How to Replace a Water Pump in a Mercury Boat Motor

Written by ian kelly
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How to Replace a Water Pump in a Mercury Boat Motor
Replace the water pump impeller every year to prevent a Mercury outboard motor from overheating. (red boat image by Alexey Gnezdilov from

A six-bladed rubber impeller rotating in the water pump housing supplies pressurised cooling water to the Mercury outboard motor. The water pump is located in the submerged propeller gear casing to supply an unrestricted amount of cold water to the engine at all times. Mercury Marine recommends replacing the impeller every 300 working hours or every three years; however, if you operate your boat in silt-laden or polluted water, protect your investment by replacing this inexpensive part at least once a year.

Skill level:

Things you need

  • Outboard motor stand
  • Socket wrench
  • Mercury water pump kit
  • Light grease
  • Oil filter
  • Oil

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  1. 1

    Drain the engine oil before removing the propeller gear case to avoid spillage. Clamp the engine to an outboard motor stand. Follow the owner's manual for the correct oil-draining procedure.

  2. 2

    Shift the outboard motor transmission to neutral. Lift the propeller end by tilting the motor as far as it will go. Lock the tilting mechanism. Remove the four bolts holding the lower gear housing to the bottom of the engine, using a socket wrench.

  3. 3

    Slide the lower gear housing, driveshaft and electrical harness slowly and carefully from the motor. You may have to gently bump the housing back and forth a couple of times to break the seal between the two faces. Unplug the electrical harness and pull the lower housing all the way off.

  4. 4

    Place the lower fin below the propeller onto a block of wood. Have a helper hold the mechanism vertically. The water pump housing is bolted to the gear casing with the driveshaft protruding through the middle of the housing.

  5. 5

    Remove the three nylon domed locknuts and washers holding the pump housing to the gear casing, using a socket wrench.

  6. 6

    Slide the pump housing up and off the shaft. Remove the impeller from inside the pump.

  7. 7

    Inspect for cracks and tears at the base of the impeller fins. Check the inside face of the pump housing for signs of wear or abrasion. Replace if badly scored or worn.

  8. 8

    Remove the gasket separating the pump housing from the lower gear casing and slide it up off the driveshaft. This is a three-part gasket consisting of an aluminium gasket sandwiched between two rubber gaskets. Install the new gaskets that came with the Mercury water pump kit over the driveshaft and water pump retaining studs. Take care to replace them in the correct order -- rubber, aluminium and then rubber.

  9. 9

    Smear a layer of light grease over the faces of all six fins of the replacement impeller, apply another layer of grease to the inside face of the pump housing.

  10. 10

    Slide the impeller over the driveshaft. Align the keyway slot on the impeller with the key on the base of the driveshaft. Push the impeller down until it rests on the base of the driveshaft.

  11. 11

    Inspect the rubber seal fitted into the top surface of the pump housing. Replace if necessary.

  12. 12

    Lower the pump housing over the driveshaft and over the three retaining studs until it is resting on top of the impeller. Take care not to loosen the seal fitted into the top of the housing. Exert moderate downward pressure on top of the pump housing while twisting the driveshaft in a clockwise direction. This will cause the impeller fins to flex sideways and slide into the housing. Press the housing down onto the top rubber gasket. Ensure that the rubber seal hasn't shifted and that it is flush with the top of the pump housing.

  13. 13

    Replace the three washers and locknuts. Snug each nut down. Gradually tighten each nut in turn with a socket wrench until all three nuts are tightened evenly.

  14. 14

    Reconnect the wiring harness. Slide the driveshaft back into the engine casing. Turn the propeller gently back and forth to line up the splines on top of the shaft. Slide the lower gear casing home. Replace the four bolts and tighten with a socket wrench.

  15. 15

    Replace the oil filter and fill the engine with approximately 7.4 qts. of oil as specified in the owner's manual.

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