The gas cap has to let air in for the gas to be able to go out the tank,,,,,,but before the emission laws came in, the cap also let air out to keep the pressure down in the tank,,,,,a hot tailpipe is the normal cause of the pressure in the tank being high,,,,,then they stopped venting the carburetor and gas cap to the atmosphere and the excess fumes and pressure went out of the tank into a vapor cannister which then goes into the engine to be burned,,,,,and thats where the non vented cap comes in,,,,,,we have a sealed tank,,,,,it builds up pressure,,,,park a full tank in the hot sun,,,,,watch the gas start pouring out of the gas cap,,,,,,now if you have problems with your van running, that another story,,,,,,so you got a choice on which gas cap you can get,,,a locking one or the stock twist on one,,,,,,,