1991 1999 ford explorer chilton User Manual

							CV-boots must be inspected periodically for damage
A torn boot should be replaced immediately
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. HOW TO USE THIS BOOK
INSPECTION 77 
						

							Cross-section of a spark plug
Click to enlarge
A variety of tools and gauges are needed for spark plug service
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 blocks 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 HOW TO USE THIS BOOK
78 Spark Plugs 
						

							
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.
Spark plug heat rangeClick to enlarge
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. HOW TO USE THIS BOOK
SPARK PLUG HEAT RANGE 79 
						

							
REMOVAL & INSTALLATION
Always twist and pull on the spark plug boot, never on the wire
Ford recommends replacing standard spark plugs every 30,000 miles (48,000km) and platinum plugs
every 60,000 miles (96,000km).
A set of spark plugs usually requires replacement after about 20,000-30,000 miles (32,000-48,000km),
depending on your style of driving. In normal operation plug gap increases about 0.001 in. (0.025mm) for
every 2500 miles (4000km). As the gap increases, the plugs 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 youre removing spark plugs, work on one at a time. Dont 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. Also, an anti-seize compound should be used before installing the plugs into the cylinder
head.
Disconnect the negative battery cable, and if the vehicle has been run recently, allow the engine to
thoroughly cool.
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.
2.  HOW TO USE THIS BOOK
80 REMOVAL & INSTALLATION 
						

							Remove the spark plug wire from the plug by twisting the boot and pulling outwards. Never pull
on the wire
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.
3. 
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.
Using a spark plug socket 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.
4. 
Using the proper size spark plug socket, loosen the plug by rotating it counterclockwise
Once the plug is loose, you should be able to remove it by hand. Compare the plug against the examples given
WARNING
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.
To install: HOW TO USE THIS BOOK
REMOVAL & INSTALLATION 81 
						

							Inspect the spark plug boot for tears or damage. If a damaged boot is found, the spark plug wire must
be replaced.
5. 
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.
6. 
Coat the spark plug threads with an anti-seize compound before installing it into the cylinder head. 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.
7. 
WARNING
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.
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 specifications provided by the vehicle or plug manufacturer.
8. 
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.
9. 
INSPECTION & GAPPING
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%.
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 youre in doubt, try one size smaller and one larger. The smaller gauge should go through easily,
while the larger one shouldnt 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. HOW TO USE THIS BOOK
82 INSPECTION & GAPPING 
						

							Checking the spark plug gap with a feeler gauge
Adjusting the spark plug gap
HOW TO USE THIS BOOK
INSPECTION & GAPPING 83 
						

							If the standard plug is in good condition, the electrode may be filed flat-WARNING: do not file
platinum plugs
HOW TO USE THIS BOOK
84 INSPECTION & GAPPING 
						

							Inspect the spark plug to determine engine running conditions
Click to enlarge
Spark Plug Wires
REMOVAL & INSTALLATION HOW TO USE THIS BOOK
Spark Plug Wires 85 
						

							Using a spark plug wire removal tool
Click to enlarge
When removing spark plug wires, use great care. Grasp and twist the insulator back and forth on the spark
plug to free the insulator. Do not pull on the wire directly as it may become separated from the connector
inside the insulator.
To install:
Whenever a high tension wire is removed for any reason form a spark plug, coil or distributor terminal
housing, silicone grease must be applied to the boot before it is reconnected. Using a small clean tool,
coat the entire interior surface of the boot with Ford silicone grease D7AZ 19A331-A or equivalent.
Install each wire in or on the proper terminal of the coil pack or distributor cap. Be sure the terminal
connector inside the insulator is fully seated. The No. 1 terminal is identified on the cap.
1. 
Remove wire separators from old wire set and install them on new set in approximately same
position.
2. 
Connect wires to proper spark plugs. Be certain all wires are fully seated on terminals.
3. 
TESTING
Only test one spark plug wire at a time. When the check is complete return the plug wire to its original
location. If the wire is defective and more wires are to be checked, mark the wire as such, return it to its
original location, then inspect the other wires. Once all of the wires are checked, replace the defective
wires one at a time. This will avoid any mix-ups.
With Distributor HOW TO USE THIS BOOK
86 REMOVAL & INSTALLATION