How to Check a Sea Water Pump on an 8.1 MerCruiser

by Will Charpentier

Spending a weekend checking the seawater pump on an 8.1-liter MerCruiser boat engine probably isn't the way you want to spend your weekend, but it requires an annual inspection. The project appears complicated on the surface, but all of the parts you need come in the MerCruiser pump replacement kit. Because doing it right is better than doing it quickly, this is a project better undertaken during the winter lay-up, rather than during those first few precious days of boating season.

Close the seacock. This through-hull valve allows seawater to come into the pump.

Note the amount of free play in the serpentine belt that drives the pump. Loosen the bolt at the top of the alternator that acts as a hinge. Loosen the alternator adjustment nut with a 19-mm socket and back the adjustment bolt out approximately 1 inch. Swing the alternator inward, slacking tension on the belt. Swing the alternator toward the engine as far as possible and slide the belt from one of the non-grooved pulleys. Remove the belt and set it aside without turning it over -- it must be reinstalled in the same orientation as it was in when you removed it.

Remove the two 19-mm nuts that hold the water pump and the 20-mm bolt holding the pulley bracket and two pulleys, with sockets of the appropriate sizes. Remove the bracket and pulleys and set them aside. Remove the two blue, plastic wing-bolt plugs from behind the pump's port side, keeping track of the O-rings. Remove the hose clamps from both the outgoing, bottom hose and the red incoming pump vacuum hose on top of the pump with a screwdriver. Lift the tab that extends the length of the sensor housing to open the housing and gain access to the wiring inside. Remove the wiring harness that runs to the back of the pump. Insert a large, flat-blade screwdriver between the pump's left side and the engine block and work the pump free.

Remove the six 10-mm nuts from the back of the pump with a 10-mm socket and separate the two halves of the pump housing. Lay the pump face down on the pulley and pull the back half free. Note that the impeller turns counterclockwise, since you are viewing it from "the bottom." Make a small mark on the side of the impeller facing you with an indelible pen, so you may return the same face to the pump. Remove the impeller, using two small pry bars to lever it if necessary.

Inspect the impeller and the interior of the pump housing for damage or wear. Replace the pump housing if it exhibits excessive signs of wear, from the contents of the pump replacement kit. Replace the impeller if it shows wear or has broken blades. Lubricate the outer edges of the propeller vanes and the impeller shaft with white marine grease, if you replace the impeller. Locate the "Direction of Rotation" arrow inside the pump. As you install the impeller onto the pump, turn the pulley so the impeller turns in the direction indicated by the arrow, while you push down on the impeller to seat it.

Rub a dab of white marine grease around the edge of the pump, where you installed the impeller. Place the new O-ring from the pump kit around the pump's edge. Put marine gasket sealer on the threads of the bolts that hold the two halves of the pump together. Thread the six 10-mm bolts back into place and tighten them securely. Thread the two 19-mm bolts through the pulley bracket and into the water pump, tightening each securely. Bolt the pulley bracket back into place with the single 20-mm bolt and tighten the bolt until it is snug. Replace the hose clamps and connect the hoses, tightening the hose clamps fully with a screwdriver. Reconnect the sensor wiring harness to the pump, then into the sensor housing. Close the housing and press the locking tab.

Replace and tighten the blue-plastic wing-nut plugs and their O-rings. Slide the serpentine belt over a smooth pulley in the same orientation in which it was removed. Feed the belt over the rest of the pulleys using the guide label or plaque on the face of the engine to set the belt over the grooved pulleys. Turn the alternator adjustment bolt clockwise to tighten the belt until it has the same free play as when you started the project. Tighten the adjustment nut until you can depress the belt only 1/4 inch with your thumb.

Items you will need

  • 10-, 19- and 20-mm sockets
  • 2 small pry bars
  • Pump replacement kit (pump housing, pump impeller, O-rings, plastic wing-nut plugs)
  • White marine grease
  • 5/16-inch box-end wrench


  • Note that the red vacuum hose to the pump is short and stiff, where the outgoing bottom hose is quite pliable. This helps to make coupling and uncoupling the red vacuum hose an adventure.


  • Disconnect the negative cable of your battery before performing any maintenance work on your outboard motor, to prevent electrical shock or accidental starting. Remove the nut from the negative post with a 5/16-inch box-end wrench. Lift the cable from your 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.


  • "MerCruiser Service Manual No. 30 - 496 CID 8.1 L Gasoline Engines, Starting Serial Number 0M000000"; Mercury Marine; 2001

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.