Yanmar 4JH57 Water Pump Impeller Replacement
A worn impeller on your Yanmar 4JH57 starves the engine of cooling water and can overheat it in minutes. Below is the water-pump kit we recommend, which 4JH57 variants it fits, a step-by-step video, and the parts you need to do the job yourself.
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Watch: how to replace it (step by step)
How to replace the water pump impeller on a Yanmar 4JH57
Changing the Raw Water Impeller on a Yanmar Marine Diesel
Tools Needed
- 12 mm ratchet
- 7 mm socket
- Needle nose pliers
- Collection bucket
Safety
- Ensure the seacock is closed before starting work to prevent water from entering the engine.
Steps
- Remove the two bolts from the pump housing using a 12 mm ratchet.
- Pull the bolts out by hand.
- Remove the belt from the pulley.
- Loosen the hose clamp on the twisting hose.
- Pry the hose off and catch any escaping water in a collection bucket.
- Remove the six small bolts from the back side of the pump.
- Remove the backing plate.
- Remove the old gasket from the housing.
- Use needle nose pliers to grip the fins of the old impeller and pull it out.
- Line up the new gasket in the housing.
- Insert the new impeller into the housing.
General Advice
- Wear appropriate eye protection throughout the procedure.
- Allow the engine to cool completely before beginning any maintenance.
- Label and organize all removed bolts and parts to ensure correct reassembly.
- Use a drain pan to catch any residual coolant or seawater.
Tips
- The impeller should be replaced every 250 hours to prevent rubber fins from breaking off and clogging the heat exchanger.
- Be careful not to scratch the brass or bronze components with your tools.
- If the impeller is difficult to remove, consider investing in a dedicated impeller removal tool.
Which part do I need for my Yanmar 4JH57?
For most 4JH57 owners the part above (YAN129670-42610) is the right choice. Price tip: the aftermarket numbers are usually noticeably cheaper than the OEM-branded box — it is the same physical part. If you need an exact match, these compatible part numbers also fit:
Show all compatible part numbers (1)
| Part number | Description | Fits | |
|---|---|---|---|
| YANX0506590-01 | IMPELLER O-RING | 4JH57 | Find on Amazon → |
Many of these are the same physical part under different brand numbers. Build years are shown where our parts data has them. Where a variant has no years, only the model code is listed. Confirm against your engine plate before ordering.
Frequently asked questions
What water pump impeller fits a Yanmar 4JH57?
The Yanmar 4JH57 uses the impeller sold as 129670-42610, plus its OEM and aftermarket cross-references (1 compatible numbers are listed above). Confirm the number against your engine's model code and year plate before ordering.
How often should I replace the impeller on a Yanmar 4JH57?
On the 4JH57 the guidance in the video is to replace it every 250 hours, whichever comes first — and sooner if you run in silty, sandy or shallow water.
What are the symptoms of a failing impeller on a Yanmar 4JH57?
A weak or missing telltale stream, engine temperature climbing above normal, or an overheat alarm are the classic warning signs. A worn impeller can overheat and damage the engine within minutes, so don't delay the job.
Do I need the full water pump kit or just the impeller?
For the 4JH57, fit the complete kit. The seals, O-rings, gasket and wear plate age alongside the impeller, and replacing them together is the reliable, do-it-once job.
How long does a water pump job take on a Yanmar 4JH57?
For a competent DIYer it's typically a 1-2 hour job of medium difficulty. The video above shows the job on a similar Yanmar diesel engine — the procedure matches, but check the 4JH57 manual for model-specific details.
Related guides
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