Low to no raw water

Black Pearl

Black Pearl

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Jun 22, 2004
47
Hunter 35 St Augustine
I am helping a friend with a Yanmar 3gmd raw water cooled engine. I stated that he would have to get a dock hose to get the raw water to come out the exhaust then it would be fine. Kind of like an air lock. Then the last time that did not help and the engine over heated.

I know enough to help a bit. The first thing I did was pull the raw water pump and check the impeller. It looked fine but went ahead and replaced it. Put it back on still no water. I chased water through the system and got blocked at the exhaust elbow. Replaced that and thought would be home free....No. Now that the raw water pump could push water the seal was leaking badly. So I got a new seal and after a lot of headache replaced the seal.

Still no raw water. At first there was a bit and I could definitely hear it gurgling in the hose but no significant discharge. I thought it might still be air locked and used dock hose to no avail. Took the discharge side hose off the pump while running and no water. The strainer is relatively high mounted so I took it loose and dropped it to the water line. No. I have sucked water through every hose and no blockage that I can find.

The only thing I can think of is for what ever reason the raw water pump is not pumping. But that is hard to believe or the exhaust box is completely stopped up.

Any thoughts on this? I am banging my head now! I was originally going to replace the raw water pump but it is not the $125 variety like mine it is the $435.

Thanks!
 
Jackdaw

Jackdaw

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Nov 8, 2010
11,386
Beneteau First 36.7 & 260 Minneapolis MN & Bayfield WI
Scale, and/or bits of broken impeller blocking passages, open and check everywhere you can, in particular at elbows.
 
Black Pearl

Black Pearl

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Jun 22, 2004
47
Hunter 35 St Augustine
That is what I was originally thinking but with the engine idling and I take the hose loose on the discharge side of the raw water pump there is no water. Now that I think about it that would rule out the exhaust box too.......

I guess it has to be the raw water pump.....
 
Jackdaw

Jackdaw

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Nov 8, 2010
11,386
Beneteau First 36.7 & 260 Minneapolis MN & Bayfield WI
Ah. the pump is belt driven on the Yanmar correct?

Not a lot to go wrong. Is the retaining pin on the impeller in place?? ;^)
 
Black Pearl

Black Pearl

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Jun 22, 2004
47
Hunter 35 St Augustine
Thanks for your replies!
I put her back together correctly and she spun freely and correctly before cover was reinstalled.

I was just thinking that I should of for just a minute taken the strainer out of line and hooked the raw water intake directly to engine just to eliminate the strainer. At one point I thought that the hoses must of been hooked up backwards on the strainer but they werent....
 
joker460

joker460

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Apr 27, 2010
968
Beneteau 352 Hull #276 Ontario
If you have a strainer in series with the intake you may have an air blockage.

If I open the strainer or the impeller and put it back together, I lose prime.

I then have to shut off the intake seacock, open the strainer and fill with water, close the strainer, open the seacock and then the pump is primed and works for the season.
 
Black Pearl

Black Pearl

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Jun 22, 2004
47
Hunter 35 St Augustine
I think that must be where the problem lies....the rw water pump has got to be spinning and pumping it just does not have anything to pump which puts me back at the strainer or intake. I thought by lowering the strainer to water line that would cure that but maybe not.

Thank you
 
B

Benny17441

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May 24, 2004
7,185
CC 30 South Florida
"took the discharge side hose while running and no water" Your problem is at the intake. Make sure there is no blockage at the intake port. Remove the hose at the thru-hull and open the valve and water should gush up an inch or two. If it just trickles you have a fouled intake clam. make sure the intake hose is not clogged. Running the impeller dry can damage it rather quickly so once you find the obstruction you may want to check that impeller again.
 
Black Pearl

Black Pearl

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Jun 22, 2004
47
Hunter 35 St Augustine
Ok taking the hose off seacock, which was the first thing a tested, does get full inflow of sea water. Hooking the in take hose directly to raw water pump I get a full discharge of cooling water from exhaust. The strainer is about 2' above water line with 4' of hose from seacock to strainer and 4.5' hose from strainer to raw water pump. Is this just to long of a run for this little pump? There is no vented loop either.

Should the strainer be relocated lower as mine is on my boat with a vented loop in line?

Thanks!
 
Allan12210

Allan12210

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Jan 22, 2008
1,670
Hunter 34 Alameda CA
Black Pearl said:
Ok taking the hose off seacock, which was the first thing a tested, does get full inflow of sea water. Hooking the in take hose directly to raw water pump I get a full discharge of cooling water from exhaust. The strainer is about 2' above water line with 4' of hose from seacock to strainer and 4.5' hose from strainer to raw water pump. Is this just to long of a run for this little pump? There is no vented loop either.

Should the strainer be relocated lower as mine is on my boat with a vented loop in line?

Thanks!
Click to expand
Make sure the lid on the strainer is seated tightly. Sometimes the gasket fails. Mine is cork and had taken a set so no matter how tight I made the two wingnuts it sucked a bit of air and the pump wouldn't prime. The wingnuts on mine have to be tightened equally as well.
 
B

Benny17441

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May 24, 2004
7,185
CC 30 South Florida
I agree the strainer should be below the water line. Don't see the need for a vented loop. It is just asking for problems. If you are worried about a breakage in the hoses and clamps, close the seacock valve and lay the keys on it.
 
joker460

joker460

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Apr 27, 2010
968
Beneteau 352 Hull #276 Ontario
I wouldn't mount the strainer below the water line unless it was designed for that. You can check that out with the manufacturer

Attached is the recommended installation for a Vetus strainer which is used in a lot of boats (mine included).
 

Attachments

Black Pearl

Black Pearl

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Jun 22, 2004
47
Hunter 35 St Augustine
The hoses to and from the strainer I just installed and are free and clear.

I guess the strainer is acting like a non-vented loop currently. If the strainer were below the water line then that is where a vented loop comes into play. I did try taking the strainer off its mounting and put it as low as possible in the boat and it seemed to help but not as good as when I connect intake directly to the pump.

Just one of these pain in the butt not logical problems.....
 
kloudie1

kloudie1

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Nov 6, 2006
10,116
Hunter 34 Mandeville Louisiana
I agree with Allan.. an air leak at the strainer (or the strainer connections) is most likely.. Mine would leak air in at high RPM, but water would not leak out when not running.. Problem disappeared after a strainer rebuild with new gaskets.
A vented loop in the suction side would allow air into the pump suction always and not let the pump work.
 
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