I have a 2001 sebring convertible v6 2.7l, after it warms up the water starts to evaporate from the…

I have a 2001 sebring convertible v6 2.7l, after it warms up the water starts to evaporate from the coolant tank (the tank that you add water to the system) and looses water. I have changed the thermostat and the problem is still there. the engine temperature is around half so it isn't overheating. what might be the problem?

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