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Old 18-01-2018, 15:45   #1
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Burned up new raw water impeller

Bought a 1970 Morgan 41 yawl and decided to change the raw water impeller on the 2007 Yanmar 3JH4E. I shut off the seawater cock and installed the impeller and after doing so we took it for a run. About 30 minutes from port the overheat alarm sounded and found the water pump">raw water pump housing got so hot it burned the paint off. Got towed in and found that the pump blades relocated to the heat exchanger. I think maybe the pump maybe somehow got an air bubble in it and ran dry. Is there a way to bleed the air out of the system? Can anyone tell me what the proper procedure is before I burn up another pump. Thanks
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Old 18-01-2018, 15:52   #2
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Re: Burned up new raw water impeller

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Bought a 1970 Morgan 41 yawl and decided to change the raw water impeller on the 2007 Yanmar 3JH4E. I shut off the seawater cock and installed the impeller and after doing so we took it for a run. About 30 minutes from port the overheat alarm sounded and found the raw water pump housing got so hot it burned the paint off. Got towed in and found that the pump blades relocated to the heat exchanger. I think maybe the pump maybe somehow got an air bubble in it and ran dry. Is there a way to bleed the air out of the system? Can anyone tell me what the proper procedure is before I burn up another pump. Thanks
Did you open the seawater cock after installing the impeller?
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Old 18-01-2018, 16:01   #3
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Re: Burned up new raw water impeller

Thanks Dot, but yes I did open the sea cock, and also bled the air out of my sea strainer.
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Old 18-01-2018, 16:04   #4
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Re: Burned up new raw water impeller

What I do when changing a raw water impeller..

Close seacock.

Change impeller.

Take the top off the raw water strainer which should be situated somewhere between seacock and pump, maybe 12 inches above the waterline.

Fill raw water strainer to the top, replace lid.

Open seacock

Start motor

Check for water in exhaust.

Sounds lucky you didn't toast your engine...... maybe you have.....
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Old 18-01-2018, 16:08   #5
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Re: Burned up new raw water impeller

You almost certainly have not hurt your engine, but your exhaust may need inspecting.
Next time, be sure you have water coming out of your exhaust before doing anything else, it should be coming out within seconds of starting the engine.
I have never had to bleed a raw water system myself.
Maybe you need to disconnect the hose from the pump at the heat exchanger and run a hose in it to pump water backwards? That ought to ensure there is no air in the system.
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Old 18-01-2018, 16:24   #6
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Re: Burned up new raw water impeller

Since this occurred immediately following an impeller change, one possibility is that the pump cover plate and gasket or o-ring may not have been properly re-installed when the impeller was replaced. An air leak at the cover plate can prevent the pump from priming fully, the impeller will run dry or partially dry and will quickly be destroyed.

Another possibility is that flow to the pump was disrupted by a blockage in the system between the through-hull and the pump. To check this, first verify that water can actually get to the pump. Check that the through-hull is clear (and open), any fittings downstream of the through-hull are clear, hoses are clear, and the strainer is clear. Make sure all connections upstream of the pump (suction side) are air-tight (hoses and strainer cover). Next, make sure that the pump cover plate is sealed tightly against its gasket or o-ring (replace these while you're at it). If located above the water line, the pump should self-prime.

A final possibility is that the system downstream of the pump is blocked, preventing air from leaving the pump. Check that hoses, heat exchanger and mixing elbow are clear and that the anti-siphon device, if there is one, is working.

Start the engine and keep an eye on the exhaust for adequate water flow and the temperature gauge, if there is one, for engine temp.
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Old 18-01-2018, 16:45   #7
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Re: Burned up new raw water impeller

The raw water pump on my Yanmar will not self prime after changing the impeller. A little dab of silicone grease on the vanes ensures that it primes properly. Yanmar impellers used to come with a little plastic packet of grease for this very purpose but apparently some brilliant accountant figured out they could save a penny by eliminating it.

Checking that you have water coming out of the exhaust every time you start the engine will save you all sorts of grief, like melted exhaust components.
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Old 18-01-2018, 17:07   #8
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Re: Burned up new raw water impeller

It's just hard to say without the system in front of you. How low is the engine? etc? Some pumps prime from opening the seacock. If your engine is high it may not. Air bubbles do happen also and until you get the pump flooded it runs dry. Cover installation issues too.
I suppose a 3 way valve between the pump and engine could tell you if it's pumping. But ALWAYS check the exhaust right after starting your engine for water flow!
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Old 18-01-2018, 17:15   #9
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Re: Burned up new raw water impeller

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Yanmar impellers used to come with a little plastic packet of grease for this very purpose but apparently some brilliant accountant figured out they could save a penny by eliminating it.
And in turn sell more impellers
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Old 18-01-2018, 17:31   #10
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Re: Burned up new raw water impeller

Bottom Line: Check your exhaust IMMEDIATELY after any raw water modifications/servicing. It's the final word.
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Old 18-01-2018, 17:56   #11
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Re: Burned up new raw water impeller

Thanks gentlemen for all the good advice. I will definatly check for water discharge at exhaust before shoving off next time. Might have to prime the pump from the heat exchanger back just to make sure it full. The cruisers forum members are the greatest! Thanks again.
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Old 18-01-2018, 18:47   #12
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Re: Burned up new raw water impeller

I think the advice of make sure the O ring isnít pinched etc on the cover plate is likely a very good one. Pump really should prime itself unless there is an air leak.
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Old 18-01-2018, 19:13   #13
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Re: Burned up new raw water impeller

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Originally Posted by rocnrichy View Post
Bought a 1970 Morgan 41 yawl and decided to change the raw water impeller on the 2007 Yanmar 3JH4E. I shut off the seawater cock and installed the impeller and after doing so we took it for a run. About 30 minutes from port the overheat alarm sounded and found the raw water pump housing got so hot it burned the paint off. Got towed in and found that the pump blades relocated to the heat exchanger. I think maybe the pump maybe somehow got an air bubble in it and ran dry. Is there a way to bleed the air out of the system? Can anyone tell me what the proper procedure is before I burn up another pump. Thanks
Have you back flushed from the sea strainer overboard?
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Old 18-01-2018, 22:58   #14
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Re: Burned up new raw water impeller

Use correct thickness faceplate joint to thick = no Prime also scored face plate = no Prime, face plate can be turned over. Missing vanes also go back down inlet side.
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Old 19-01-2018, 04:55   #15
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Re: Burned up new raw water impeller

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Thanks gentlemen for all the good advice. I will definatly check for water discharge at exhaust before shoving off next time. Might have to prime the pump from the heat exchanger back just to make sure it full. The cruisers forum members are the greatest! Thanks again.


This should be one of the two things you check every time you start your engine.

Oil pressure, and water from exhaust.
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