fuel tank filler vent ?? or fuel pump bad??

[ivanlee190e]

I have an '88 190E 2.3. According to the Haynes Manual, one of the reasons my car misfires before cutting out is that "Fuel tank filler vent blocked (suction will be evident on releasing cap)." I am experiencing exactly that when I remove the cap, there's a lot of pressure and sucking when I remove it. The problem is that I have no idea how to unblock that fuel tank filler vent. What and where exactly is that???

Or could it be the fuel pump that is going bad and lacking appropriate pressure??? I purchased this new fuel pump no more than 9 months ago. Is it possible that it's already dying ???