My sons 300E was running warm and overheating. I would add water/coolant and 2-3 days later add again. I saw no H20 leaks and was completely baffled. Well finally, you know what happened. On a very hot day in LA, the car starting to blow steam out of the exhaust. Luckily I stopped the engine quickly and saved the motor. "Long story shortened". After removing the head I discovered there is a engine block water drain plug/petcock that is located under the exhaust manifold at the rear of the block that I could see or be seen, at least I could not see it, until I removed the head. The water being lost form this plug/petcock was leaking at he same firewall location as the AC condensate. The plug had rusted through and progressed to constant water loss. I suggest do not drive the car, try to look way up under the car under exhaust and see if this is it. good luck Paul in LA
89 420SEL
89 300E
93 500SEL
65 Mustang Fastback
2002 Explorer