Originally Posted by blueblisteringbarnacles
Last few weeks the plastic top has been blown off of the overflow container on the boat and dumping anti-freeze into the bilge, about a cup/round trip. Don't motor much: 5 minutes out and 5 minutes in usually.
Any ideas? I see references in the Yanmar help section to faulty heat exchange caps and faulty thermostats, but not in response to this exact issue.
BTW, per the owner's manual the antifreeze in the overflow container is supposed to cycle back into the heat exchanger as the engine cools. How does it do that when the only line into the container drops in from the top.
The correct name for the overflow container is actually expansion bottle/tank. The pipe that drops into the top should reach down to about 2/3rds or more of the way down inside the tank.
The pressure cap on the heat exchanger is designed to allow coolant (which is not compressible) to go somewhere when it expands. It goes into the expansion tank. When the engine cools the pressure cap allows the resultant vacuum to draw coolant back from the expansion tank. Consequently an amount of coolant continually flows back and forth.
If the pipe does not reach down far enough, the cooling of the engine will cause the system to draw air into to cooling system. The next time the engine gets warm, the air will expand more than the coolant and a greater volume of coolant will be pushed out because there is more expansion happening. So check that the end of the pipe is never hanging in air, it must always be in coolant.
If you fill the expansion tank so that there is inadequate space to accommodate the coolant that is oscillating back and forth, it will overflow into your bilge (coincidentally about a cup-full
). But it should do that only once. If you then top it up when it appears to be about half full, it will do it again and again until you stop topping it up.
If it overflows even after you have not
topped it up, then the chances are there is excessive pressure building in the cooling system and that points to a leaking head gasket. Alternately you need to try and get all of the air out of the cooling system.
So the short story is: Try not topping up the expansion tank, keep it about half full, make sure the end of the pipe is always in the coolant and observe the result. React accordingly.
Edit: Incidentally, the purpose of a pressure cap is to create a constant pressure in the closed-circuit cooling system because the coolant can then go to a much higher temperature before it boils (in excess of 100 degrees C). If the cap is dodgy and is not maintaining pressure, there is a chance that the engine could boil off coolant.
My first synopsis is the most likely but this edit just demonstrates that it is not the only scenario.