Sounds as if engine has a possible cracked cyl head or blown head gasket. Is engine gasoline or diesel? 2010 diesel had a long history of cracked heads
By outflow, do you mean the tube connected to the top of the radiator?...If so, get a new radiator cap first...Newly acquired 2010 is dumping anti freeze out the overflow. It was just idling, not under load. I filled it back up and it did the same thing. I don't know if the temp gauge works but the engine didn't act or seem hot. Any thoughts?