On Tue, 21 Sep 2004 16:13:55 -0700, wrote:

I have two different after-market fuel caps on my C172. The right tank
cap is vented; the left is not. Should both be vented? Is a properly
functioning vented cap one that only allows air to flow into the tank?

That's probably right. Check your ADs. I seem to recall there being
one from the 1970s that deals with fuel caps. Something to do with
vapor lock above 5000 feet.

Don