"I'm very slowly losing coolant. Ex., I fill the overflow container in the cockpit locker. Some months later, it's gone"

I have a '99 C36MKII M35B engine and that exact same thing occurs to me. It has for years, pretty much since I've had the boat. I have no clue why? I have searched and searched and now have just written it off and live it. Maybe I have a small leak or tiny pinholes in the heat exchanger? It doesn't seem to be causing any cooling issues with my boat.

You can drain Antifreeze from the heater Hose going to the Hot water tank if you want. Thats one of the lowest points that the antifreeze goes. When I changed the Antifreeze a while back thats what I did. (lots of crud in the hot water tank antifreeze I fould out too)

We had this problem for several years from the time we took delivery. Turned out to be a tiny pinhole in the heat exchanger. We found out when we overheated the engine which caused the H/E to fail. When we replaced it (thankfully the only damage) the problem was solved. Incidentally, we found a much less expensive H/E on the web than our repair shop could manage, and better quality than the original.

I have had the same problem since I have owned the boat. A couple of times each summer I have to add a cup of premixed antifreeze to the overflow container . If it is leak it is so small that I cannot find it and have stopped worrying about it.

Ditto on above, I add a little each year also.

[QUOTE=davecask;5781]We had this problem for several years from the time we took delivery. Turned out to be a tiny pinhole in the heat exchanger. We found out when we overheated the engine which caused the H/E to fail. When we replaced it (thankfully the only damage) the problem was solved. Incidentally, we found a much less expensive H/E on the web than our repair shop could manage, and better quality than the original.[/QUOTE]

Dave do remember the web site and item number of the heat exchanger that you found on the web? Was it basically a plug and play per say fitting in the same place as the old one? I am sure there are a lot on here as myself would like to know of the site as I am one that also has to add coolant at times.

Randy

There are a number of manufacturers for direct drop in replacements, if you google Universal M35B heat exchanger. I think ours came from [url]http://www.manifoldwarehouse.com[/url] but I'm not sure. We ordered the copper/nickel one which is better than the original. and it was a direct and easy fit, and about $400.00, a few years ago. By the way, I don't think those minor leaks are inconsequential. When the engine heats up, the cooling system is designed to become pressurized (don't remove the rad cap!) which it can't do if there's a leak allowing coolant to be forced out of the system into the raw water exhaust. When the system cools down again, it can draw raw water (in our case, salt water) back into the cooling system, as the antifreeze contracts. So you lose some of the design efficiency of the cooling system (which can lead to easier overheating - our problem) and your coolant gets contaminated over time, neither of which is good.

I had this problem too on my 2002 Mk II. It was the heat excchanger but the problem was the bracket which holds the exchanger to the back of the engine had seperated and the H/E was rubbing ever so gently against the engine. This caused a very slow leak which got worse over time.

I'd check that area. The new exchangers do not have the H/E brackets welded onto the exchanger so you don't get this problem.