Without the constant flow of water while running, an outboard motor will overheat and suffer permanent damage. Some boaters advocate opening the water pump housing and turning the impeller over each year, so that a new face is exposed for use, but Mercury Marine, in its shop manuals, actively discourages this practice. A used impeller, even one that still has a "good side," can't guarantee a sufficient flow. Changing the water pump's impeller on your Mercury outboard requires removing the water pump.
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Things you need
- Socket wrench
- 3/8-inch socket
- 3/8-inch open-end wrench
- 2 small pry bars
- Utility knife
Raise the motor to the full "Up" position with the tilt control. Move the tilt lock lever to lock the motor in position. Shift the motor into "Neutral."
Remove the plastic cover from the access panel on the driveshaft housing between the power head and the lower unit of the motor to expose the retaining bolt which secures the trim tab from above. Loosen and remove the tab from the anticavitation plate. Remove the centre locknuts, one on each side of the centre of the anticavitation plate.
Disconnect the speedometer hose at the front of the lower unit mount. Press in on the end of the hose at that junction while pulling out on the hose. Remove the top locknut at the front of the lower unit mount. Slide the lower unit away from the driveshaft housing, allowing the driveshaft and shifter levers to disengage as you pull the lower unit free, and set the lower unit on a stable work surface.
Remove the centrifugal slinger from the lower end of the driveshaft. Remove the four fasteners, usually three nuts and a bolt. Break the pump housing free of the gasket by lifting it with a small pry bar at the front of the housing and one at the rear. Remove the water pump cover and gasket from the driveshaft; discard the gasket and clean its remains from the adjoining surfaces with a utility knife.
Inspect the water pump cover for damage or deformation. Remove the impeller from the driveshaft and remove the impeller's Woodruff key, which holds the impeller in place on the shaft. Stash the key where it can be found later. Remove the face plate and the gasket from the pump base bolted to the lower unit housing and discard the gasket.
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