The fuel tank on my 1.4 AUA petrol (X-reg) has been developing a partial vacuum as fuel is used, evidenced by the fact that when the cap is opened after a tank-to-tank brim, petrol vapours will vent out of the tank for at least 15 seconds (displaced by nearly 35 litres of incoming/mixed air), whilst at the same time the decompression of the tank can be heard and felt through the chassis of the car during the tank in-breathing.
I've had plenty of other A2's and other petrol and diesel cars to recognise that this is not the initial one or two second venting (out-breathing) that might happen on a hot day.
The lack of pressure which can develop in the tank has also caused fuel supply problems - on occasion the car would not pull cleanly between 1.5k and 4k rpm after an EML was triggered, which I was able to resolve (at partial tank level) by stopping the car, opening the tank cap, and allowing normal atmospheric pressure to be restored.
So, I'd like to understand - what are the designed routes for atmospheric air into the tank during normal operation? Is it through the fuel cap (I have a fuel cap from a 1.6 petrol, but would assume they're the same), or is there also another breather (but not vent) pipe - and if so, does the breather work passively (and so may be blocked) or actively (e.g. by a solenoid)?