Water Vapor and Its Relationship to "Overheating"

The temperature at which an engine using a water-based coolant "overheats" is the temperature at which vapor in the cooling system presents itself. That temperature is the boiling point of the water for the pressure of the system.

When water-based coolant boils, the vapor generated is almost entirely water vapor, no matter how much glycol is in the mixture. The glycol part of the vaporized coolant condenses immediately, but not the water vapor component. The water vapor cannot condense unless the immediate environment is colder than the boiling point of water, a temperature that is lower than the boiling point of the glycol-water coolant.

Water vapor in a cooling system always occupies a volume that displaces a like volume of liquid coolant from the location of the water vapor. The hottest parts of the cylinder head are the likeliest locations for localized boiling of coolant and water vapor creation. If the nearby surrounding liquid coolant is hotter than the boiling point of the water at the local pressure, the vapor will remain at that location, forming an insulating barrier between the hot cylinder head metal and the liquid coolant. How good is the insulation? Water vapor has just four percent of the thermal conductivity of a liquid coolant that is half water. With that insulation present, less heat is conducted from the hot metal, the metal temperature rises, and a "hot spot" forms.

A cylinder head hot spot in any engine, stresses the metal, possibly causing the head to wrap or crack. In a spark-ignition engine, the hot spot can be a site for pre-ignition and detonation.

Evans waterless coolants don't contain water and they have a boiling point considerably higher than the temperature at which they are operated. The huge separation between operating temperature and boiling point means that any locally generated vapor immediately condenses into nearby liquid that is much colder than the coolant's boiling point. There is no vapor to contend with.