Q: I have a coolant leak...recently changed the expansion tank...now I have a leak right in front of the radiator 2008 BMW 750Li

Hello - there is an air bleed hose that runs across the top of the radiator (under a plastic cover) and into the expansion tank. There is a coolant level sensor, and a coolant transfer hose (expansion tank to the water pump) that could be leaking onto the undertray. Any of these connections can cause leaks where you describe - but, my money is on the air bleed hose - it is terribly easy to cut when replacing the expansion tank. I recommend a coolant leak inspection performed by a mobile, professional mechanic, such as one from YourMechanic, who will come to your location, confirm this diagnosis, and give you an accurate assessment of damage and cost estimate for repairs.