Gone Outdoors

How to Fix a Mercury Outboard Motor Water Pump

by Don Bowman

The water pump in a Mercury outboard motor cannot be fixed--it must be replaced, which is not a complicated process. Once familiar with the procedure, it can be accomplished in about 30 minutes. This is not an expensive project and should be done once a year if the boat is used in salt water (the salt corrodes many parts). This procedure will increase the life of your investment for many trouble-free years.

Place an oil pan under the lower unit. Remove the oil by removing the two standard screws in the side of the lower unit. The lower screw is the drain and the upper screw is the vent.

Place the morse control into forward at your throttle. Just push far enough to be in forward, there is no need to push full throttle. Mark the location of the trim tab so it can be installed in the same location.

Remove the trim tab by taking out the torx head screw. Remove the bolt under the trim tab using a ratchet and socket. Remove the two 5/8 nuts on both sides of the trim tab. Loosen the top forward 5/8 nut on the top front of the lower unit, but leave it on a few threads to keep the unit from falling off when the last nuts are removed.

Remove the bottom forward 5/8 nuts. Wiggle the lower unit until it slides loose and begins to drop. While supporting the lower unit remove the upper 5/8 nut (the one you left on with a few threads) holding it up and slowly lower the unit. The lower unit is not light so be prepared and have something to sit it on so not to damage the scag.

Look for the little o-ring that is located on the right forward side in a groove. It is used to seal the oil passages between the upper and lower unit. It could stick on the upper unit and will fall off. Put it aside and do not lose it.

Remove the top o-ring on the shaft drive, the thick rubber gasket and the plastic pump housing by removing the two 7/16- and one ½-inch nuts and the 5/16-inch bolt. In this order, lift off the upper pump housing, the impeller with the woodruff key, the top gasket, the stainless steel wear plate, then the lower gasket.

Install the new parts in reverse order--the bottom gasket, stainless steel wear plate and the impeller. To correctly install the impeller, hold the key it in its cavity and push the impeller down unto the key making sure the key stays in place. Slide the upper housing down while rotating the shaft slightly clockwise to ease it down. Make sure you do not turn the shaft counterclockwise because it will kick it out of gear. If this does happen then the short shaft in front of the unit must be turned counterclockwise to put it back in forward and you should turn the prop clockwise to be sure it is in forward position prior to installing.

Tighten the nuts and bolt with the following torque: 35 inch pounds on the ½-inch heads; 30 inch pounds on the 7/16-inch heads and 20 inch pounds on the 5/16-inch bolt. Replace the plastic alignment tube on the pump housing, the thick rubber gasket and the top o-ring.

Place the small o-ring used for the oil seal in its location on the forward right and in its groove. To keep it there, coat it with Vaseline or grease.

Install the lower unit, sliding it up. Position the driveshaft in the splines. If it is does not slide into place, turn the prop slightly clockwise to engage the splines. Do not turn the prop counterclockwise, as you will kick it out of gear. Once the shaft starts to slide in, continue and line up the water tube and shift shaft. Push up until you can install two of the lower 5/8 nuts. Put the nuts on a few threads and look to make sure the water tube lined up.

Move the throttle in the boat to neutral and walk back and make sure the prop turns freely. Move the throttle to reverse and check for the same at the prop. If it shifts and goes into gear properly then it is lined up. Continue to tighten the bottom nuts. Replace all the nuts and torque to these specs: the 5/8 nuts get 35 foot pounds and the 9/16 head bolt gets 28 foot pounds. The bolt under the trim tab gets 80 inch pounds.

Cut the tip off at the end of both bottles of gear oil. Get the two cap screws for the drain and vent. Hold the bottle in the bottom hole and fill the unit. When you change bottles, hold your finger over the top hole and it will not run out the bottom as fast when you switch to a full bottle. It will take approximately 30 oz. so fill it until it starts to run out of the top hole. When it is full put the plug in the top hole first and then the bottom.

Items you will need
  • Set of 3/8- and 1/4-inch drive sockets
  • Screwdriver
  • Water pump impeller
  • Water pump housing
  • Gaskets
  • 30 oz. 85 to 90 weight gear oil

About the Author

Don Bowman has been writing for various websites and several online magazines since 2008. He has owned an auto service facility since 1982 and has over 45 years of technical experience as a master ASE tech. Bowman has a business degree from Pennsylvania State University and was an officer in the U.S. Army (aircraft maintenance officer, pilot, six Air Medal awards, two tours Vietnam).