Where does coolant go?

[FirstOwner69]

Originally Posted by litew8

I can't type all this out so I'll copy/paste.
http://auto.howstuffworks.com/how-do...r-cap-work.htm

"a protective cap that’s actually a pressure valve and a tank on each side to catch the coolant fluid overflow."

"When coolant fluid overheats, it expands, causing the fluid to become highly pressurized. When it enters the radiator, the pressure increases even more because it’s in an enclosed space. The radiator cap acts as a release valve set to open at the maximum pressure point. Usually this is set at a density of 15 pounds per square inch (psi). When the fluid pressure inside the radiator exceeds 15 psi, it forces the valve open, allowing heat to escape and excess coolant fluid to overflow into the tanks on either side of the radiator."

I agree 100% with this, except it should say pressure, not density. However, it was written after overflow tanks became standard issue. Before overflow tanks, radiators were not filled to the top leaving an air cushion at the top. If the air pressure reached the pressure of the cap due to coolant expansion, the air would escape, hopefully, allowing enough room for coolant expansion without overflowing. If this were not the case, coolant would have been constantly puked on the roads. Certainly, it was more common than today with overflow tanks installed, but it wasn't an everyday occurrence.