Sea water cooling system

[Endeavour]

We have a Volvo penta D2 40 and we are losing sea water out of the strainer. There are no leaks in the engine bay, but before starting the engine we have prime the pump. The heat exchanger was tested at the weekend and was OK. I have no idea why the water is leaving the strainer anyone got a cue?

[Salty]

Is your sea water inlet valve open or shut at the time when you notice the loss of water from the strainer? If it is open, you may have air leaking into the raw water system thereby allowing the water to run back to sea. If the sea water inlet valve appears to be shut, and water is being lost there are two possibilities. One is that the sea water valve is defective in one way or another, the other is that water is being lost within the cooling/exhaust system. To check that the sea valve is working properly, put it in the shut position, then disconnect the sea water inlet pipe from where it connects to the strainer. If the pipe is already empty then fill it with water from a jug, and while holding the end of the pipe as high as it will go, watch what happens to the water level in the pipe. If the level starts to drop away, then you have either a leaking sea valve, or one that is not fully shut. If the water level remains constant with the valve shut then you have ruled out the leaking/not fully shut possibilities, but from this point on things are starting to look expensive. Good luck.

[Symphony]

Check the seal on the lid of the strainer. Common for these to not seal properly, either because of wear or more likely distortion because the cap is not screwed on properly. It only needs a very small amount of air to be sucked in for the strainer to empty back through the seacock.

[Endeavour]

Thanks Just trying that now. I will post if its a positive outcome

[MarkTheBike]

Hi Endeavour
How old is the engine? I agree with Symphony about the cross-threaded strainer cap. Ours is an MD2030D/2001 and we had the same problem all the time. I checked the seating and cleaned it all up. The problem got much better but didn't stop altogether; every now and again, it would drain away, even after stopping for a couple of hours. A few weeks after I'd sorted the cap, we were hoofing it at 3/4 throttle and the higher than normal pressure in the heat exchanger blew an existing tiny corrosion pinhole through into the manifold, losing all the coolant and raw water into the exhaust gas stream. I'm not saying it's the same, but if you clean up the cap and there are no splits or leaks and it still loses water, then it may be worth having the heat exchanger pressure tested (a manual tester is fine), just to see if there's any pressure loss. Quick and inexpensive and might save you from enjoying that same heart-stopping moment when the overheat alarm starts yelling. But clean the cap first! Common things happen most commonly.   

[Spirit of Mary]

Hi,

I would consider it as normal, because the seawater cooling circuit is provided with an aerator (located in the top of your engine room). This device takes care of that your system is not siphoning when the engine is not running.

Ger

[Salty]

True Ger, but where is the water going from the filter? Is it going back out to sea through an open or leaking sea valve, or elsewhere?

[Spirit of Mary]

The aerator is between the seawater pump and the heatexchanger. So part of the water is passing the seawaterpump when it has some leakage. Part of the water is goingvia the heatexchanger into the exhaust. Maybe when the seawaterpump is not in an optimal condition and the engine is standing for a long period, you loose almost all the water via the inlet strainer back to the sea. When you close the valve then you can prevent this. But don't forget opening this inlet valve when you start the engine again.

Ger

[Salty]

Endeavours problem was that he was losing water out of the strainer. If the sea valve is open or leaking and IF the vacuum relief valve allows air into the line, then water from the strainer can run back to sea. Likewise if the lid on the strainer is not properly shut or it is leaking. If the sea valve is shut and not leaking, water cannot leave the strainer to a point lower than the bottom of the outlet line between the strainer and the vacuum relief valve, no matter what happens unless the inlet line from the sea valve is leaking. But Endeavour noted that there were "no leaks in the engine bay." Also, on my boat the vacuum relief valve is located between the strainer and the raw, sea water pump, not between the pump and the heat exchanger.

[MarkTheBike]

...Also, on my boat the vacuum relief valve is located between the strainer and the raw, sea water pump, not between the pump and the heat exchanger...

+1

[Spirit of Mary]

Also, on my boat the vacuum relief valve is located between the strainer and the raw, sea water pump, not between the pump and the heat exchanger.

-1
When this would be trough, the aerator or like you call it the vacuum relief valve, would let air in the sea water system because in this part of the system (the suction site of the raw sea water pump) there is likely an under pressure (suction pressure). So I don't believe that the aerator valve is located there. The vacuum relief valve needs an over pressure to close and that is what you have at the discharge side of the raw sea water pump. I am not on the boat, so I can't verify this.

Ger

[MarkTheBike]

Oops - Ger, you're absolutely right! My memory is worse than I thought. I've just checked a photo of the engine bay and sure enough: Flow = seacock -> filter -> raw pump -> relief valve -> heat exchanger. My apologies to all.

[Spirit of Mary]

In the mean time I checked several sources and indeed the anti siphoning valve should allways been located downstream the raw sea water pump, upstream or downstream the heat exchanger, which to my opinion is logical. With Bavaria there is no space downstream the heat exchanger. So it is located between the raw sea water pump and the heat exchanger at least 300mm above the water line (in the top of the engine room).
So if the raw sea water pump is slightly leaking also the water upstream the anti syphoning valve (vacuum breaker) can leak back to sea via the inlet strainer! The water downstream this valve is draining towards the exhaust (limited amount). When this valve is not functioning/blocked seawater can siphon into the engine when sailing under heel.

Ger