<VV> DIsappearing Water Question - No Corvair, obiouvsly

[Grant Young]

Having had no luck with the PT Cruiser Forum, I am hoping to find some experience or knowledge on a water cooled mystery I have with my wife's '05 PT Cruiser GT. I noticed that every time I checked it out that the coolant overflow tank was below the low level line. I would top it up to the full when cold line, but would find the same problem the next time I checked, often the very next day. I have not ever had to add water to the radiator as it is always visible when the radiator cap is removed. I had the radiator out a few years ago to replace the A/C system, but bled it properly and there has never been an overheating problem. I removed the overflow tank today to confirm it did not have a leak. It is a simple system with a single hose from the radiator fill neck to the overflow tank. The tank has a vent near the top. There is no staining anywhere that gives a clue about the water leaving the overflow tank. I quit adding antifreeze and simply top it up with water now due to the cost. While it has not caused a problem, I am not one to simply leave well enough alone when it doesn't do like it should. It has not always been this way, just during the last year. Any ideas? No I can't get rid of it because it only has 84K and the wife likes it too much and wouldn't know about the issue otherwise and doesn't like to drive my Jag. Thanks for any thoughts.
Grant