T&T: Onan cooling problems

[Ken Bloomfield]

Hi David,
I believe the problem is that your raw water pump has become a poor air 
pump, even though it still pumps water just fine once primed (or at 
least I suspect so).  It sounds like not only is the raw water pump 
above the water line of your boat, but also the strainer is.  This means 
that after you shut down the generator, the water in the system slowly 
leaks backwards (drawing air from the water-lift muffler, probably a 
Vernalift).  Then, when you go to start the genny again, the raw water 
pump must pump air (i.e. suck air) until the water fills (a) the 
strainer, and (b) all the pipe up to the pump.  An agingpump does not 
like to do this for several possible reasons:
1.  There is end wear in the pump, either in the body or on the cap, 
allowing the air it is trying to pump to short cycle.  This I believe is 
the greatest likelihood.  Water won't short cycle so much as the CV of 
water is much greater, so once the water gets there it pumps.  The 
solution, depending on your pump, may be:

2.  Anew end cap (if worn), or

3.  A new wear plate (if the pump has one, some don't), or possibly both.

4.  The most expensive solution is to replace the pump.

I once has a Chris Craft Roamer that had that problem, and effected a 
"work around" by putting a large ID non-return valve in the hose 
bringing the water to the generator, just after the strainer.  This kept 
the water in the pump and the system worked OK till I could effect the 
proper repair.   There are some drawbacks to this approach, which I am 
sure you will hear about, but in a pinch it could get you going.  The 
real proper fix is to make your pump a good air pump once again.

Ken Bloomfield
M/V Tellico Lady, 50' Marine Trader
Maryville, TN

David Poorvu wrote:
> I am having a problem with my Onan MDKAE-3376937 generator. It looses raw
> water prime if not running while underway.