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Old 27-09-2016, 20:32   #1
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Onan Raw Water Blockage

Hi,

I have a 7kva Onan Marine Generator which I just removed broken pieces of impeller out of inlet to heat exchanger.

Now when I start this unit, motor runs as normal but there is minimal outflow of raw water plus the outlet hose from impeller housing is very firm (as if there is excess pressure in this hose).

With unit shut down & this output hose disconnected from impeller housing, I can not blow through it (ie through heat exchanger). With outlet side of heat exchanger removed, I can blow through this hose. Is this normal? I assume not. What could be causing this blockage?

Note, before I cleaned out heat exchanger, generator was working normally.

One step forwards, two steps backwards!

I would appreciate any input,

Brett


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Old 27-09-2016, 23:41   #2
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Re: Onan Raw Water Blockage

More rubbish inside the heat exchanger. Remove it & give it a decent clean including the tubes.
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Old 28-09-2016, 01:05   #3
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Re: Onan Raw Water Blockage

The restriction could be anywhere after the pump. Heat exchanger is obvious start place but could be mixing elbow or anywhere there is a fitting or bend that debris can collect.
You could try reverse flow by forcing water with a pump or dock hose.
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Old 28-09-2016, 01:16   #4
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Re: Onan Raw Water Blockage

Yep I gather that is the issue. Thanks for your help.

Have removed raw water exit hose & can not blow air through in normal flow direction. Using my RIB inflation pump, I am able to push some water through ( reverse flow ), but blockage is still there when I try it in normal flow direction.

Is it possible to push some wire through heat exchanger, with both end caps removed, in an attempt to push out blockage?

If not, I shall remove heat exchanger & give it a complete clean.


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Old 28-09-2016, 01:20   #5
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Re: Onan Raw Water Blockage

If you have good access this is probably good as you won't have to drain your coolant. Do it once, do it right.
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Old 28-09-2016, 02:38   #6
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Re: Onan Raw Water Blockage

NoTies,

You hit the nail on the head - I am trying not to drain coolant as I just changed it two months ago - just before changing impeller. To try & collect to reuse coolant may be difficult with access being tight.

So a new fella to this procedure has just learnt something - have a good look at heat changer before adding new coolant....

Thanks again.



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Old 28-09-2016, 04:14   #7
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Re: Onan Raw Water Blockage

This is common problem and lots written already in these forums. Here is how many pieces I picked out in one go plus the anode. Get a 'run dry' impeller or suffer forever. dave
Click image for larger version

Name:	ImageUploadedByCruisers Sailing Forum1475057633.941087.jpg
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Size:	84.5 KB
ID:	131942Click image for larger version

Name:	ImageUploadedByCruisers Sailing Forum1475057661.691911.jpg
Views:	46
Size:	89.5 KB
ID:	131943


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Old 28-09-2016, 11:11   #8
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Re: Onan Raw Water Blockage

Hi Dave,

I emailed run dry impeller manufacturer & unfortunately they don't make one foe my model generator.

Cheers Brett


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Old 28-09-2016, 11:47   #9
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Re: Onan Raw Water Blockage

Quote:
Originally Posted by DavefromNZ View Post
This is common problem and lots written already in these forums. Here is how many pieces I picked out in one go plus the anode. Get a 'run dry' impeller or suffer forever. dave
Attachment 131942Attachment 131943


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Or ditch the impeller pump and use a March or other airconditioner pump.


If you chose to keep the impeller pump you can install a strainer after the pump, but before the heat exchanger, then you can just look every now and again at the strainer, if there are pieces of impeller in there, time to change your impeller.
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Old 11-10-2016, 14:51   #10
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Re: Onan Raw Water Blockage

So problem was when previous owner had an anode changed - zinc had broken off holding cap & was left sitting in heat exchanger - I gather it wasn't blocking flow in heat exchanger until I cleaned out the supply end of heat exchanger & increased flow caused broken anode to move slightly & block most of flow.

So moral of story, when an anode is changed & you see anode cap without anode (like DavefromNZ second photo) don't assume anode has been eaten away!

Thanks NoTies for your advice - removing heat exchanger exposed problem & allowed for a complete clean.


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Old 12-10-2016, 15:11   #11
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Re: Onan Raw Water Blockage

So earlier in this thread I stated that the manufacturer of run dry impellers don't manufacture a run dry impeller for my model Onan.

Well this is so if you ask them if they have a match for Onan part number 0132-0498.

Ask them if they have a match for Sherwood 10077 (which is the Sherwood part number for my Onan) then yes they do, it is Globe Composite Model 1130 impeller.


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