. Gap growth will not average more than approximately 0.001 in. (0.025mm) per 1000 miles (1600 km) or operation. Spark plugs that have normal wear can usually be cleaned, have the electrodes filed and regapped, and then reinstalled.
Some fuel refiners in several areas of the United States of America have introduced a manganese additive (MMT) for unleaded fuel. During combustion, fuel with MMT coats the entire tip of the spark plug with a rust color deposit. The rust color deposits could be misdiagnosed as being caused by coolant in the combustion chamber. MMT deposits do not affect spark plug performance.
SPARK PLUG HEAT RANGE
See Figures 6 and 7
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, 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.
Some later model vehicles may use Platinum tip type spark plugs. Use care not to damage the Platinum tip. The Platinum type spark plugs can usually be identify with 5 blue lines around the the spark plug porcelain insulator.
REMOVAL & INSTALLATION
See Figures 8, 9 and 10
Rough idle, hard starting, frequent engine miss at high speeds and physical deterioration are all indications that the plugs should be replaced.
The electrode end of a spark plug is a good indicator of the internal condition of your car's engine. If a spark plug is fouled, causing the engine to misfire, the problem will have to be found and corrected. Often, "reading'' the plugs will lead you to the cause of the problem.
A small amount of light tan or rust red colored deposits at the electrode end of the plug is normal. These plugs need not be renewed unless they are severely worn.
- Before removing the spark plugs, number the plug wires so that the correct wire goes on the plug when replaced. This can be done with pieces of adhesive tape.
- Next, clean the area around the plugs by brushing or blowing with compressed air.
- Disconnect the plug wires by twisting and pulling on the rubber cap, not on the wire.
- Remove each plug with a rubber-insert spark plug socket. Make sure that the socket is all the way down on the plug to prevent it from slipping and cracking the porcelain insulator.
- After removing each plug, evaluate its condition. A spark plug's useful life is approximately 30,000 miles with electronic ignition. Thus, it would make sense to replace a plug if it has been in service that long. If the plug is to be replaced, refer to the Tune-up Specifications chart for the proper spark plug type. The numbers indicate heat range; hotter running plugs have higher numbers.
- If the plugs are to be reused, file the center and side electrodes flat with a fine, flat point file. Heavy or baked on deposits can be carefully scraped off with a small knife blade or the scraper tool on a combination spark plug tool. It is often suggested that plugs be tested and cleaned on a service station sandblasting machine; however, this piece of equipment is becoming rare. Check the gap between the electrodes with a round wire spark plug gapping gauge. Do not use a flat feeler gauge; it will give an inaccurate reading. if the gap is not as specified, use the bending tool on the spark plug gap gauge to bend the outside electrode. Be careful not to bend the electrode too far or too often, because excessive bending may cause the electrode to break off and fall into the combustion chamber. This would require removing the cylinder head to reach the broken piece and could also result in cylinder wall, piston ring, or valve damage.
- Clean the threads of old plugs with a wire brush. Lubricate the threads with a drop of oil.
- Screw the plugs in finger tight, and then tighten them with the spark plug socket to 20 ft. lbs. (28 Nm) for all engines except for the 3.0L in the Premier and Monaco. For the Premier and Monaco, tighten the spark plugs to 11 ft. lbs. (15 Nm) of torque. Be very careful not to overtighten them.
- Reinstall the wires. If, by chance, you have forgotten to number the plug wires, refer to the firing order illustrations.