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The two black hoses meet at the top. One goes to exhaust elbow, the other comes from thermostat. Th smaller hose coming out of top is not connected to anything and drips water when engine is running. What is its purpose and should it be connected to something?
Thanks. Sorry picture upside down
 

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the Y fitting that has the small vent line on top is a anti siphon Y to prevent sucking water into the engine when stopped, if the engine is mounted below the waterline. the small line should have a duckbill valve in the top of the Y fitting that allows water to enter the Y when the engine is stopped, when running the valve is closed. if there is a duckbill valve it is leaking. some do not have a valve and let a small amount of water to run out when the engine is running and should be piped over board. some just let it drip into the bilge.
 

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Yes, if that hose is dripping, then the little rubber duckbill valve probably has something caught in it. Common thing. Unscrew the fitting at the top of the Y and look for a small rubber piece and clean it up. If the valve was missing completely, I'd expect a stream of water, and not just drips.

Mark
 

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I've never really understood this system.
Since the raw water intake and the motor are below the waterline, the water pressure is coming from the motor, and exists as long as the intake valve is open, correct?
I assume that the theory is that, without the engine running, the exhaust water hose will fill the exhaust system, without exhaust pressure to remove the water. Then, water will enter the cylinders via the exhaust manifold.
How does the anti siphon valve differentiate between the natural water pressure ( When the motor is not running), and the pressure from the water pump (when it is running)?
Edit: Having gone through all this, I think I just figured it out. The valve allows the pressure in the hose to equalize with the pressure at the water line. The added pressure from the water pump when the motor is running overcomes this equalization, and allows the water to get over the loop. Is this correct?
 

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the duckbill valve on top of the y does not see any pressure when the engine is stopped because it is above the waterline and it can allow air into the system so it will let the water drop back to waterline . no water enters the exhaust elbow . when running the valve is closed so water stays in the system
 

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Whether its a duckbill or little butterfly valve on a high loop, its purpose is to allow air back into the system, when the engine stops, preventing further syphoning ,filling the exhaust and then the engine and then the oil sump and then the bilge This could get serious. Seem like a lot to ask of a bit of neglected rubber.Understanding the system may help and a properly designed pisser has no moving parts and even imparts valuable information if visible.(sort of like the stream of water coming out the back of an outboard)
 

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The problem with the anti-syphon valve is they often stick closed flooding the engine with back syphoned water. I do away with the valve entirely and replace with a hose barb on the top of the anti-syphon 'Y' then run the hose coming out of it overboard with a loop as high as you can get it.

Had a factory specified anti-syphon valve on the Volvo engine in my boat that kept sticking closed and flooding the engine. After the third flooding, did away with the valve on 'Y', ran the hose overboard and never a problem for the 10 years we owned the boat.
 
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