How to Change a Water-Pump Impeller
If you’ve spent any time in a boatyard during spring commissioning season, you won’t find it out of place to hear the roar of the engine and see a cloud of blue smoke erupt followed by a mechanic craning over the stern to look at the exhaust. What he’s looking for is signs of water exiting with the exhaust: Water exiting means that it is being sucked up and pushed through the engine appropriately — this is the only visual clue that the raw-water cooling system is working.
This is true for inboards, outboards and stern-drives. The marine engine’s cooling system is pretty basic. Water gets sucked up from the water surrounding the boat, runs through the engine, and exits through the exhaust system. There aren’t many parts to the system and one necessary seasonal maintenance task is checking the impeller. It’s a small rubber device that looks like a paddlewheel. It is spun by the engine and sucks in water to circulate through the engine.
The impeller is pressed tightly into the water-pump housing and over time it loses its fit, and the rubber vanes can crack or degrade. Additionally, if the water pump overheats, it will cause the impeller to break up into small bits. The impeller is the only device to provide the suction and pressure to move the water necessary for engine cooling. It if malfunctions, the engine will overheat.
Depending on the type of engine, the impeller could be located in a few different places:
Inboards — like those found on most sailboats and on powerboats with a prop and shaft set-up. To find the water pump, trace the hose that runs from the seacock (where water enters the boat) to the first place that it clamps onto the engine. This is the water pump. The impeller is behind the flat plate on the face of the water pump.
Outboards — Outboards draw cooling water through a vent at the bottom of the lower unit. Halfway up the lower unit, there are bolts that connect it to the upper. The water pump is found inside gearcase, between these two halves. To get to it, the bolts securing the lower unit need to be removed (the owner’s manual should be consulted). The lower unit is removed and the plastic casing of the water pump unbolted. The impeller is beneath the casing.
Sterndrives — Sterndrives function similarly to an outboard. Water is sucked up through the vents near the propeller and forced through a water hose into the engine. Each major sterndrive manufacturer uses a different way to mount its water pump.
OMC: The water pump is behind a casing at the top of the drive. Three bolts hold the casing in place. Once removed, the water pump is accessible. A few bolts later, the impeller can be accessed and changed.
Volvo Penta: These have vents similar to outboards and OMC drives, but the water pump is at the front of the engine. To locate it, follow the hoses that run from the stern drive along the underside of the engine. The water pump can be taken off the engine by removing a few bolts. To access the impeller, remove the back of the pump.
Mercury/MerCruiser
- Alpha: Alpha drives have a set-up similar to most outboards: The lower unit of the drive needs to be removed to access the water pump
- Bravo: Bravo use an engine-mounted water pump.
When removing an impeller, space is sometimes limited. There are many handy tools that can be used to extract a stuck impeller (in a pinch, a pair of screwdrivers tend to work well). If the impeller is cracked, or has pieces missing, try your best to find any missing pieces. If they aren’t visible, they may have made their way into the engine. Take care to flush the engine through to ensure that rubber bits aren’t blocking any cooling passages.
When installing the new impeller, there are a few things to watch for:
- Take note of which direction the impeller blades were bent on the original impeller. Try to mount the new impeller with vanes bent in the same direction
- Most impellers that are mounted to a shaft are held in place by a small metal key. Make sure the key is replaced in the same orientation that it was removed.
- Lithium grease is your friend. It will help you to slide the impeller in place and will help it to turn freely once installed.
Most service shops with a parts department have ready access to impellers of all shapes and sizes. If in doubt, the engine’s model and/or serial number can be used to easily look it up.
Understanding how your engine’s cooling system works can help you catch and prevent costly damage in the event of a problem. Additionally, knowing how to check and change an impeller on your own can help you maximize your time on the water.
Andrew McDonald is the owner of Lakeside Marine Services — a boat repair/maintenance firm based in Toronto. He has worked in the marine industry for 12 years and is a graduate of the Georgian College Mechanical Techniques - Marine Engine Mechanic program.