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3.8 Ignition System
Service spark plugs at the interval specified in the
service schedule using the following procedure.
Spark Plug Service Procedure
1. Remove spark plug wires by grasping the spark
plug boot and turning slightly while pulling. Do not
pull the wire. Pulling on the wire rather than the
boot may damage the wire or terminal.
2. Loosen the spark plug with a ratchet and 5/8-in.
spark plug socket with a rubber insert to prevent
spark plug damage.
Problem/Condition
Gap-bridged spark plug
Oil-fouled spark plug
Carbon-fouled spark plug
Lead-fouled spark plug
Preignition damaged spark plug
Overheated spark plug
Worn spark plug
Figure 3-12 Engine Evaluation Using Spark Plug Condition
TP-5985 10/02
3. Use compressed air to remove dirt from around
4. Remove spark plugs, one at a time, and examine.
Means of Identification
Built-up deposits and gap between
electrodes closing.
Wet, black deposits on the insulator
shell, bore, and electrodes.
Black, dry, fluffy carbon deposits on
insulator tips, exposed shell surfaces
and electrodes.
Dark gray, black, yellow, or tan deposits;
or a glazed coating on the insulator tip.
Melted electrodes and possibly blistered
insulator. Metallic deposits on insulator
suggest internal engine damage.
White or light gray insulator with small
black or gray/brown spots with bluish
(burned) appearance on electrodes.
Severely eroded or worn electrodes.
each spark plug to prevent dirt particles from falling
into the combustion chamber.
Identify a normal spark plug in good operating
condition by observing a light tan or gray deposit on
the firing tip. See Figure 3-12 to evaluate engine
condition by color/condition of a problem spark
plug.
Possible Cause/Solution
Oil or carbon fouling. Clean and regap
the spark plug.
Excessive oil entering combustion
chamber through worn rings and
pistons, excessive clearance between
valve guides and stems, or worn or
loose bearings. Replace the spark plug.
Incorrect spark plug, weak ignition,
clogged air intake, defective fuel pump,
overrich fuel mixture, or excessive
no-load operation. Clean and regap the
spark plug.
Caused by highly leaded fuel. Replace
the spark plug.
Wrong type of fuel, incorrect timing or
advance, too hot a plug, burned valves,
or engine overheating. Replace the
spark plug.
Engine overheating, wrong type of fuel,
loose spark plugs, too hot a plug, low
fuel pump pressure or incorrect ignition
timing. Replace the spark plug.
Caused by normal wear and failure to
replace spark plug at prescribed
interval. Replace the spark plug.
Section 3 Scheduled Maintenance
19