Uh oh. Green coolant dripping onto your Cole-Haan loafers while you're driving your M3 is not a good sign-in fact, it's a sign your BMW heater core has sprung a leak. That's the unfortunate thing about BMW heater core design (to be fair, all heater core designs): It places hot engine coolant inside the passenger compartment of the car. Granted, that coolant is usually contained in the maze of pipes and passageways that make up your BMW heater core. But as liquids tend to do, after tens of thousands of miles of driving the coolant may just find a way to escape from the BMW heater core, where it ends up leaking onto your wool carpets. Don't think a BMW heater core leak is just an inconvenience, either. Not only will the antifreeze dripping (or running, or spraying, or pouring) from your BMW heater core wreck your interior in short order, but it will also make its way to the lowest point on the inside of your car: The floorpans. The coolant coming from your BMW heater core is essentially water and your floorpans are metal. You do the math-your car can develop terminal rust in a hurry. Of course, leaks aren't the only thing that can kill a BMW heater core. Your car's radiator can clog up if the coolant isn't flushed out regularly, and the same goes for the BMW heater core. Particles of rust and corrosion enter the cooling system and lodge in the small passages inside the BMW heater core, restricting flow and reducing the amount of heat you can get from the system. So an unexplained reduction in the temperature of your heater is another sign that your BMW heater core needs attention. Preventive maintenance in the form of regular coolant changes can go a long way towards protecting your BMW heater core. If it's too late, though, change your BMW heater core and remember to maintain the system this time around.