f you have ever been to a parts store, you probably noticed about a million radiator caps. While some of this is due to OEMs not using a standard cap size (Asian cars have very small caps for example), there is also a difference in the pressure relief. Heat means pressure, and your cooling system has a limit on how much pressure it can contain; this is where the cap relief valve vents to purge that excess pressure. When you get steam pouring out from under the hood, that is the cap venting.
