Well, It's a Stant cap marked "Caution: Rotate cap one-half turn to vent." and "After Hissing Stops Continue Turning to Remove."
Seems like it's supposed to "hiss" or Swoosh.
Fuel tanks need to be vented to allow air to replace fuel drained from them and thus prevent tank collapsing not to mention compromising fuel pump suction. Irrespective of what is printed on your present cap which is likely not OE, it's a simple test to go for a short drive with cap removed and some form of plug from a freely breathing rag in its place.
BTW I have heard of ethanol bloating seals in caps that otherwise allow some air flow.
I've often wondered about this as my fuel cap is none vented, but I suspect the the vaccum that would be created by using fuel must be relieved via the evap cannister. I think that there is a one way valve on the cannister, because if there wasn't, it would defeat the purpose of it think.
I get a whoosh when I open my fuel cap, but I'm sure this is the build up of fumes coming out as opposed air being sucked in. I wonder if those with a vented fuel cap suffer from fuel evaporation.
