Venting the fuel tank

[Idle]

Derr.. Or the pump simply won't pump fuel against the lowered pressure.

I've found from another site that the MkVI Golf's fuel tank cannot be vented when filling. Thanks for everyone's "help"

I don't quite get that — if it can't be vented whilst filling, where would you suppose the air goes?

It doesn't start to be compressed until the fluid level reaches the expansion (vent) space, so the displaced air simply comes back up the filler tube until such time as inflow ceases and the filler cap is replaced.

It's my understanding that this is how the auto shutoff nozzles work — when fluid reaches the vent hole in the end of the nozzle the back pressure triggers the release. That's also why many poorly designed filler tubes won't take the full flow and have to be filled slowly, and why you can usually get a few more litres in with a trickle flow.