Then I can pretty much guarantee that's ur problem....a few years ago I had an S2 with one of those chavvy filler caps fitted. The problem with them is that they dont allow the system to breath and just cause pressure to build up. I bet when you remove it at the petrol station it makes a loud hiss as you remove the cap? Mine was so bad it was bursting the pipe that goes from the pump to the tank because of this built up pressure Changed it to a standard filler cap and all was sorted