How to Replace a MerCruiser Water Pump Impeller

by Chris Stevenson

The MerCruiser outdrive, or sterndrive, motor has been used on marine watercraft for decades. Like all marine engines, the MerCruiser relies heavily on its water pump impeller to send outside water through the engine manifold, jackets and passage to keep it cool. The rubber impeller inside the pump housing suffers continuous use while the engine runs, and can fail from age, wear or overheating. Replacing the water pump impeller on the outdrive requires removal of the lower unit case.

Place the engine in a half-tilt position. Remove the ignition key and disconnect the negative battery cable with a socket. Use a socket to remove the lower case oil plug and drain the gear case oil into a pan. Replace the plug and tighten it. Place the shifter in forward gear. Unscrew the anode underneath the lower unit, with an Allen wrench. Mark the trim tab position with chalk. Use an Allen wrench to unscrew the trim tab, but only loosen the screw to pull the trim tab free.

Unscrew the lower unit mounting nuts and bolts, located under the cavitation plate and through the side case, using a socket and wrench. One of the bottom screws has an Allen head, so use an Allen wrench to remove it. Gently pry the seam between the the upper and lower case with a thin-blade screwdriver, being careful not to damage the mating surface.

Have an assistant help you lift the lower unit and place it in a marine engine stand. If the copper water tube came free of its grommet hole in the upper case, remove it from the lower case housing and stick back up into the upper case grommet seal. Look for an O-ring on the driveshaft that sits in a groove. Slide the seal up over the shaft and keep it close by. Pull up the rubber slinger seal from the shaft and set it aside.

Use a socket to remove the nuts on the plastic water pump housing. Pry the housing loose, and lift it up off the driveshaft. Turn the housing upside down. Note the orientation of the impeller blades; you will install the new impeller with the blades facing the same direction.

Pull the impeller out of the housing bore. Remove the impeller drive key, a gasket, the pump face plate and the last gasket. Clean the inside of the the pump housing with a gasket scraper, engine cleaner and a rag.

Place a new kit gasket down into the pump housing, then the face plate. Place another kit gasket down over the face plate. Set the impeller drive key in the new impeller and position it into the plastic water pump housing. Remember the correct orientation of the impeller blades.

Slide the pump housing down over the shaft and onto the studs. Replace the stud nuts and tighten them with a socket. Place a new slinger seal over the shaft and flush it with the top of the water pump housing. Place a new upper O-ring on the drive shaft groove.

Move the lower unit under the upper unit case. Have your assistant raise the engine stand gradually while you align the drive shaft splines with the upper socket and the copper water tube that must fit into the lower pump grommet.

Twist the propeller if you need to mesh the drive shaft splines, to mate the two cases. Start all of the lower until bolts and nuts in by hand, starting with the trim tab anode nut. Use a socket or Allen wrench to tighten them gradually, rotating from one to another as your assistant raises the lower case in tiny increments. Tighten all the bolts with the Allen wrench or regular socket.

Remove the lower case gear oil filler cap with a socket. Fill the gear case with oil, according to the amount prescribed in your owner's manual. Do not attempt to start the engine until you submerge the lower unit in water, or use a garden hose flush device. Reconnect the negative battery cable with a socket.

Tip

  • check Check the lower case oil level after your first use with the new impeller.

Items you will need

About the Author

Chris Stevenson has been writing since 1988. His automotive vocation has spanned more than 35 years and he authored the auto repair manual "Auto Repair Shams and Scams" in 1990. Stevenson holds a P.D.S Toyota certificate, ASE brake certification, Clean Air Act certification and a California smog license.

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