Fuel Filler Caps

[RABBITT_Andrew at mail.orbeng.com.au]

>> in the tank, as every time I open mine to refill (Lincoln
>> Continental) I hear this "ffffst" sound.
>
>A small point, I think it's actually a vacuum.  The charcoal 
>cannister and engine vacuum are used to maintain a slight negative 
>pressure in the tank in order to reduce evaporative emissions.  
>Supposedly a defective cannister system can collapse a gas tank by 
>applying too much vacuum, but that may be an urban legend.

During running and hot soak, the fuel tank pressure will be positive 
from the vapour pressure (that's how the carbon canister is charged). 
The engine applies a vacuum to the canister to purge it, but this flow 
rate is not enough to create a vacuum in the tank itself.  When the 
fuel in the tank cools down, say overnight, a vacuum may form if the 
vapour pressure of the fuel drops below atmospheric pressure.