Spark Plug Analysis; Normal Condition; Chipped Insulator; Wet Fouling (Oil Deposits) - Mercury MerCruiser MCM 454 Mag MPI Service Manual

TROUBLESHOOTING

Spark Plug Analysis

Normal Condition

Few deposits are present and probably will be light tan or gray in color. This plug shows that
plug heat range is compatible with engine, and engine is electrically and mechanically in
good running condition. With proper plug servicing (clean, file and re-gap), this plug can be
reinstalled with good results.

Chipped Insulator

Chipped insulator usually results from careless plug re-gapping. Under certain conditions,
severe detonation also can split insulator firing ends. Replace spark plugs.

Wet Fouling (Oil Deposits)

Plug becomes shorted by excessive oil entering combustion chamber, usually in engine with
many hours of operation. Worn piston rings, cylinder walls, valve guides or valve stem seals
are causes of oil entering combustion chamber. Only engine repairs will permanently relieve
oil wet fouling.
IMPORTANT: New engines or recently overhauled engines may wet foul plugs before
normal oil control is achieved with proper break-in procedures. Such fouled plugs
may be serviced (clean, file and re-gap) and reinstalled.
Index Index
Page 1C-4
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SERVICE MANUAL NUMBER 23
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90-861326--1 MARCH 1999