Replacing an Evinrude 25 HP outboard motor impeller can be done in about an hour or two. The Evinrude impeller sits inside the water pump housing at the top of the gear case. You can access the part by removing the gear case from the engine housing. You should replace the impeller about every two years, depending on how often you run the outboard motor.
Remove the nuts from underneath the top lip of the Evinrude 25 HP gear box with a socket wrench. Sslide the gear case away from the motor housing. Unfasten the bolts on the water pump housing on the top of the gear case with a socket wrench, and lift the pump housing off the motor shaft.
Turn the water pump housing over and remove the impeller from inside the pump housing. Remove any debris or residue from inside the pump housing with a clean cloth.
Apply a thin layer of marine grease to the inside edge of the pump housing and along the motor shaft. Place the new impeller inside the water pump housing upside down and set the pump housing back over the motor shaft.
Slide the pump housing and new impeller down the motor shaft and secure it to the top of the gear case with the bolts. Slide the gear case back onto the motor housing and attach the case to the Evinrude 25 HP housing with the nuts.