Hello, my question is about air in engine cooling systems. Coolant is formulated to resist boiling, and furthermore kept under pressure to increase its boiling point. I think this is to control its phase. In other words we must prevent coolant from boiling (vaporizing) because vapor does not conduct heat away from the engine very well. Too high of temperature, too low of pressure, and you change phase from liquid to vapor. Now what I want to know is how does air affect the coolant's tendency to boil, and does it reduce its boiling point or reduce its ability to conduct heat? There are tools available for draining and refilling the cooling system that work by drawing a vacuum of about 26inhg and "sucking" all of the coolant out, then using the vacuum to draw the new coolant in. Their manufacturers claim that cooling systems can have air pockets trapped in high points in the engine which lead to overheating. Revisiting pressure, temperature, and phase, by reducing the pressure enough we can boil the coolant at ambient temperatures, is that how the tool works? Ordinarily air and water would remain separate from each other, but if there is enough pressure air can actually dissolve into a liquid. I've experienced this when I accidentally overpressurized an engine cooling system. It was fine until I went to release the pressure and then it boiled chaotically. An engine has a circuit of coolant that is controlled by a temperature operated valve - the thermostat. The thermostat can stay closed and allow pressure to build inside the engine. Then once it opens the system is controlled by the radiator cap which is a pressure related valve. So I'm looking at this situation with all of the temperature and pressure effects and wondering about the impact of air contamination on the overall performance. My gut feeling is that if the normal expansion and contraction of the coolant pulls air into the system from a leak or from a low level that the air will dissolve into the coolant when the system increases in pressure, and migrate into the engine, where it will separate and dwell. My intuition says that air contamination will increase the tendency to boil. Based on that I'm thinking I should pressurize the system and cap to its rating to check for leaks, but also drain and refill it with fresh coolant and check for leaks under vacuum as the best maintenance practice. What do you think guys? Feedback appreciated.