M-25XP Raw Water Blocked

Jan 30, 2021
Catalina 30 Savannah
I just replaced my Oberdorfer raw water pump on my Catalina 30, same pump as before. Never had any issues with previous pump, but the weep hole had just started weeping so I figured better change it now. Worked fine for a few days, then it stopped and I found that the impellor was "spun". The shaft and inner part would spin, but the rubber didn't move. Scratched head, replaced the impellor, worked fine for a few days. Now it intermittently stops spitting water out the exhaust. I start troubleshooting by pulling hoses and running the engine all the way from the pump to the exhaust inlet. Everytime I pull the hoses it is pumping fine. Heat exchanger is also only 100 hrs old.

I've started to think there is some type of air block in the system. This could keep the water from pumping until I pull the hoses and relieve the pressure. I reckon enough back pressure could've led to the initial impellor spinning, because this all has to be related somehow. I've taken the impellor cover off and bumped the starter, impellor spins fine now. The raw water system is open, so I'm not sure how water could get trapped. I've checked and replaced any problem hoses. I'm not entirely clear on how the aqua lift muffler works so maybe that? Any thoughts would be greatly appreciated!
Jun 11, 2004
Oday 31 Redondo Beach
Have you pulled off the heat exchanger cover to check for debris there?
Last edited:
Nov 13, 2013
Catalina 34 Tacoma
Did you replace the impeller with the same brand that was "spun"? I had the same problem with the black impellers but not the blue.
Jan 25, 2011
S2 11.0A Anacortes, WA
Where is your raw water strainer wrt the waterline? If it is high, do you have air in it?
Feb 26, 2004
Catalina 34 224 Maple Bay, BC, Canada
Are you sure your thru hull is clear? Have you blown back through it? Looks like you're approaching it systematically, but only halfway there.
When you replaced the pump, did it have a new faceplate or did you use the old one?
Jan 4, 2006
Hunter 310 West Vancouver, B.C.
I start troubleshooting by pulling hoses and running the engine all the way from the pump to the exhaust inlet.
I see you've proven everything up to the inlet of the exhaust. Looks like you've eliminated pretty well everything to that point. Only thing I can add is:

- what is the pressure like coming out of the hose attached to the Xhaust inlet when the engine is revved ?
- any chance there is a blockage IN the Xhaust system ?


Jan 20, 2005
Nauticat 321 pilothouse 32 Erie PA
2 thoughts: 1/ i ve seen old raw water feed hoses delaminate from the inside . w good suction from a new pump the inner lining actually pulls away from the exterior, partially collapses under pump suction, and reduces h 2 0 flow into the pump, causing you to think it's a lousy pump or worn impeller. replace that length of raw water feed hose, it's cheap. 2/ volvos were notorious for defective impeller hubs that spun freely causing zero water flow. i now test the 'new ' impellers by trying to twist the vanes off the hub by hand . (a few times i succeed.)

Ward H

Nov 7, 2011
Catalina 30 Mk II Barnegat, NJ
I had l issues with water flow on my M25XP and OB pump. Intermittently I would have no water flow like it lost the prime and could not pull the water into the pump.
Changed impellers, pulled hoses, checked HX, etc. It mostly occurred after a day of sailing.
The pump cover plate O-Ring look to be in good condition but I installed a new one, lubed with dish soap anyway. That appears to have solved the problem.