Re: MC 3.7 (470) closed cooling question
Have to agree with the above linked post pointing to the heat exchanger...if you're not seeing coolant leaking anywhere and not in the oil (definitely check the bilge though and feel around the drain screws on the bottom of the heat exchanger)...it's probably leaking into the raw water inside the heat exchanger and then going out your exhaust along with the raw water.
Manual says that if you can manage to figure out into which raw water tubes inside the heatexchanger the coolant is leaking into (probably difficult)...that you can theoretically pinch and seal up to 2 or 3 of them shut at both ends. Close any more than that and you're going to start seriously impairing your raw water flow and thus heat exchanging. How to find, pinch, and seal these leaking tubes however if it's a leak that's only going to show up when things are heated up a bit, it might be pretty tough to find.
I suppose you could pull both end caps off with the boat not running, borrow (for free) an automotive coolant pressure tester and gauge from (i.e. from AutoZone) and pressurize the refilled system to some above-normal pressure while looking through the tubes for fluid. I'm cheap so that's just what I'd try first before replacing it. If you can find the leaking tubes...and are handy enough at brasing/welding metal ends of tubes...you may just save yourself $600-$900 on a new heat exchanger.
