Here's a link to a really good basic explanation of the wide range of terms used to confuse people about cooling systems - https://jimdinner.wordpress.com/2012...aps-explained/

Go to your local parts store and tell them you want a radiator cap for a "closed" or "vented" system, and no these terms are not in conflict even though they sound like they are opposites. A ‘Vented’ cap = ‘Closed’ cap – ‘Vented’ means is sucks from the ZERO PRESSURE tank as it cools down. ‘Closed’ means it does not go to the ground.
A ‘Non-Vented’ cap = ‘Open’ cap- ‘Non-vented’ means it does not suck coolant back into the cooling system. ‘Open’ means when the pressure gets too high it dumps onto the ground.

Spend a dollar or two more and buy the name brand cap, like a STANT or equivalent.