TP-6277 10/03 Scheduled Maintenance 25
3.6 Ignition System
3.6.1 Spark Plugs
Service spark plugs at the interval recommended in the
Service Schedule.
1. Remove the spark plug wires by grasping the boot
and turning the spark plug slightly while pulling. Do
not pull on wire.
Note: Pulling the wire rather than boot may
damage the wire or terminal.
2. Loosen the spark plug with a ratchet and 5/8 in.
spark plug s ocket with a rubber insert to prevent
spark plug damage. Where possible, use
compressed air to remove dirt from around each
spark plug to prevent dirt particles from falling into
the combustion chamber. Remove spark plugs
one at a time and examine. See Figure 3-17 to
evaluate engine condition by color/condition of
spark plugs.
3. Clean spark plugs by wiping them with a rag. File
the center electrode so that it is parallel to the side
electrode. Should replacement be necessary, see
Specifications for s park plug type.
Note: Do not sandblast, wire brush, scrape, or
otherwise service a spark plug in poor
condition. Obtain a new plug for best
results.
4. Before installing any spark plug, check the gap.
See Figure 3-18. The correct gap is attained when
the feeler (or wire) passes between the spark plug
electrodes. It should pass easily, but with some
resistance or drag; otherwise adjust as necessary.
The correct gap is 1.0 mm (0.040 in.).
Problem/Condition Means of Identification Possible Cause/Solution
Normal spark plug Light tan or gray deposit on the firing tip. Good operating condition and
maintenance.
Gap bridged spark plug Deposits built up and gap between
electrodes closing.
Oil or carbon fouling. Clean and regap
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 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, inoperative fuel
pump, overrich fuel mixture, or
excessive no load operation. Clean and
regap 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
spark plug.
Pre-ignition 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 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 spark plug.
Worn spark plug Severely eroded or worn electrodes. Caused by normal wear and failure to
replace spark plug at prescribed
interval. Replace plug.
Figure 3-17 Engine Evaluation Using Spark Plug Condition
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