Here is the problem. I have a 525 with closed cooling system. The last two winters I have lost about a gallon of coolant from the motor each winter. It leaks externally into the bilge. My first thoughts were that I had either a cracked block, cracked head, or freeze plug issue. It passed both a pressure test and a vacuum test on the coolant system. The oil never got milky. Last summer after running the motor, it never lost any coolant and ran fine. It seems to happen during the colder parts of winter, usually in the 20's or below. Where could this coolant be coming from?

I live in Sweden and this problem isn’t that unusual here. I’ve had it on a couple of boats and it can be hard to find.

I think you got a hose or connection somewhere that when get cold the material shrinks and start leaking. It’s usually a hose clamp that needs an extra turn.

It may be leaking around the rubber grommets that the exchanger ends plug into.

Yup, same issues 496's have. Happened to mine every winter. Just topped off coolant tank each spring and boated. I will say though a gallon is excessive. I at most lost 3/4 the coolant tank's worth.

If not there (at cooler ends) . check the weep hole under the snout of the recirculation pump.

Ok, glad I am not the only one, 496HO sitting at the dock for the last couple of months and the port reservoir is half full now. No sign of coolant in the bilge and doesn't disappear when running..

Thanks for reminding me to go get some DexCool..

The 496's especially in cold winter storage all do it. You can usually see traces of the Dexcool in the bilge at Spring commissioning. It's always a little disconcerting to see the levels drop over the winter, but it's just the nature of expansion and contraction of the gaskets on the pressure side of things.

If the coolant reservoir is empty with no signs of coolant stains in the bilge, I might be worried. Check the oil level if that is the case and see if the coolant worked its way into the pan.

I have, oil is clean no pink/orange in the bilge. Temps are in the 60s to 80s. No clue where it is going!!

I had the same thing happening on my 525's. It was the circulating pumps that went bad letting coolant drip through the weep hole. I replaced mine with new ones when I had the motors rebuilt and the problem is gone now.

https://www.perfprotech.com/mercury-...9/80?model=664