Quote Originally Posted by VX KAT View Post
Would that trigger a CEL because of the vapor or fumes?
That's why I suggested a screw plug or a schrader valve. It'll act just like an un-modified cap until you release the pressure (or vacuum). The only problem I can foresee is that a schrader valve functions on positive pressure - it may leak if it's seeing negative pressure (vacuum). On the other hand, that might be mo betta.