How to Change the Impeller on a Yanmar Diesel

by Kyle McBride

Yanmar diesel engines have been one of the more popular power sources for mid-sized sailboats over the last two decades. Prestigious manufacturers such as Beneteau and Endeavor use them, as well as many of the other boat makers. The engines are compact, light, dependable and easy to maintain and repair. A fresh-water, closed circuit cooling system cools the engine. A positive displacement, impeller vane type water pump forces the engine cooling water through the engine water jacket and through a raw-water cooled heat exchanger.


Loosen the hose clamps on the water pump with a flat screwdriver. Pull the hoses off the water pump nipples.


Remove the two water pump mounting bolts with the 12mm wrench. Disengage the pump belt from the pulley and pull the water pump off of the engine.


Remove the water pump cover bolts with the 9/32 inch wrench. Pull the cover off the pump and then scrape the gasket off of the cover and the pump body with a gasket scraper.


Note the direction of rotation of the pump. Remove the old impeller with a pair of pliers. Grab an impeller vane and pull the impeller clear of the body and off of the impeller shaft.


Inspect the impeller key for damage and replace it if necessary. Clean the pump body and remove any debris.


Wrap a cable tie around the new impeller and compress the vanes in the direction of rotation by tightening the cable tie. Slide the impeller onto the shaft and line up the key with the key slot. Push the impeller fully into the pump body. The cable tie should slip off as the impeller goes into the pump body, releasing the vanes.


Install the new gasket on the water pump cover. Install the cover and the cover bolts and tighten the cover bolts with the 9/32 inch wrench.


Install the water pump pivot bolt and tighten it with only your fingers, or tight enough that the pump is held onto the engine but able to rotate while adjusting the belt tension. Install the drive belt onto the pump pulley. Install the belt tension adjuster bolt. Rotate the pump body until the belt is tight and tighten the adjuster bolt and the pivot bolt firmly.


Pour some fresh water into the water pump hoses and the pump body to prime them. Install the hoses onto the pump nipples and tighten the hose clamps with a screwdriver. Top off the cooling water level in the engine. Test run the engine and check for leaks.

Items you will need

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