The function of the water pump impeller on your Yamaha 90 Outboard is to draw water through the water pump, which prevents the engine from overheating by cooling it down. The impeller can be found inside the housing of the water pump. Removing and installing a new impeller can be a tricky task, as you must first remove the motor housing from the gear box in order to gain access to the water pump. The task will take you the best part of two hours to accomplish.
Locate the gear box (it is attached to your Yamaha motor). Use a socket wrench to remove the four bolts that hold the gear housing in place. Lift the gear box away from the motor.
Use a socket wrench to remove the bolts that fix the water pump to the top of the gear box.
Pull the water pump away from the gear box and move over to your work area. Slide the impeller off the drive train spindle, using your hands. Use a putty knife to take the gasket off the top of the gear box and replace it with a new one.
Apply a layer of grease on to the new impeller and the spindle. Slide the new impeller onto the spindle.
Place the water pump back onto the gear box and secure it in place with the mounting bolts. Lift the gear box back onto the motor and attach the four bolts that secure it in place.
