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Go Back   SailNet Community > On Board > Gear & Maintenance > Engines > Diesel
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Diesel This is a new forum dedicated to diesel engines and their applicable accessories.


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Old 07-30-2009
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Air blockage in cooling line.

I just rebuilt the raw water pump on my Yanmar 2GM20. The pump worked fine when I took the boat out, I probably motored about 30 minutes. Went for a sail, and when I fired the motor up to come back, no water was coming out of the exhaust. I took off the raw water strainer top, ran the motor, and still no dice. I ended up disconnecting the water hose where the pump supplies water to the motor and the pump was then able to supply water again. I motored back without any problem. I assume there must have been an air blockage somewhere in the line (probably at the impeller?). If so, how would it develop after initially running for half an hour? If not, what might have caused it. Is my method the best way to fix the problem should it happen again?
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Old 07-30-2009
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some people lube the heck out the impeller when they change it to help it prime. i have heard of some using lithium grease and some Vaseline. i dont know if anything is wrong with either some body else will know for sure.

also you might have just picked a trash bag they fell away when you shut it down to check it out
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Old 07-30-2009
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Both of my raw water strainers work on a suction principle. The are mounted above the water line. With the top off, they would not work at all. Once the water already in the strainer drained out, the engine would be dry.

Is yours mounted below the waterline?

If you ran the engine for an extended time without water in the pump, you probably damaged the impeller. The instructor of the engine course I took at Mack Boring recommended coating new impellers with olive oil. This makes them easier to install and keeps them lubed during the initial startup while the pump is dry.
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Last edited by HerbDB; 07-30-2009 at 07:16 AM.
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Old 07-30-2009
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Quote:
Originally Posted by HerbDB View Post
Both of my raw water strainers work on a suction principle. The are mounted above the water line. With the top off, they would not work at all. Once the water already in the strainer drained out, the engine would be dry.
Is yours mounted below the waterline?.
Yes it is. In fact, when I do it, I have to control the flow with the intake valve. A while back the same thing happened and someone suggested taking the top off and it worked. Not sure why, but I think it reduces the surface tension in the rest of the line. Doesn't really make sense to me because I would think the water pressure from the intake would offset any surface tension. Anyway, it didn't work this time!
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Old 08-05-2009
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Still having the problem! The boat sat for a week after the last episode. Motored out, no problem. Went for a sail, and started the motor to come back in.... no water! Same solution. Seems like heeling while sailing must have something to do with it. But that makes no sense to me. The only thing I changed (other than the rebuilt pump) was the 1/2" hose that goes from the pump to the motor. Bought it from an auto parts store, but It's cooling hose and seemed Identical to the hose that came off. Any ideas?
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Old 08-05-2009
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Sounds like the pump may be losing its prime. Most likely cause is wear in the pump that has opened up the clearances and prevent the impeller from sealing correctly and drawing water in from the suction line. Check for scoring in the cover plate. If its scored, either file it smooth or replace it. An air leak in the suction line could also be part of the problem.
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Old 08-05-2009
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ok ive had this problem recently,after sailing u probably have heeled the boat quite a bit & in doing so dumped out all the water in exahust line from the water lift back....it takes a few minites to fill up again !! try pullins off the sea water line going into the exhaust if there is water there than just wait a minite or so till water apears at stern[after re connecting line]
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I too suspect the pump. An impeller pump is designed to be self priming. If it won't pull the air out of the hose, something is wrong with it, or you have an air leak in the system.

You did not have the problem with the boat sitting a week, so an air leak is unlikely. I would focus on the pump. Many people carry a spare anyway so they can change it out and rebuilt the spare pump later.
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Old 08-05-2009
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Quote:
Originally Posted by norsearayder View Post
ok ive had this problem recently,after sailing u probably have heeled the boat quite a bit & in doing so dumped out all the water in exhaust line from the water lift back....it takes a few minutes to fill up again !! try pullins off the sea water line going into the exhaust if there is water there than just wait a minute or so till water appears at stern[after re connecting line]
Interesting. So, this would mean that the boat has always done it, but that I've become aware of it now, because I've rebuilt the pump. Classic mechanics syndrome, where a mechanic fixes one thing, and the vehicle owner suddenly becomes hyper sensitive about things that have gone on for years. Drives my brother (a mechanic) crazy! Come to think of it, I'll usually check for flow at the dock, but seldom when I'm out, so this makes sense. I don't think the pump is the problem, as it is rebuilt (including a new shaft), and draws strongly when I go out and when I flush the system at the dock. I'll test the theory and report back. Hopefully, I won't report that I don't have the problem anymore because I have a new engine !
Thanks!
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Old 08-12-2009
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Quote:
Originally Posted by norsearayder View Post
ok ive had this problem recently,after sailing u probably have heeled the boat quite a bit & in doing so dumped out all the water in exhaust line from the water lift back....it takes a few minutes to fill up again !! try pullins off the sea water line going into the exhaust if there is water there than just wait a minite or so till water apears at stern[after re connecting line]
You are right! The engine on my Yankee 30 is midships, so the exhaust water has to travel over half the length of the boat, to exit. After the water in the exhaust line has drained by the heeling of the boat, it takes about 2 minutes to get to the Stearn. When you've just rebuilt the water pump and are sensitive to cooling issues, that seems like an eternity! All is well, thanks for the insight!
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