How to change a water pump impeller...

The following was contributed by Randy Smith, owner of Attitude, a 1997 Beneteau 351 Oceanis. About 90% of overheating problems on diesel engines can be traced back to a bad raw-water impeller. Since the Yanmar 3GM30 is perhaps the most common diesel engine on mid-size sailboats built in the last 15 years, we thought it would be helpful to show you how to replace the impeller!

Even if you don't have a Yanmar 3GM30 on your boat (even if you have a gasoline powered boat, for that matter), the impeller replacement is essentially the same on every engine. Find the raw water pump, take it apart, replace the impeller, go have a beer. Just like that!

Randy says:

The Yanmar 3GM30F is a wonderful 27 hp diesel used during the 90’s. She’s a solid performer if we stay on top of maintenance.


My Yanmar is a fresh water cooled engine, and the fresh water passes through a heat exchanger that is cooled with raw water. The most common failure of this system is the little rubber raw water pump impeller. Here’s how to replace it.

List of parts and tools:
Impeller: Yanmar part # 128296-42070 (Johnson #827)
Gasket: Yanmar part # 124233-4211
12 mm wrench
9/32” wrench
Impeller puller or needle-nosed pliers
Difficulty: Easy, less than 30 minutes.

The raw water pump has its own pulley and dedicated belt. The pump is on the forward starboard corner of your engine, or lower left as you look at the front.


It is necessary to remove the pump to get to the impeller. There are only two bolts and two hose clamps. Loosen the hose clamps (no need to remove). Remove the two bolts and lift the belt off of the pulley.


Pull the pump away from the engine far enough to spin it over so you can remove the 6 small bolts holding on the back cover. Be prepared for water from the hoses to run out of the pump when the cover is removed (Shouldn’t be more than a couple of cups).


Note the direction of the impeller flanges before pulling it out with an impeller puller or a pair of pliers. Also notice there is a key notch in the impeller when its time to install the new impeller.


Make sure the pump is clean and the pulley spins freely. Install the new impeller, again paying close attention to the direction of the flanges. I find it easier if I put a zip-tie around the impeller tight enough to get the flanges to lay over in the correct direction. The zip-tie will slip off as I push the impeller into the pump.

Using a new gasket, replace the cover and tighten the six small bolts. Spin the pump back around and slide the belt over the pulley. (Make sure it’s in the correct groove on the main pulley.) Install the two bolts, and adjust the belt as you tighten the bolts. Don’t forget to tighten the hose clamps.

I like to finish by cleaning out the strainer and filling up the hose with some water. It’s now time to fire up the engine and look for leaks!

For complete instructions on changing water pump impellers and any and all other repairs to Yanmar diesel engines, the new Yanmar shop manual was released on August 21, 2010. YOU CAN BUY YOUR COPY HERE.