How to Change Out the Impeller on Marine Outboards
The impeller on marine engines serves the same purpose as the water pump on an automobile. Although somewhat different in design and location, the impeller functions as the main cooling device for the outboard. The rubber blades on the impeller act as a rotor confined in a smooth, round bore, which forces outside cooling water through the jackets and passages of the engine. Impellers reside in the lower unit of most outboard motors, and can be easily replaced with some simple tools.
Place the motor in the full tilt-down position and disconnect the negative battery cable with a socket. Use an Allen wrench to remove the lower gear case oil plug, and drain the oil into a pan. Place the shift selector in "forward" gear. Refer to your owner's manual for the location of your lower unit mounting bolts. Some bolts will be in the side case, and some will be underneath. Use a socket or Allen wrench to loosen and remove the bolts, depending upon your fastener types.
Wedge the upper and lower case halves apart with a screwdriver. Set the motor stand under the lower unit as you drop it slightly. Check your motor manual for the type of transmission linkage. To remove the interior coupler type, place a socket through the seam of the case halves and unscrew the coupler bolt that holds the linkage together. If you have a side case plate, unscrew the plate with a screwdriver and disconnect the coupler bolt with a socket.
Drop the lower unit down into the motor stand and pull it away from the boat, with an assistant's help. See if the copper water tube came down, attached to the lower unit grommet. If it did, place the tube back up in the upper case grommet hole, twisting it in firmly.
Remove the upper O-ring from its groove on the driveshaft. Look for any other seal or O-ring further down on the shaft and pull that off. Both the O-ring and lower seal should come with the kit as new replacements.
Remove the water pump housing bolts from the lower unit case with a screwdriver or socket. Pull the pump housing up and off the shaft. Some housings will have only a face plate that you remove, while the housing remains fixed to the lower unit.