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Old 29-07-2016, 07:44   #1
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raw water impeller on Onan

I have a 2016 L450 and the water pump">raw water pump failed at 346 hrs on the factory installed Onan Cummins 13.5 kw generator. Since then I have had 2 impellor failures both at approx 100 hrs. There is a lift pump to get water up to the sea strainer but when generator is shut down the sea strainer goes dry. I believe the problem is the "dry" start up. Is it ok to replumb the intake line so it goes above the sea-strainer so water is always sitting in it?
Anyone else having a similar problem?
Capt Larry
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Old 29-07-2016, 07:56   #2
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Re: raw water impeller on Onan

I put a check valve in my line between the strainer and engine. I was getting less than 50 hrs per impeller before, about 400 after. Some people have plumbed an ac pump below the waterline to replace their raw seawater pump. My 5kw requires 5gpm of cooling water, your 13kw probably need about 12-15 or about an 8-900 gph pump.
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Old 29-07-2016, 08:06   #3
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Re: raw water impeller on Onan

We have a March 510 gph pump in-line which is tied into the genset outlet panel and so starts and stops with the genset. Changed our impeller at 1,100 hours - little wear.
Will be putting similar system on the new boat.


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Old 29-07-2016, 19:14   #4
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raw water impeller on Onan

Hi,

This is a well known Onan problem and you will find many discussions in these forums and many solutions offered. I have also had two failures at about 150 hours each and spent a lot of time contorted while accessing the heat exchanger to pick the impeller blades out.

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I seriously suggest the easiest and - so far - successful solution is the run dry impeller:

https://www.gcsmarine.com/content/impellers/impellers

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I would not waste my time with major replumbing, although Capt Bills check valve sound good as it is after the strainer, and non return valves as the impeller will probably still start dry on the L450.

I had the factory NRV, it filled with fine grass weed jammed and water ran out. I replaced with another model, same thing happened. I removed it. The electric pump pushed water up to the strainer ok before the generator starts. However the air lock will likely mean that the impeller will start dry.

ps don't forget to replace the anode. I figure every 1 year to 18 months max.

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Cheers, Dave


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Old 29-07-2016, 19:48   #5
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Re: raw water impeller on Onan

Check out Speed Seal, they extend the life of impellers and allow thwm to run dry for longer periods of time.

See more @ redemptiverepair.com
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Old 29-07-2016, 20:36   #6
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Re: raw water impeller on Onan

I looked in to it. Can't be fitted to the Onan.


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Old 30-07-2016, 05:05   #7
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Re: raw water impeller on Onan

Oh, that's a shame..

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Old 30-07-2016, 08:36   #8
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Re: raw water impeller on Onan

Dave from NZ-- I have an Onan 7.5 Khw and impeller loss seems to occur every 30 days when cruising. I welcome another, more durable impeller. Thanks for the tip.
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Old 30-07-2016, 08:54   #9
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raw water impeller on Onan

Couple of things,
You can just remove the mechanical water pump and replace it with an electric one, March AC pumps work really well, although the the pump must be below water line.
Secondly if you want to keep your mechanical pump, fitting a strainer just after the pump will catch any impeller parts keeping them out of the heat exchanger, and provide a quick, easy way to check impeller condition, no pieces it can be assumed the impeller isn't coming apart yet.


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Old 30-07-2016, 09:17   #10
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Re: raw water impeller on Onan

Like the idea for a strainer between the pump and heat exchanger. Never thought of that, but way easier to corral the blade parts.
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Old 31-07-2016, 00:51   #11
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Re: raw water impeller on Onan

We have a March 510 gph pump in-line which is tied into the genset outlet panel and so starts and stops with the genset. Changed our impeller at 1,100 hours - little wear. Will be putting similar system on the new boat.

I can concur in this post by Catalysis. The impeller problem is an Onan constant. The March pumps utilize magnetic impellers which will be trouble free for at least 1,000 hours. Throw out the installed impeller and connect the March marine pump between the strainer and raw water inlet. Use them also for your air conditioner compressor motors. Connect leads direct to generator.
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Old 31-07-2016, 05:09   #12
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Re: raw water impeller on Onan

Question: in attaching the March pump between the raw water strainer and the raw water inlet (bottom of stern), on my sailboat, it may not be "below the water line". Is this a non-starter?
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Old 31-07-2016, 07:47   #13
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Re: raw water impeller on Onan

March Air con pumps are generally not self priming (you can buy other self priming pumps but I wouldn't use them in this service) and so you need to locate the March pump after the through hull strainer in the supply line to the genset.



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Old 31-07-2016, 08:44   #14
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Re: raw water impeller on Onan

Thanks for the responses.....does anyone know the pump rating on the "stock" lift pump from Lagoon? The water flow seems very good so i am inclined to think re-plumbing so sea strainer is higher than raw water pump sounds like the best solution. I think the screen to catch broken vanes is a must as Onan
access to the heat exchanger is very difficult. What is best way to access anode? I have an owners version and access from the owners bathroom is very difficult.
Capt Larry
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Old 31-07-2016, 14:40   #15
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Re: raw water impeller on Onan

On my L450 I access the anodes when changing the oil so the oil filter is removed. I take the front and top panels off and loosen the aft panel at the bottom so it can be rotated a bit aft to give room. I take the battery and cover out. I then squeeze myself in and reach around with a socket (10mm I think).

There is an access hole for the anode but I find more space by loosening the side panel.

At the same time get a plastic tube that fits over the coolant water drain that is on the heat exchanger. You can then suck it out and change it with no mess.


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