There are more than a few possibilities. If the thermostat is stuck shut, the coolant can't circulate. Same issue if the water pump is not turning due to a broken belt. If it's leaking, and you top it off with water, the coolant will be diluted and will boil at a lower temperature than it should. If the engine is just plain making too much heat than the system can handle, the exhaust may be obstructed by something like a melted catalytic convertor or damaged muffler baffle. I would start by fixing any leaks, replace the thermostat, and make sure the coolant is at adequate strength, then run it and make sure the radiator gets hot and the electric cooling fans are turning on when they should.
Posted on September 07, 2016