Putting a new water pump on your Mercury outboard means tinkering with the engine's peripheral workings that, well, keep it working. The pump's driven by the same drive shaft that drives the propeller and the pump itself is little more than an impeller in a housing with a couple of gaskets. Nonetheless, it's something you have to do at least once yearly, so make it a winter project that won't cut into your boating fun.
Drop the Lower Unit
Shift the engine into forward gear. Tilt the motor up, using its trim-tilt system. This gives you room to remove the lower unit, the bottom part of the motor. You won't have to bend over to remove the unit and you'll have room for a second person to support it during removal. Open the vise on your workbench and pad its jaws with a thick towel to avoid scratching the paint on the lower unit.
Unbolt the trim tab from the bottom of the plate above the propeller using an appropriately sized combination wrench or socket. Remove the bolt under the trim tab. Remove the nuts from the bolts holding the lower unit
Stand on one side of the outboard, at the joint between the lower unit and the rest of the outboard. Position your helper on the opposite side. Remove the bolts holding the lower unit in place, beginning with the bolts nearest the floor. If you fail to pull the shaft and rod away from the outboard's motor housing, you can bend the drive shaft or the shift rod. As you remove the two bolts, have your helper support the unit.
Slide the lower unit straight away from the outboard. The water tube, which moves water from the water pump to the rest of the motor, will pull loose from the top of the pump housing on the top of the lower unit. Move the lower unit to the workbench. Place the unit into the vice so the drive shaft and shift rod point straight up. Tighten the jaws of the vice sufficiently to prevent the lower unit from moving.
Replace the Pump
Look at the top of the lower unit; you'll see the water pump surrounding the drive shaft, where it enters the lower unit. Remove the bolts that hold the pump housing in place. Pull the pump housing from the drive shaft to expose the impeller.
Remove the pump's impeller. As you do so, a small, square piece of metal, called a Woodruff key, will fall from the driveshaft; this is the key that holds the impeller to the driveshaft. Save the key and open the pump replacement kit. Lay the kit's contents out on the workbench.
Remove only the parts of the pump that are included in the replacement kit. Use a paint scraper to clean any bits of gasket from the top of the lower unit. Use a marine gasket compound, such as Mercury's Perfect Seal, to attach the gaskets from the kit to the lower unit.
Grease the inside of the new pump housing with white marine grease. Use a bit of grease on the Woodruff key and return the key to the driveshaft. Put the impeller into place on the driveshaft. Apply Perfect Seal to the threads of the pump housing bolts, thread the housing bolts through the bores in the pump housing and into the lower unit.
Bolt the pump housing in place, tightening the bolts to 60 inch-pounds of torque, using a torque wrench. Spread a small amount of gasket compound onto the water tube stub on top of the pump housing and press the water tube connector onto the housing.
Reinstall the Lower Unit
Apply Mercury Perfect seal to the threads of the lower unit bolts. Loosen the jaws of the vise and move the lower unit to the outboard. Slide the drive shaft and shift rod into the outboard without touching the sides of the exhaust housing. Slide the drive shaft upward until it engages the engine splines.
Replace the upper two lower unit bolts in the order in which they were removed. Place the nuts onto the bolts and ensure the water tube connector has engaged the water tube. Install the remaining nuts and bolts (except the bolt that goes under the trim tab) that hold the lower unit to the engine and tighten to them 35 foot-pounds.
Tighten the bolt under the trim tab to 80 inch-pounds. Replace the trim tab and lower the engine using the trim-tilt system.
Items you will need
- Thick towel
- Combination wrench
- Pump replacement kit
- Paint scraper
- Mercury Perfect seal
- White marine grease
- Torque wrench
- Owning an outboard means owning tools. Have a selection of open- and box-end wrenches of various sizes and a wide selection of sockets, up to 1 1/2 inches.
- Disconnect the negative cable of the boat’s battery using a 5/16-inch box end wrench before performing any maintenance work on the boat’s engine. Lift the cable from the battery, move it outside of the battery box and close the lid of the battery box. After the work is complete, reconnect the negative battery cable.
- When you work on your boat’s engine, remove the propeller nut with a propeller wrench. Slide the thrust hub, propeller and washers from the propeller shaft. Failure to remove a propeller before working on an engine that’s out of the water may lead to a propeller-strike injury, which can injure or kill.
- Mercury Marine: Mercury/Mariner Outboards Service Manual, Models 70,75,80,90, 100,115 - 1987-1993 70, 75, 75 Marathon 75XD, 75 SeaPro, 80, 90 3-Cylinder, Serial Numbers 0A9960142 Thru 0D283221 and 1988 through 1993 100, 115 (4-Cylinder), SN 0B209468 Thru 0D283221
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