Fresh Water Cooling - loss of antifreeze

[musella]

Hi. I am new to this so excuse the terminology if I have it wrong

I have a 1 year old mercruiser 5.0l i/o, with fresh water cooling. Since it was new, I would check the fresh water tank before starting each trip, and it was always a little low - so I would add about an ounce of antifreeze/water mix.
The temp gauge never has gone above the normal range.
I brought it back to the dealer and he told me it was normal and not to worry about it.
This year, it has been getting worse - sometimes it would take 3 ounces. (I went through 1.5 gallons this season so far) So I took it to another mechanic and he said it is definately not normal. He did a pressure check, and there was no leak. He changed the radiator cap and that didn't help. So he connected a plastic reservoir to the end of the "overflow"? tube. (A clear plastic tube that goes from just under the radiator cap, straight down to empty into the bilge).
Turns out the missing fluid is going into the reservoir. Is this normal?

[Don S]

Turns out the missing fluid is going into the reservoir. Is this normal?

Yes, it's normal. Just like your first mechanic told you.
The problem is you are overfilling the heat exchanger. (You should NOT fill it all the way to the top) Then when the engine warms up and the anti-freeze expands it has to go some place.
The "Radiator cap" is also a pressure relief valve. When the pressure builds too high it releases some out that vent you now have going to the plastic reservoir.

[musella]

Thanks..
You would think the dealer would have told me that