FAQ: Raw Water Pump
Why do I Loose Priming in my Engine Raw water pump ?
Here is the collected works of assorted contributors to date. Submitted
by Charlie Pearsall
Stephen Moseley wrote:
We've got a cooling problem that has me stumped. We've got a Universal
25XP with an Oberdorfer raw water pump. The pump seems to have lost the
ability to self-prime. When primed, everything is fine, the engine runs
cool and plenty of water flows out in the exhaust. But if we stop the
motor and sail without closing the intake thru-hull, the pump loses its
prime when the intake comes out of the water on port tack, and won't
re-prime when the engine is started again. I've checked all the fittings
and the strainer O ring as suggested in this month's Mainsheet, and I'm at
a loss. We've had the boat 5 years and never encountered this problem. I'd
appreciate some advice.
Steve, C34 "Sunshine" #542 SMTP:firstname.lastname@example.org
I had the same problem on my boat. My problem was two of the blades on the
impeller were missing. I replaced the impeller and that solved my
problem.Hope this helps.
Chap Hodges, C34 Kemosabe email@example.com
The gasket is good, and the impeller is almost new, I replace it once a
year "whether it needs it or not." There are no leaks between
the intake and the pump, and it seems to be airtight. At least when I blow
on the pump end of the intake hose I can't detect any air leaks.
Steve, C34 "Sunshine" #542 SMTP:firstname.lastname@example.org
Somewhere in that system air is getting in. Thoughts--If you haven't done
this lately install a new gasket on the Oberdorfer faceplate and also for
the heat exchanger end caps. Have you ever smoothed out the ridges worn
into the inside of that faceplate?
Ron APACHE #788 email@example.com
Had the same problem a few years back? I relocated the raw water intake
hose from the thru hull to the pump so that it was always below the
waterline. No rerouting was necessary; I just lowered the hose and
refastened it. You might want to check for leaks on the suction side of
the pump before you reroute the hoses. Since these hoses are not under
pressure they don't necessarily show leaks but they can suck air.
Sounds like you are losing the siphon prime because air is entering at the
impeller or to the engine side of the system from there. Is the gasket
good on the impeller cover, or could there be some foreign crud on the
cover, gasket or housing that is preventing a seal? Can you see any water
leaks when the engine is running and water is flowing? How old is the
impeller? It's not mentioned in your letter, so at the risk of sounding
insulting: when was the last time you replaced yours? They only live two
to five years, depending on how long they last.
Charlie Pearsall firstname.lastname@example.org
Excepting the common obstruction type causes for this problem a very
likely candidate for your problem is scoring of the pump cover. This cover
needs to be perfectly smooth otherwise it will not pump. I know, been
Dave Smith, C34 Celestial Melody email@example.com
From: firstname.lastname@example.org (Steve Moseley)
Thanks for the replies on my pump priming problem. I checked the face of
the pump cover for scoring and there was some, so I smoothed it using fine
emory cloth on a flat surface. I was really hoping this was the answer,
but when I reassembled the pump and fired up the engine, it still did not
prime. The problem started shortly after I replaced the impeller, which I
do every year, and it finally occurred to me that one difference this time
was that the new gasket they sold me with the impeller seemed thicker than
usual. The pump gaskets I have used in the past have been really tissue
thin, flimsy, easy to tear little devils. This time the gasket was more
substantial. Guess what. I took the gasket off and put the face plate back
on without a gasket, fired up the engine, and the pump quickly primed
itself and that lovely sound of water gushing out the exhaust was back. Of
course now the pump leaks with no gasket. So tomorrow I'm off in search of
a thinner gasket, or some thin gasket material I can cut to fit. Or I
could grind the pump body down a little bit. This is a standard
maintenance procedure on Norton motorcycle centrifugal oil pumps, but I
hate to do that to the Oberdorfer if I don't have to.
Steve, C34 "Sunshine" #542
You could try the old cereal box cardboard emergency gasket system and
make a gasket using the old one as a pattern. My '71 Dodge Demon used to
thrive on these (boy, I miss the slant-six). If it works, great. If it
doesn't, what have you lost?
Charlie Pearsall, email@example.com
I'd go with one of the "liquid gaskets" available in most auto
Gary Wiseman, Up Spirits #894, firstname.lastname@example.org
Call these guys, they are great and the prices or great as well. 707
426-6670. Co name?
David, C34 #247 DAucella@aol.com
In my original draft to your question I had "paper thin" new
gasket, but took it out--sorry! The thicker gasket or worn groves on the
inside of the pump face plate (Oberdorfer OR Sherwood) will not give the
impeller a good seal inside the pump housing. If you can't find the
correct gasket or in an emergency use a THIN film of #2 NON-harding
Permatex to seal the face plate to the pump body. You can order the
correct size gasket from any dealer that stocks Universal engine parts or
DEPCO 1-800-445-1656. Tell them you want a #6599 gasket for an Oberdorfer
Over the years I've been told that impellers are like light bulbs-some
last longer than others. I always spray my new spares with silicone and
keep them stored in a ziplock bag. Over the winter I remove the impeller
and keep it stored in a shallow cup under the engine after I give it a
shot of silicone. This keeps the "rubber" soft and plyable. The
problem is you don't know how long they've been sitting on the store self
drying out. I used one impeller for 4yrs. changing it because I started to
This fall I changed out a 2-1/2yr. old impeller on general principles.
Three weeks later I was winterizing the engine and had sucked up one
gallon of the pink antifreeze. I shifted the hose to the second jug and
Nothing happened! I revved the engine-still nothing! Shut the engine down
and sucked the antifreeze to the intake of the pump and replaced the
intake hose. Started the engine-still nothing. Shut the engine off and
scratched my head!?! Can't call the C34 Tech. Ed.! -- so I removed the
Oberdorfer face plate. NO fluid ran out while loosening the screws! The
impeller was missing 2 blades and the 3rd was ready to go. I was lucky to
find the missing blades in the output elbow and remove them with a
hemostat. Otherwise I would have had to retrieve them from the heat
exchanger. Moral of the story -- just because you recently replaced a part
don't let the OBVIOUS cloud your mind.
Ron Hill Apache #788 (Mainsheet C34 Tech Editor)
My priming problem turned out to be entirely due to a gasket that was
too thick. I bought a paper gasket at the "authorized parts
dealer" for Universal, and the pump primed itself fine after that.
The difference in thickness between the gasket that worked and the one
that didn't was VERY slight, but enough to keep the pump from priming. It
occurs to me that a check valve in the inlet line would be a good thing.
Is there any reason not to install one?
Steve, C34 "Sunshine" #542 email@example.com (Steve Moseley)
Your raw water pump is not meant to be self priming. You might have
solved the symptom but you could still have the problem. Each time you
lose prime your pump is forced to run dry. Every revolution without the
lubrication of water shortens the life of your impeller. If water does not
freely flow from your pump with the cover off (with the engine not
running) you are asking the pump to overcome head pressure.
A check valve will ADD resistance and create more resistance to flow.
Besides, a check valve stops water from going in the other direction, only
permits flow in one direction. What would be the purpose here in the raw
water intake line, water can only flow in one direction to the pump. Only
place for a check valve is in bilge pump outlets to prevent water flowing
back in the boat, and in the drinking water system.
Stu Jackson, #224 Aquavite San Francisco, Dosgoats@aol.com
Important: The opinions expressed here are those
of the individual contributors to this page, and not those of the Catalina
34 National Association or Catalina Yachts, Inc. Additionally, this
material has not been reviewed by Catalina Yachts, Inc. for technical
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information or the desirability of suggested modifications or upgrades.
Please obtain assistance from a competent marine mechanic or boatyard
prior to making any significant modifications to your vessel.