1998 Mercury 9.9/15HP 4-stroke outboards factory service manual, Page 258Get this manual
NON BIGFOOT GEAR HOUSING
Water Pump, Drive Shaft and Shift Shaft
1Remove four (4) bolts from pump cover
a-Bolts (4) 2Remove cover, nylon washers (above and below impeller), impeller and drive key from drive shaft3It is recommended that impeller be replaced whenever gear case is being serviced
NOTE: If impeller is not going to be replaced, DO NOT install impeller in reverse rotation to its original state as vanes have taken setVanes will crack and break shortly after outboard is returned to service