From my experience Shiny, Fuel tanks have to be vented in some way shape or form. I've never had any type of issue what so ever with a vented tank but I have had all kinds of weird issues with tanks not vented properly, with the biggy being fuel supply issues that aggravate other things like vapor lock, WOT fuel delivery etc..... So bad to the point it'll make you think you have a different problem.

When I get something in here with a fuel delivery issue the very first thing I check is that the tank is vented properly. You can have fancy fuel pumps and big fuel lines and still have delivery issues if the tank isn't vented. It's a very simple concept that is easily missed.

When did pontiac stop using vented caps?

About 1965, I believe. A pre-production 65 Vent system was installed on my 64 GTO BRASS HAT convertible vs the stock vented cap after the wife complained the car was stinking up the garage.

Tom V.

My 69 GTOs were vented.My 9 bird is not.I might pull and drain the tank,fill with water and put a vent fitting in the neck.Drill a hole and put in a fitting.Tom