Page 35 - REPOWER REFERENCE GUIDE (2020)

Engine Failure Diagnosis

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

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Chipped Insulator
A chipped insulator usually results from careless plug regapping. Under certain conditions, severe detonation also can split the
insulator firing ends. There is also the special case of a broken insulator called a "rattler" that can occur from detonation,
over‑torquing, or dropping the spark plug. In any of these cases, check compression on all cylinders, and replace the spark
plugs.

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Wet Fouling (Oil Deposits)
The plug becomes shorted by excessive oil entering the combustion chamber, usually in the engine with many hours of
operation. Worn piston rings, cylinder walls, valve guides or valve stem seals are causes of oil entering the combustion
chamber. Only engine repairs will permanently relieve oil wet fouling.
IMPORTANT: New engines or recently overhauled engines may wet foul the plugs before normal oil control is achieved with
proper break‑in procedures. Such fouled plugs may be serviced (clean, file, and regap) and reinstalled.

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90-8M0149179 eng NOVEMBER 2018 © 2019 Mercury Marine Page 3A-9