1987-1993 Mercury Mariner Outboards 70/75/80/90/100/115HP 3 and 4-cylinder Factory Service Manual, Page 147Get this manual
Check Valves DESIGN (MODELS WITH CHECK VALVE ONLY) Located in crankcase cover under center main bearingsThese can be damaged by hot combustion blow-by into crankcaseTo Check: Look through holeif you see light, the nylon ball is badprobably melted; REPLACE valveIf you see no light, insert fine wire into check valve hole to see if there is slight movement of the nylon ball; if ball moves, valve is o.kotherwise, replace check valve
This kit is for use in three cylinder 71 cubic inch 70, 75, 80 and 90 horsepower models and four cylinder 105 cubic inch 100 and 115 horsepower models
NOTE: Three cylinder and four cylinder outboards with starting serial number OC123214 and above have these kits installed from factoryInstallation Instructions: 1Referring to appropriate service manual, remove powerhead from driveshaft housing2Remove all assemblies as required to obtain access to crankcase cover3Remove crankcase cover from cylinder block4Using 532 (3.9mm) punch, carefully tap bleed check valves out of crankcase cover(Tap check valve towards crankshaft side of crankcase cover