top of page

Yanmar 4JH4 Sea Water Pump Impeller Service

An Important component of boat parts is the raw water pump on a 4JH4 Yanmar series engine of the neoprene impeller type, driven by the cam gear, is located on the right-hand side of the engine, same side as the starter motor. The impeller, if bought originally, is of high-quality material that lasts many engine hours. According to the manufacturer, the inspection intervals are 250 engine hours, or once a year, depending on which occurs first, and the replacement intervals of the neoprene impeller are 1000 engine hours or 4 years. These recommendations are good and well for boat parts, but sometimes reality is different, it is common to replace the neoprene impeller before long voyages, depending on sailing conditions and impeller age, if you expect to make a long passage under power then replacing the impeller beforehand is good practice.

The pump, in some instances, is on the further and less accessible side of the engine, and so complicates changing the parts; the easiest route for this service, though longer, is removing the entire pump altogether and working on it on a bench.

For this service a Yanmar Raw Water Pump Impeller Kit is required and includes the impeller, O Ring and glycerine. Some boat parts you would need such as service and repair kits for the Yanmar Impeller Pump 129271-42502 are available as follows:

Removing the Yanmar 4JH4 Sea Water Pump

Start by closing the seacock to the pump and then removing the hoses leading to and from the hose fittings on the pump. Then remove the three bolts and one nut that hold the pump assembly in place, make sure not to damage the gasket when removing the pump, you may reuse it if it’s in good condition.

Yanmar 4JH4 Sea Water Pump

The Pump diagram is as follows:

Pump diagram

7. Washer

9. Retaining Ring

10. Key

13. Shaft

15. Nut

16. O-Ring

17. Bearing

18. V-Ring

19. Seal

20. Retaining Ring

21. Retaining Ring

Removing Impeller for inspection

Remove the four hex head screw that holds the face plate in place

Remove the faceplate gasket, and clean the sealing surfaces of the face plate and pump housing. You will see remains of dirt, calcium and neoprene (as in photo) and these may cause a lack of seal.


A good addition to your boat parts and tools would be using the extractor tool, carefully remove the neoprene impeller. It is very important not to use leverage or excessive force using screwdrivers and other instruments to avoid damaging the pump housing sealing surface, if the surface does get damaged then the pump will not have proper sealing. Inspect the neoprene impeller for any cracks or excessive wear on the blades themselves and if any cracks are apparent on the blades then the neoprene impeller must be replaced.

Moreover, is to make sure the neoprene surface of the hub that touches the face plate, and on the opposite side on the wear plate, are not worn down. These surfaces seal the low-pressure chamber of the pump housing for the pump to function properly. As you can see, in a comparative photo of new and used, with the used impeller, the sides of the blades and hub have been rubbed and hairline streaks can be seen. As opposed to the new impeller where the sides of the blades and hub are smooth and shiny.


Fitting an Impeller

If installing a new neoprene impeller or fitting the used one, make sure to use glycerine or silicone spray to lube the neoprene impeller, this is important because up until the point that seawater reaches the pump chamber, the neoprene impeller might overheat from friction and lose sealing efficiency.

There are several techniques to install a new impeller in its housing. The first would be using a special tool designed to contract the blades so that the impeller will slip easily into its housing. Another would be the use of a zip tie to contract the blades, just make

sure that while inserting the impeller, do so in a rotating motion in the direction of the pump's direction of rotation.


Then make sure the faceplate and housing surface are clean for the new gasket, a thin application of multi purpose grease will suffice, this you can find in any boat store. Tighten the screws evenly and replace the pump back to the flange, not to forget to replace the flange mounting gasket in case damaged

Fit the pump assembly with the mounting gasket, tighten screws and the nut, reconnect the hoses and open the seacock.

Finally, prime the pump to make sure of a good flow of water before returning to normal engine use and don't forget to stock up on spare boat parts so you can perform this service whenever you are in need.


Recent Posts

See All


bottom of page