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
"Thanks Charlie"

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:moseley@u.washington.edu

Steve,
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 kemosabe@digitalexp.com

Chap,
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:moseley@u.washington.edu

Steve,
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 ronphylhill@erols.com

Steve,
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.
Trrizz@aol.com

Steve,
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 cpearsall@smcplus.com

Steve,
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 there.
Dave Smith, C34 Celestial Melody dsmith1@amica.com

From: moseley@halcyon.com (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

Steve
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,   cpearsall@smcplus.com 

Steve,
I'd go with one of the "liquid gaskets" available in most auto stores.
Gary Wiseman,   Up Spirits #894,   gwiseman@ftn.net

Steve,
Call these guys, they are great and the prices or great as well. 707 426-6670. Co name?
David, C34 #247 DAucella@aol.com

Steve,
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 202M-15 pump.
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 feel bad.
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 moseley@halcyon.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.
Trrizz@aol.com

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 accuracy. This page's maintainer cannot guarantee the accuracy of this 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.