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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 3-17 Engine Evaluation Using Spark Plug Condition
5. Clean spark plugs by wiping them with a rag. File
the center electrode to keep it parallel to the side
electrode.
Note: Do not sandblast, wire brush, scrape, or
otherwise service spark plug in poor
condition.
results.
6. Check the spark plug gap before installing any
spark plug. See Figure 3-18. Attain a correct gap
when the feeler (or wire) passes between the spark
plug electrode. It should pass easily but with some
resistance or drag; otherwise adjust as necessary.
The correct gap is 0.76 mm (0.030 in.).
TP-6591 10/13
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.
Obtain a new plug for best
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.
1. 0.76 mm (0.030 in.) gap
2. Spark plug electrodes
Figure 3-18 Spark Plug Gap Inspection
7. Use a gapping tool to gently bend the side
electrode closer to or farther from the center
electrode to set the correct gap. See Figure 3-19.
Position the side electrode directly over the center
electrode.
Section 3 Scheduled Maintenance
1
2
1-514
35
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