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Changing the impeller on your Yamaha 225-horsepower four-stroke outboard motor is "must-do" maintenance every three years, according to Yamaha Motor. Changing the impeller not only allows you the opportunity to perform required maintenance on the water pump, but also to check the general condition of the lower unit. You can inspect the propeller shaft seal, remove fishing line from around the propeller shaft and check the condition of your propeller.
Items you will need
Pump replacement kit
Waterproof marine grease
Rotate the motor to the full "Up" position using the trim and tilt system and engage the tilt lock lever. Move the gear selector into "Neutral." Remove the round zinc anode to access the lower unit retaining bolt with a 15-mm socket. Remove the rubber plug from the rear edge of the lower unit housing above the cavitation plate. Loosen the bolt in the recess, which secures the zinc anode from above.
Remove the zinc anode from the cavitation plate. Remove the previously obscured lower unit retaining bolt with a 20-mm think-walled socket. Clip the wire securing the speedometer hose to the fitting on the lower unit with a pair of wire cutters and remove the hose from the fitting.
Loosen the four lower unit mounting lock nuts, two on either side, evenly, without removing any nut before you have loosened the opposite side, lest you damage the driveshaft. Once all are sufficiently loosened, pull on the lower unit to separate it from the exhaust housing, and fully remove the lock nuts. Pull the lower unit out of the exhaust housing while guiding the shift rod and driveshaft straight out of the the exhaust housing. Set the lower unit onto your work bench.
Slide the driveshaft seal to the top of the water pump housing, then slide the seal up and free of the driveshaft. Lift and slide the water pump cover and its gasket free of the driveshaft.
Check the cover for grooves, other than the circular impeller sealing groove, which are 0.030 inch or deeper. There is no insert in the water pump cover on the lower unit for this horsepower motor. Lift and slide the impeller free of the driveshaft and remove the impeller Woodruff key from the driveshaft cutout.
Apply a dab of waterproof marine-grade grease onto the Woodruff key, and place it in the driveshaft keyway. The grease holds the Woodruff key in place while you install the impeller. Slide the new water pump impeller from the pump replacement kit down the driveshaft, with the keyway in the impeller aligned with the Woodruff key in the driveshaft. Press the impeller into place with the keyway aligned.
Lubricate the inner pump surface with a light coating of waterproof marine-grade grease. Slide the pump cover down the driveshaft until it makes contact with the impeller. Apply a small amount of downward pressure on the water pump cover, while rotating the driveshaft clockwise until the cover seats on the plate.
Start the water pump fasteners by hand. Slowly and evenly tighten them to 60 inch-pounds with a torque wrench, using a criss-cross pattern.
- "Yamaha/Mercury/Mariner Outboard Repair Manual 2 - 250 HP 1995-2004"; Seloc Marine; 2007
Will Charpentier is a writer who specializes in boating and maritime subjects. A retired ship captain, Charpentier holds a doctorate in applied ocean science and engineering. He is also a certified marine technician and the author of a popular text on writing local history.