Raw Water Pump Priming Problem

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 Moseley, Sunshine #542

Responses from the Message Board:

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, Kemosabe

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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 Moseley, Sunshine #542

Cover seal:

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 Hill, APACHE #788

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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.

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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, Celestial Melody

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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 Moseley, Sunshine" #542

More on the cover seal:

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

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

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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 pliable. 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

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MYSTERY PROBLEM SOLVED ON #299

I've followed this discussion with great interested because I was experiencing the same "mystery" problem.

I changed all the hoses on both sides of the Oberdorfer Raw Water Pump from the intake through-hull to the mixing elbow on the exhaust system. I rerouted the intake hose to the front of the engine (to shorten the distance of the run) as someone suggested, and still experienced the problem with failing to have water coming out the exhaust at random times.

BOTTOM LINE MYSTERY SOLVED: The cover plate on the Oberdorfer Raw Water pump showed "circular wear groves" and this caused the pump to fail to pick up or prime itself at engine startup. The front edge of the impeller must make a tight seal to the cover plate in order to create suction and the ability to prime and pump water. I replaced the pump with a new one and the problem has not reoccurred.

What I've learned is that the raw water pump on the M25 in the 1987 Catalina 34 sits a few inches above the water line of the boat. This is easily determined by disconnecting the water line from the pump and when held at the same level as the fitting on the pump no water came out. When the hose is lowered a few inches I could see water starting to appear in the hose. When lowered below the water line water will overflow the hose.

This essentially means that the pump must prime itself every time the engine is cranked. I wonder how many C34 owners think that the raw water pump on the engine is below the water level - as did I?

I plan to resurface the face plate on the old Oberdorfer Raw Water Pump and keep it on board as a spare.

James H. Newsome, CaiLeigh Anna #299

Other Impeller Issues

Besides wear grooves on the pump plate check to make sure the metal center of the impeller is not free spinning inside the rubber impeller itself. This required some amount of head scratching and a bit of luck to even notice the internal portion of the impeller wasn't still firmly attached. I'd call this a bad design.