A 15-horsepower outboard boat motor is cooled by a freshwater system that consists of a very simple aluminum water pump housing with a little rubber wheel inside, called an impeller, which has multiple fins that spin like a fan inside the housing. When the impeller gets damaged or overheats, it can't pull water into the engine compartment and the temperature alarm will sound. It only takes a few minutes of running without water to destroy an expensive engine, so you should replace the water pump impeller as part of your regular yearly maintenance.
Things You'll Need
- Socket wrench
- Impeller housing gasket
- Gear lube
Remove the lower unit from the outboard. The upper and lower units are joined by four bolts on the rim. There's also one screw on the underside of the lower unit where the sacrificial anode attaches to the bottom of the fin. Remove the anode to get to the screw to detach the lower unit. Place it on the ground near the boat or on a nearby workbench so you do not have to disconnect the wiring. There is enough wiring to allow you to move the lower unit five feet from the back of the boat.
Remove the bolts fastening the water pump housing to the top of the lower unit where it surrounds the shaft, using a socket wrench. Lift the housing up and off of the shaft.
Pull the impeller from the inside of the water pump housing. Inspect it for wear, including cracks or broken fins.
Remove the three gaskets that are on top of the lower unit where the water pump housing was. Two rubber gaskets sandwich an aluminum gasket. Replace both rubber gaskets, and inspect the aluminum gasket for signs of wear, burn marks or nicks. Replace the aluminum gasket only if necessary. Slide the aluminum gasket over the shaft and the mounting bolts of the pump housing.
Apply a light coat of gear lube to the old impeller (or to a new one if being installed). Insert the impeller over the shaft and down until the grove of one fin hooks into the grove of the shaft.
Lube the inside of the water pump housing with a light coat of grease. Slide the housing over the shaft and down over the impeller. Tighten the bolts to seal the housing to the lower unit with a socket wrench.
Reassemble the lower unit to the upper unit of the outboard by replacing the four bolts on the rim with a socket wrench until snug. Use a screwdriver to replace the screw under the tail fin. Replace the sacrificial anode above the rudder.
- "How Boat Things Work"; Carlie Wing; 2007
- "Practical Boat Mechanics"; Ben L. Evridge; 2007
- Duckworks: The Online Magazine for Amateur Boat Builders: Obsolete Outboards: Bringing a 1956 Johnson 15 hp Back to Life
- Photo Credit Jupiterimages/Photos.com/Getty Images
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