Any ideas what could cause this ?

[YBNormal07]

at the bottom of the fill pipe..underneith the car..where the tank hose and filler neck hose meet...just disconect the rubber hose from the metal and you will see a little plastic piece inside the metal filler..the only reason the valve is there is in case you roll over your vehicle the gas cant leak out..its a one way valve set up..hense the name roll over valve

A flapper type check valve or a float type check valve? Now you've got me curious. Gonna see if I can find a diagram of this.

[Okeyd57]

A flapper type check valve or a float type check valve? Now you've got me curious. Gonna see if I can find a diagram of this.

I didn't see it in the engine service manual for 06 Impreza. I have them all, so I'm going to check the mechanisms and function book.

Update: After checking mech. and functions, the only valve on the fill pipe is the shut valve. It is not to keep gas from coming out, but rather to close the evaporation line when you stick the pump nozzle in the tank. I really don't see how this could cause slow re-fueling ! Back to square one.