Yanmar 3YM30 Water Pump Impeller Replacement
A worn impeller on your Yanmar 3YM30 starves the engine of cooling water and can overheat it in minutes. Below is the exact water-pump kit that fits this engine, 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 Impeller – yanmar YM30
Raw Water Pump Rebuild Yanmar 3YM30 Johnson F4B903
Tools Needed
- Socket wrench with 10mm and 12mm sockets
- Screwdriver (flathead)
- Hammer (preferably plastic or rubber)
- Spanner
- Circlip pliers
- Puller tool (optional)
Safety
- Wear gloves to protect your hands from sharp edges and saltwater.
- Be cautious when handling tools, especially around the engine.
Steps
- Remove alternator belt cover using a 10mm socket.
- Loosen and remove the top alternator bolt with a 12mm socket to allow for belt slackening.
- Disconnect the water pump hose clamp and remove the old water pump.
- On the workbench, remove the six bolts on the top of the water pump and pull out the impeller.
- Inspect the lip seal; if the spring is snapped, replace it as it causes leaks.
- Remove the pulley system by undoing screws and circlip, then gently tap out the bearings using a hammer or puller tool.
- Separate the bearings using a screwdriver and spanner.
- Order replacement parts based on the manual (Johnson F4B903): shaft (5), O-ring (15), two sets of ball bearings, lip sealer (18 & 21), and impeller.
- Assemble the water pump in reverse order using new parts.
- Reinstall the water pump onto the engine and reconnect the hose clamp.
- Replace and tighten the alternator belt, ensuring it's not overtightened to avoid excessive wear on bearings.
Tips
- Yanmar parts are expensive; consider generic or aftermarket alternatives for savings (e.g., bearings from Bolton Engineering Products).
- Keep all components together during disassembly to ensure correct reassembly.
- Tighten belts properly to prevent excessive wear on engine bearings and alternator.
Which part do I need for my Yanmar 3YM30?
For most 3YM30 owners the part above (YAN128990-42570) 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 | |
|---|---|---|
| YANX02173476 | IMPELLER O-RING | Find on Amazon → |
Many of these are the same physical part under different brand numbers. Confirm against your engine plate before ordering.
Frequently asked questions
What water pump impeller fits a Yanmar 3YM30?
The Yanmar 3YM30 uses the impeller sold as YAN128990-42570, 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 3YM30?
Replace the water pump impeller every 1-3 years or roughly every 200-300 running 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 3YM30?
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 3YM30, 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 3YM30?
For a competent DIYer it's typically a 1-2 hour job of medium difficulty. The step-by-step video above covers every step.
Related guides
- Yanmar 3GM30F water pump / impeller
- Yanmar 2GM20F water pump / impeller
- Yanmar 3GM30 water pump / impeller
- Yanmar 2GM20 water pump / impeller
- All Yanmar water pump guides
- Free Yanmar boat valuation
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