Air in raw water strainer

[fiddlersgreen]

Air is leaking into the raw water cooling system.

Background information: The raw water cooling system for a Volvo Penta MD 11 C that is 32 years old consists of:

through hull,
seacock,
hose to a raw water strainer,
raw water strainer,
hose to the engine water pump and impeller.

I fill the raw water strainer to the top with water. (This raw water strainer is above the waterline while the raw water intake and the engine are below, or at the water line). I screw the lid on the strainer and use all the sealant as instructed in the manual. Then I run the engine. After a couple of hours everything looks fine. After 5 hours the water level in the strainer begins to go down. There is about a half inch of air within the top of the strainer. As time goes on there is more air in the strainer. By the time 10-20 hours go by there is half air, half water in the raw water strainer. If I let this go on the cooling system will be sucking all air and the engine heats way up.

What do you think???? There are no visible water leaks in the hoses, through hulls, or raw water strainer. The system should be a closed water system where a vacuum is in the hose. Question: Can air be sucked into this vacuum system without there being a water leak?????
Where is the air leak coming from? How should I seal that air leak?

Thanks for any suggestions you can give me.
fiddlersgreen-Mazatlan Marina, Sinalo, Mexico

[Charlie]

I had this problem while in Canada. I'm not an expert but this is what I did. I took apart all the hose clamps on the water hoses. I looked for any cuts in the hoses that could have been caused by old hoses or by the hose clamps themselves. I then had to make up new gaskets for the raw water strainer. This worked for awhile. After about 8 hours of motoring it started leaking again. Finally I moved the raw water strainer just below the waterline. This finally worked. Now I have to shut off the raw water strainer everytime I turn the engine off. Just so I don't forget to open it up I attach the keys to the handle for the raw water seacock. I hope this helps. If the boat is as old as the engine then it may be time to replace the gaskets on the water strainer so that you can keep the striner up over the water line. Good luck and let us know how things work out.

[AnchorageGuy]

Charlie, Why would you have to turn off the water strainer when you turn off the engine? If everything is below the water line it would seem easier to put an anti-siphon in the raw water line as it goes into the exhaust elbow. It would drive me crazy to constantly have to open and close the valve.

[Chief Engineer]

What type of strainer?

[James S]

Has any work been done on any of this system recently...in other words is this a new problem....if so, what has changed.
How much higher is the strainer than the water line?
What is its relationship height wise to the water lift/muffler?

[richardhula]

I have a similar problem with my Volvo TMD31B installation. Raw water system needs priming every so often, particularly after sailing hard to windward with lots of heel for extended time. Once primed it will work fine.

This problem started after changing the impeller. Checked the seal of the filter housing but the problem intermittently persists. I had changed the impeller several times before & it always self primed. My conclusion is that this one has some defect or other & is not allowing the pump to work at full efficiency such that it will not always pull up the head of water to the filter screen housing once the prime is lost.

Just bought another impeller so hopefully that will solve the problem.

[Sandero]

You have some sort of air leak which prevents the pump from circulating the water. Moving the entire system to below the water line will only mean that the leak, however small will continue, but you will not have the appearance of a circulation problem.

My guess is that you need to change some hoses and the strainer. If you had a leaking seal on the RW pump it would all work but drip into the bilge.

Check the rubber gasket O ring type thingies at the copper pipes which in the RW circuit as well. They are cheap and you should get a dozen or so

[Chief Engineer]

Test those donuts or as the Brits call them (olives i think) dont last forever.

Also the pre formed piping on Volvos cannot be bent out of the way and then put back....not even a little......if you muck up one one the suction side all you will get is air....no amount of goop will seal it.

Do you have a vetus strainer on it? If so it is a bad gasket or o-ring on the lid...or a crack in the housing.

[S/V Antares]

Does the level in the strainer drop to the waterlevel of the boat if left unattended with the seacock on?