Since we had our lesson on thermostats, here is a companion lesson. A radiator cap has a pressure setting on it that allows the system to bleed off pressure above that point. They have different levels beginning around 8 pounds and going on up. I'm not sure of the highest pressure on newer cars but the average for a long time was around 15 pounds.
The purpose for a cap isn't to close the filler neck. For each pound of pressure on the cap, it raises the boiling point of the coolant 4 degrees. Sorry, I grew up in the US, and I can't convert temps for you... Raising the boiling point is necessary because once liquid boils it forms bubbles and they don't transfer heat to help cool the engine once it gets to the radiator. In fact enough bubbles can form a block and not let enough coolant get through the radiator, or in the case of filling, not let you get the entire amount of coolant into the system. Engines run so hot today, they average right close to the boiling point. So without the extra pressure we'd have a melt down. So there is your very basic lesson in cooling systems. Feel free to add to it and we'll all pick something else up that you may not even care about. Laughing here! [Non-text portions of this message have been removed]
