i use a dodge caliber and I have noticed the water boils in the reserve tank when the engine is hot and also when I park it overnight ,I have to refill the reserve tank the next morning because it is empty. what can be causing this problem ??
A strong possibility is that you have a small leak in the head gasket allowing combustion gas into the coolant system. These usually leak both ways and so you may see excessive steam out the exhaust especially when first started.
You could also be simply overheating, but you don't mention any overheat light etc.