Product Manual

TP-6905 7/18 61Section 4 Scheduled Maintenance
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, 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.
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 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 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 4-10 Engine Evaluation Using Spark Plug Condition