Were you having drivability problems by using a non-vented cap? This may answer the three ??? query posted by
@1966newport -Using a non-vented cap on a non-vented tank would gradually have the fuel system developing enough of a vacuum in the tank that the fuel pump would not be able to overcome it, and the fuel delivery to the engine would cease or cavitate. As well, conversely, the tank could experience expansion issues when no venting exists, and spray gas and fumes out when removing the cap to fill.
Both issues were eliminated with a vented cap as originally designed. However, part of the issue for moving to a tank design with venting tubes and a non-vented cap was to permit less sloshing of fuel out the filler tube that occurred with a vented cap when the tank was full, and to prevent the potential for water to get in the same way. As well, as the design evolved in conjunction with emissions laws, the venting tubes were able to be used with vapour recovery systems and charcoal cannisters etc. in the aid of environmental concerns.
