Hi and thanks for using 2CarPros.com.
It is not uncommon for a vehicles to lose a little coolant over time due to evaporation from the reservoir, but a significant loss of coolant over a short period of time usually indicates a leak, a radiator cap that is not holding pressure or a cooling system that is running too hot.
Since you are seeing steam from the overflow and the vehicle is not overheating, chances are the radiator cap is bad and not holding pressure. That will allow coolant to evaporate via the overflow. However, as I mentioned, if you are losing a lot of coolant, check for a leak.
Please keep in mind, coolant does have a boiling point. For it to be its strongest, it must be mixed 50/50 with water. If the coolant you have is weak (too much water), the boiling point will be reduced. For example, a 50/50 mixture will boil at around 270 degrees F. Water, on the other hand, boils at 212 degrees F. That is much lower and aprox the temp at which your engine operates.
Here are a couple links that may be helpful:
Let me know if these help or if you have other questions.
Wednesday, August 29th, 2018 AT 6:59 PM