Perhaps you live in a warm climate, but in an area where it freezes, putting plain water in the coolant resevoir can lead to a broken tank and or frozen hoses. With a frozen hose, the fluid may not be able to expand into the tank, and the pressure will find another less pleasant outlet.

The tank does exchange fluid with the main circuit, but only a little, and the water may never become "freez proof".

As for using coolant. My 04 MCS had a proper level for the first 8 months. The first time the car was taken out for some aggressive excercise, the level dropped to "empty tank". I added fluid, and it never dropped again. It is not uncommon for engines to have an air bubble that is very hard to "burp" out. I had a Dodge that had to be burped 3 times when ever the coolant was changed.

John

 

I had the same problem regarding no coolant in the expansion tank. After refilling the tank with some coolant, I checked several times a day thereafter to insure the tank did not have any noticeable cracks. It wasn't till a couple weeks later that I noticed this white film on the expansion tank that I realized the reason for my leak was due to the cap. Once I took a look at the cap more closely, I realized it was one of those pressure release types and since I didn't notice anything visually wrong with it, I decided to screw it back on more tightly and ever since then I haven't noticed my coolant level drop. I typically check my fluids every couple weeks now and it's maintained the same level for several months now. This could be due to the cooler weather we are having, but it certainly doesn't seem to be giving me any issue. If the problem arises again, I suppose the next step will be to see if I can just purchase a new cap and see if that resolves it. I'd love to buy one of those stainless steel expansion tanks, but I'm not so keen on having to spend that much just for an expansion tank.

GotMINI