if you look at about the 5 oclock position on the pic i posted, you can see a small hole in the cap, that is the vent. hard to tell if your cap has that, if it doesnt its not a vented cap. on the inside it has the hole like yours.
you can test it by sucking in on the inside hole & it should flow air without too much suction, then blowing out it will stop air flow, kinda like a PCV valve or one way valve. yours looks pretty dirty so if it is a vented cap maybe its plugged up. the new caps are cheap.
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