IDK, I'm actually interested of the answer here... my engine will go dead if I try to add fuel while it's running, so IDK what kind of venting malfunction I might have. I was looking at a locking gas cap, but I haven't bought it yet.
I went to the LPS for a locking cap. Took me a minute to decide if I should buy vented or not? What are your thoughts on this subject?
The fuel cap has a relief valve in it, designed to open at a pressure of 17kpa (2.5 psi) or a vacuum of 2kpa (0.6 in hg). ----2002 and 2003 FSM. Your '04 may have different specs but I'm quite sure it will also have a relief valve in it. If you use the wrong cap, you will have a check engine light constantly on while driving.