How to Change a Water Pump on a 40 HP Johnson Seahorse

How to Change a Water Pump on a 40 HP Johnson Seahorse

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Long winter days may keep you off the water, but that doesn't mean you have to ignore your boat. Changing the water pump for a 40-horsepower Johnson Seahorse is a project you can undertake during your motor's winter storage. You are supposed to inspect the water pump at least yearly, and it only makes sense to do so -- you've got the lower unit off and you have to take the pump apart, so changing the impeller is as easy as inspecting it.

Items you will need

  • Adjustable wrench

  • 1/4-inch open-end wrench or 1/4-inch socket

  • Pump replacement kit

  • Needle-nose pliers

  • Marine grease

  • Gasket sealer

  • Small-jawed seal puller

  • White marine grease

Removing the Old Impeller

Shift the motor into "Neutral." Remove the lower engine cover and loosen the shift rod lock-nut, so that you can loosen the turnbuckle on the shift rod. Take note of the number of turns of the turnbuckle needed to disconnect the shift rod, to ensure proper adjustment upon reassembly. Locate the three bolts -- on each side of the outboard -- that hold the lower unit to the motor and use an adjustable wrench remove them. Pull straight down on the lower unit to separate it from the motor.

Locate the driveshaft, protruding from the lower unit, and the water pump housing, on top of the lower unit, through which the driveshaft runs. Take lock nuts off the studs in the corners of water pump housing with a 1/4-inch open-end wrench or -- probably easier -- a 1/4-inch socket and set the lock nuts and the washers aside.

Discard the impeller -- the pump replacement kit contains a replacement impeller. Pull the impeller drive pin out of the driveshaft with a pair of needle-nose pliers.

Pull the impeller plate and wear plate gasket from the lower unit and discard them, or clean them up as spares. Remove and discard the driveshaft seal, the water tube grommet and the O-ring.

Installing the New Impeller

Install the new drive seal into the water pump housing so that the seal's lips will face downward when installed on the lower unit. Grease the seal, using marine grease, once it is in place. Install the water tube grommet from the pump replacement kit into the smaller of the two holes on the housing's top.

Spread a thin coating of gasket sealer onto both sides of the new wear plate gasket and position the gasket atop the lower unit. Settle the new impeller plate onto the top of the wear plate gasket. Push the drive pin back into its location on the driveshaft. Slide the new impeller into place, turning the driveshaft, as necessary, to move it past the drive pin, and settle it onto the plate.

Lower the housing into place, so that it covers the impeller. Turn the driveshaft clockwise to seat the housing as it covers the impeller. Twist the pump, clockwise only, to position it on the retaining studs (turning it counterclockwise will damage the vanes on the impeller).

Smear a bit of gasket sealer onto the threads of the four studs, place the washers over the studs and thread the nuts onto them. Tighten the nuts with a torque wrench to between 70 and 80 inch-pounds.

Reinstall the lower unit. Reconnect the shift rod and reinstall the lower motor cover.


  • Retain any serviceable parts removed, as spares.
  • If the driveshaft grommet is difficult to remove, a small-jawed seal puller will do the job handily.
  • Before you reinstall the impeller drive pin, dab its end into white grease to hold it in place as you slide the impeller and pump housing over it.
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