Skip to content
Home \ Sailing \ Technical \ Changing the Impeller and Belts on our Yanmar Diesel Engine

Changing the Impeller and Belts on our Yanmar Diesel Engine

Both the impeller and its drive belt are critical parts of a marine Diesel engine and should be inspected regularly for signs of wear and tear. A failure in either would cause the engine to overheat.

There is quite a bit of conflicting information online as to the lifespan of these components. We’ve read comments where people change the impeller each year, some after 150 hours or 300 hours or 500 hours, one comment even mentioned 10 years! Likewise there seems to be similar conflicting info regarding the lifespan of the belts. As the parts are relatively inexpensive and the process is fairly straight forward, we opt to change the impeller and both belts fairly regularly, once a year to every two years, even when they show little sign of wear.

Raw water hose leading into water pump on the left, belt on the right

What is the purpose of an impeller?

The impeller is housed in the salt water pump and, quite simply, pumps raw sea water into the engine’s heat exchanger in order to cool the engine’s coolant. Most modern engines have a rubber impeller which over time can degrade leading to a loss in pump performance and a failure to circulate the water properly. They can also suffer a sudden failure whereby the blades (or vanes) break off and can get sucked up into the heat exchanger section of the engine. Boats that sit for long periods of time can end up with their impeller ‘bonding’ to the impeller housing, others can become brittle. Therefore many experts recommend changing the impeller once a year, regardless of hours.

What is the purpose of the belts?

The belts drive the salt water pump, alternator and coolant water pump. Like the impeller, belts deteriorate over time and can fail. Poorly tensioned belts can cause a belt to slip whereas old belts could snap at any time, not what you want as you’re manoeuvring through a busy harbour!

How to change the impeller and belts?

First up close the seacock which is responsible for drawing the sea water up into the engine. Loosen the bolts which tension the belts with the aid of a 12mm socket. (These bolts are the ones on ‘sliders’)

The bolt on the ‘slider’ above the impeller belt should be loosened in order to release the tension
The bolt on the ‘slider’ should be loosened in order to release the tension of the alternator belt

This should now give you movement in the pulleys so that the belts can be easily removed.

Yanmar Engine with belts removed

Remove the bolts that hold the salt water pump on.

You should now be able to bring the pump forward to loosen the Jubilee clips (or hose clamps as they are sometimes referred to) that secure the water hoses.

Gently prise off the hoses – do expect some water to come out so ideally avoid this coming into contact with the metal engine!

Inspect the ends of the hoses for any chafe or splits, and give them a little clean up. If they show any signs of damage then replace.

Remove the wear plate on the back of the pump and remove the existing O ring.

Using a pair of mole grips (or similar) gently pull out the existing impeller. Be careful not to damage any of the surrounding housing as this is considerably more expensive to replace than a rubber impeller!

Check the unit to ensure no obvious damage or signs of any leakage. Remove any debris and insert the new impeller. We’ve found that putting a cable tie wrap around the new impeller can help to bend all the blades at once and helps to ease it into the pump. The new impeller cost us £27.69 from Bottomline Marine. It is a Genuine Yanmar Water Pump Impeller 128990-42200 / 128990-42570.

Once the impeller is in position gently lubricate the inside of the pump with glycerin (this should be included with the new impeller).

Insert the new O ring and apply a thin layer of gylcerin. Then, working in a reverse order, re-attach the salt water pump.

Attach the new belts. Create tension in each belt by prying the pulley apart and tightening the bolt on the ‘slider’. We use the rubber end of a hammer to act as a breaker bar to lever this apart. We like to be able to create a 1/4 turn in the longest run of each belt and/or a 1cm deflection.

We used a Genuine Yanmar Marine Engine Water Pump Belt 104511-78780E, again from Bottomline Marine, costing £14.18. For the alternator belt we used a toothed belt (V belt). We switched to a toothed belt about 3 years ago after finding that the standard Yanmar belt would endure excessive wear and would produce an awful lot of black dust over the engine. Our existing belt was a BANDO RPF 3375 13X925Li however Bottomline Marine sold a Genuine Yanmar Marine Engine 2YM15 – 3YM20 (LATER) Alternator Belt 129612-42290E costing £23.58 so we opted for that. As it turned out the new belt we were sent had BANDO wrote on it also, so it appears these are the same. In all honesty we have been much happier with the toothed belt over these past few years and would highly recommend one.

Remove the strainer from the water filter and clean. Prime the system by refilling until the water is level with the intake tube in the filter. Return the filter and replace the cap.

Once you are satisfied that all bolts are tight, the belts are tensioned and the system re-primed, it’s time to give the engine a run. Re-open the seacock and fire her up! Make sure to check all areas for signs of any leaks and check to ensure water is coming out the back of the boat.

It is also recommended to regularly check the tension of your belts, especially in the first 50 hours or so after changing, as they may need re-tensioning. If your engine starts to frequently overheat or you notice that the water isn’t pumping back out at the back of the boat then check your impeller, it may be starting to fail.

We hope this post has been of help however if you have any concerns then please consult a marine engineer, our blog is for guidance only!


The information contained in this post is for general information purposes only. Information provided is deemed accurate at the time of posting, however this cannot be guaranteed. Text, including views, thoughts and opinions, along with photographs are our own (unless stated otherwise). Whilst we endeavour to keep the information up to date and correct, we make no representations or warranties of any kind, express or implied, about the completeness, accuracy, reliability, suitability or availability with respect to the website or the information, products, services, or related graphics contained on the post for any purpose. Using or acting upon any content shared on this website is entirely at your own risk and you free us of any responsibility for any loss or damages as a result of taking action from this content.