fuel tank pipes question

[javalin]

Hello,

Not strictly a VW question, but if the tank is effectively sealed to prevent fume venting, and uses the EVAP valve is used to de-pressurise the tank when hot fuel gasses - how does the tank allow air in when fuel is used?

I know there is a breather for when you fill the tank, but isn't that shut off by the filler cap?

Reason for asking - Wife's Fusion appears to run quite a strong vacuum in the tank.,

Not covered in Haynes, or Hilliars Fundementals....

Thanks,
James

[javalin]

Shameless Bump. Anybody?  Or have I stumped you all?   

[AudiA8Quattro]

The car will be fitted with a one way valve and pipe under the car, this will be connect to the carbon cannistor box.
How it works?
When vacuum in the inlet manifold is high, at idle and low revs, fuel vapour is drawn from the tank via the filter, also any collected vapour from the filter box will be drawn into the engine. This only occurs when the engine has reached 80 degrees.
When vacuum in the inlet manifold is low, at high revs, or when the pressure in the tank exceeds the inlet manifold, the one valve opens and allows air in from the outside. This will go back up the line back to tank.
The same line that feeds the cannistor with vapour, it works both ways, depending on how much vacuum is in the tank versus the inlet manifold.
That probably makes no sense at all 

[javalin]

Ok....  So when the car is at low revs the ECU will open the EVAP valve to allow gasses to be sucked out.  Where is the valve that allows air back in - is it the EVAP valve again or something on the tank itself?

Cheers!

James

[AudiA8Quattro]

No. The ecu operates the EVAP valve once engine oil reaches 80 degrees.
The filter box is either used to draw any fuel vapour into the engine or to draw air from the outside back to the tank, depending on which side(inlet manifold or tank) has the highest vacuum.