Several days ago, I had a large coolant leak on my 2000 ML320. The leak originated from the hose which leads to the expansion tank. The coolant was spraying out at the location where the hose connects to the tank. Thinking that a stuck thermostat might be the culprit, I replaced the thermostat. Everything seemed fine for several days, until today I had the same problem. Does anyone have any ideas? The water pump was replaced last year, and the thermostat last week. Could it possibly be the expansion tank valve stuck, or is it a hose issue? Any help would be greatly appreciated.