I have a 2006 3.5L Dodge Magnum. The car started overheating about a month ago. We went and tested the radiator cap at AutoZone and they said it was not holding pressure, so we bought a new cap. The car has been not overheating for about 2-3 weeks now. Yesterday the car overheats. So I checked the coolant and it was empty. I filled it up last night and have not drove the car since then. I just went out to check the coolant level and it was empty again. Where is all this coolant going? Because it is not leaking into the oil and it is not leaking on the ground. Can you help me figure this out?