Hi guys, rackin up the posts I have a 66 Impala, and on hot days gas pours out of the vent tube and their is a lot of pressure in the tank. I currently have a non-vented gas cap per spec, any suggestions on resolving the issue?
A 1966 used a vented gas cap. Non-vented gas caps weren't introduced before 1972 when the charcoal canister first appeared under the hood to hold all of the gas tanks evaporative fumes between the times the engine was running. Your car needs a vented cap to let air in (it also lets gas fumes out; which is why they were banned in California first), but that technology hadn't been invented yet in 1966.
1965-1967 used Non-Vented gas caps and had the vent on the tank itself. The full size cars with these gas tanks would spill gas out the back under acceleration if they were vented. Previous Impalas with side fill tanks were vented.