TP-6390 1/06 35Section 3 Scheduled Maintenance
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.
3. Use compressed air to remove dirt from around
each spark plug to prevent dirt particles from falling
into the combustion chamber.
4. Remove spark plugs, one at a time, and examine.
Identify a normal spark plug in good operating
condition by observing a light tan or gray deposit on
the firing tip. See Figure 3-14 to evaluate engine
condition by color/condition of a problem spark
plug.
Problem/Condition Means of Identification Possible Cause/Solution
Gap-bridged spark plug Built-up deposits and gap between
electrodes closing.
Oil or carbon fouling. Clean and regap
the spark plug.
Oil-fouled spark plug Wet, black deposits on the insulator
shell, bore, and electrodes.
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.
Carbon-fouled spark plug Black, dry, fluffy carbon deposits on
insulator tips, exposed shell surfaces
and electrodes.
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.
Lead-fouled spark plug Dark gray, black, yellow, or tan deposits;
or a glazed coating on the insulator tip.
Caused by highly leaded fuel. Replace
the spark plug.
Preignition damaged spark plug Melted electrodes and possibly blistered
insulator. Metallic deposits on insulator
suggest internal engine damage.
Wrong type of fuel, incorrect timing or
advance, too hot a plug, burned valves,
or engine overheating. Replace the
spark plug.
Overheated spark plug White or light gray insulator with small
black or gray/brown spots with bluish
(burned) appearance on electrodes.
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.
Worn spark plug Severely eroded or worn electrodes. Caused by normal wear and failure to
replace spark plug at prescribed
interval. Replace the spark plug.
Figure 3-14 Engine Evaluation Using Spark Plug Condition
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