See Figure 1
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 degrees angle so its tip is just past and parallel to the tip of the center electrode. The distance between these 2 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 20,000-50,000 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.
SPARK PLUG HEAT RANGE
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.
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, 4, 5, 6 and 7
A set of spark plugs usually requires replacement after about 20,000-30,000 miles (32,000-48,000 km), depending on your style of driving. 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 2 to 3 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).
All 1996 models were originally equipped with platinum-tip spark plugs which can be used for as-long-as 100,000 miles (161,000 km). This holds true unless internal engine wear or damage and/or improperly operating emissions controls cause plug fouling. If you suspect this, you may wish to remove and inspect the platinum plugs before the recommended mileage. Most platinum plugs should not be cleaned or regapped. If you find their condition unsuitable, they should be replaced.When removing the spark plugs, work on 1 at a time
. Don't start by removing the plug wires all at once because unless you number them, they're going to get mixed up. On some models though, it will be more convenient for you to remove all of the wires before you start to work on the plugs. If this is necessary, take a minute before you begin and number the wires with tape before you take them off. The time you spend here will pay off later.
- Disconnect the negative battery cable, and if the vehicle has been run recently, allow the engine to thoroughly cool. Attempting to remove plugs from a hot cylinder head could cause the plugs to seize and damage the threads in the cylinder head.
- Check for access to the plugs on your vehicle. The wheel wells of most vans covered by this information are designed to allow access to the sides of the engine. A rubber cover may be draped over the opening, and it may require removal of 1 or more plastic body snap-fasteners (which are carefully pryed loose using a special C-shaped tool) before you can move it aside for clearance. If this is your best access point, raise and support the vehicle safely using jackstands, then remove the front tire and wheel assemblies.
On some models, the engine cover may be removed to provide additional access to the spark plugs. This will be necessary if you also plan to check the spark plug wires at this time anyway.
- 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.
A spark plug wire removal tool is recommended as it will make removal easier and help prevent damage to the boot and wire assembly.
- Using compressed air (and SAFETY GLASSES), 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 plug removal 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.
- Using a spark plug socket (usually a 5 / 8 in. socket on these engines) 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.
INSPECTION & GAPPING
See Figures 8, 9, 10, 11, 12, 13, 14, 15, 17, 16, 18 and 19
If the specifications on the underhood tune-up sticker in the engine compartment disagree with the Tune-Up Specifications chart in this Section, the figures on the sticker must be used. The sticker often reflects changes made during the production run.
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%.
All 1996 models were originally equipped with platinum-tip spark plugs which can be used for as-long-as 100,000 miles (161,000 km). This holds true unless internal engine wear or damage and/or improperly operating emissions controls cause plug fouling. If you suspect this, you may wish to remove and inspect the platinum plugs before the recommended mileage. Most platinum plugs should not be cleaned or regapped. If you find their condition unsuitable, they should be replaced.
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 (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.
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. The correct gauge should pass through the electrode gap with a slight drag. If you're in doubt, try 1 size smaller and 1 larger. The smaller gauge should go through easily, while the larger 1 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.
- Inspect the spark plug boot for tears or damage. If a damaged boot is found, the spark plug wire must be replaced. As mentioned earlier, this is an excellent time to check each of the spark plug wires for proper resistance and/or for damage.
- Using a wire feeler gauge, check and adjust the spark plug gap. When using a gauge, the proper size should pass between the electrodes with a slight drag. The next larger size should not be able to pass while the next smaller size should pass freely.
- 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.
- Carefully tighten the spark plug. If 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 11 ft. lbs. (14 Nm) for all engines except 1996 models. On 1996 model engines, tighten the plug to 14 ft. lbs. (20 Nm) on used cylinder heads or to 22 ft. lbs. (30 Nm) on new cylinder heads.
- 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.