1998 Mercury 9.9/15HP 4-stroke outboards factory service manual, Page 252Get this manual
NON BIGFOOT GEAR HOUSING
Draining and Inspecting Gear Lubricant
WARNING
If gear housing is installed on outboard, disconnect (and isolate) spark plug leads from spark plugs before working near the propeller1Remove both VENT and FILL screws with gaskets2Inspect gear lubricant for metal particlesPresence of small amount of fine metal particles (resembling powder) indicates normal wearPresence of large particles (chips) or large quantity of fine particles indicates need for gear housing disassembly and component inspection3Note color of gear lubricantWhite or cream color indicates presence of water in lubricant
NOTE: When draining gear case for the first time, lubricant may appear cream colored due to the mixing of assembly lubricant and gear lubricantThis is NOT an indication of water intrusionIf, during subsequent draining of gear case, lubricant appears cream colored or milky, water may be presentGear case should be disassembled and all gaskets, seals and o-rings replacedInspect all components for water damage4Check drain pan for water separation from lubricantPresence of water in gear lubricant indicates the need for disassembly and inspection of oil seals, seal surfaces and gear housing componentsPressure check gear case prior to disassembly