How to Replace the Impeller on a 150-HP Mercury Outboard

by Will Charpentier
Replace the Impeller on a 150-HP Mercury Outboard

Replace the Impeller on a 150-HP Mercury Outboard

The maintenance schedule for the 150-hp Mercury outboard motor calls for replacing the water pump's impeller after every 100 hours of operation, or once a year. Unlike many of Mercury's larger motors, the lower unit found on this model is equipped with a water pump housing that uses a replaceable cartridge insert. The cartridge insert, new gasket and a new impeller are part of the required pump replacement kit, along with a new housing and face plate.

Impeller Removal

Lift the centrifugal slinger off the driveshaft. Remove, with an adjustable wrench, the two 3/8-inch nuts, the one 1/4-inch nut, the bolt and their washers that hold the water pump cover to the top of the lower unit.

Use two small pry bars, one at the front and one at the rear of the pump cover, to break the cover and its gasket free from the lower unit. Lift the water pump cover and gasket off the driveshaft. Discard the gasket and scrape any remaining gasket material from the lower unit's surface with a putty knife.

Lift the impeller up and off the driveshaft. Remove the Woodruff key from the driveshaft keyway and tuck the key in your pocket until needed. Lift the face plate from the driveshaft; discard the faceplate.

Inspect the water pump cover for cracks, distortion, grooves or rough surfaces. Replace the cover, if necessary.

Impeller Installation

Slide the replacement gasket and face plate down the driveshaft and onto the water pump base. Index the gasket and place the dowel pins on the pump base. Slide a new face plate-to-water pump cover gasket onto the face plate and index it with the dowel pins.

Remove the Woodruff key from your pocket, dab a bit of white marine grease onto its rear edge and insert it in the keyway on the driveshaft. Slide the pump impeller down the driveshaft, aligned so it slips over the Woodruff key. Push the impeller into place.

Apply a thin coating of gasket sealing compound to the outer diameter of a new pump cartridge insert, align the insert tab with the slot on the pump cover and press the insert into the cover. Wipe off any excess sealant. Lubricate the inner surface of the pump cover with a thin coat of white marine grease..

Slide the cover down the driveshaft and over the impeller Press on the cover while rotating the driveshaft clockwise until the cover seats on the face plate. Reinstall the three nuts and the bolt that secure the water pump in place, tightening the nuts to 50 inch-pounds and the bolt to 35 inch-pounds with a torque wrench, using a crossing pattern.

Apply a thin coat of white marine grease to the inside of the water tube seal. Slip the centrifugal slinger down the driveshaft and against the pump cover.

Items you will need

  • Pump replacement kit
  • Adjustable wrench
  • 2 small pry bars
  • Putty knife
  • White marine grease
  • Gasket sealing compund
  • Torque wrench


  • "Mercury Marine Outboard Repair Manual 65-300 HP 1992-2001"; Seloc Marine; 2007

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.

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