My 3.6 had same problem. Coolant kept getting low. Couldn't see a leak and car would overheat about once a week. Turned out it was the " oil/coolant cooler located on top of the engine block bolted to it. Its made of hard plastic and I believe the extreme heat from bock wears out this plastic cooler and I never knew about these until I got this car. Ever since I replaced it, no more problems .