Cooling System Pressure Loss
Now onto the more likely causes of overheating… The cooling system is designed to operate under pressure. The pressure allows the coolant to reach higher temperatures before boiling. As a safety feature, there is an over pressure release mechanism. But there are a few other factors to cooling system pressure loss.
The pressure release mechanism has springs inside of it. These springs are calibrated to hold the correct amount of pressure at a particular temperature. The radiator cap is a dynamic component, and might not hold full pressure despite sealing correctly. Additionally, a cracked overflow tank will sometime cause pressure loss even without coolant loss.
When the system is open and pressure is released, coolant will boil at a significantly lower temperature. As coolant is comprised of 50% glycol, its boiling point is lower than water. Another factor that lowers the boiling point is elevation. So reaching an engine temperature of only 85 C could result in a boil over.
It’s important to know that pressure loss is not always apparent at lower temperatures. However, if the pressure loss occurs at a temperature above the unpressurized boiling point, the coolant will boil instantaneous. In short, a boil over can occur without extreme temperature. But as boiling fluid has a decreased ability to transfer heat out of the system, should the coolant start boiling, the engine temperature will rise quickly. This is when an overheating alarm can minimize engine damage.
There is a complete article that explains how coolant mixture affects boiling point.