Hello, I would like to know the function of the small lever (knob) inside the fuel filling neck (marked with the red arrow)

 

 

Welcome. Which type of Fabia do you have? Engine, year etc.

I think that's the vent valve, to do with opening and sealing the tank, for the evaporative emissions control system.  Normally unscrewing the cap changes it from open to closed (or vice-versa, not sure) and refitting the cap changes it back.

Some people have found, by accident or deliberately, that pushing that with the fuel filling nozzle as you refill the tank lets more air out and increases the amount of fuel you can get in there.  For petrol models this can have bad consequences, to do with the charcoal canister.

Thank you Wino, 

Engine 1.4 16V 2005

 

Definitely a vent valve that opens when you refit the tank cap.If I fill the tank to the brim on my 6Y5,and then press the small lever with the fuel nozzle,the fuel in the filler neck disappears and I can get a few more litres in,I keep pressing it until I cannot get anymore in.It never gives me any fuel expansion/overflowing problems as the filling station is a few miles from my home and an amount of diesel is used to lower the level as I drive home.

Thank you Maccy

 

Diesel does not expand much and so brimming the tank does not cause any issues.

 

I have removed the vent valve in the filler neck which makes it much quicker to brim the tank to the max, never had any issues doing this over the last few years.

As stated earlier it's only a potential problem on petrol cars because they use manifold vacuum on startup to purge the vapours from the tank through a charcoal canister which can be flooded with liquid petrol causing a significant hazard.