How to Change a Water Pump on a Mercury Outboard

Mercury Marine's maintenance interval chart for its outboard motors cautions you to check the water pump impeller after the initial 10 hours of operation and every 100 hours, or 12 months, thereafter. The water pump, located atop the lower unit, acts as the primary machinery for the cooling system, moving the water captured by the raw water cooling system intakes through out the motor, dissipating the heat as the water stream leaves the motor.

Remove the 1/4-inch mounting bolts that hold the pump cover in place using a 1/4-inch socket, and slide the pump cover up the driveshaft and off the lower unit. Remove the impeller and stash the Woodruff key the removable metal tabe on the driveshaft that turns the impeller with the driveshaft.

Remove the gasket and the faceplate that serving as the interface between the pump and the lower unit. Use a utility knife to remove any dried grease or other buildup where the pump was seated.

Push the new gasket and faceplate onto the driveshaft and down to the top of the lower unit. Dab the Woodruff key into white marine grease and place it into the driveshaft key way. Align the impeller with the Woodruff key, slide it down the driveshaft and press it into place.

Apply a touch of white marine grease to the inside of the pump cover surface and position the gasket on the cover. Slide the cover down the driveshaft to the impeller. Press down on the cover while rotating the driveshaft clockwise until the cover settles into place on the pump's base plate.

Apply a thread locking adhesive to the water pump bolts' threads, thread the bolts through the pump cover and torque to 60 inch-pounds. Apply a thin coat of white marine grease to the water tube seal.


  • "Mercury Outboards 2001-09 All Engines 2 Stroke"; Seloc Marine; 2009

About the Author

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.