Sounds like the breather pipe has come away from the inside of the fuel filler cap mounting. I had the same problem on my S4 (916). I ended up losing a couple of litres every time I filled up until I realised what had happened. All you need to do is take the fuel filler mounting plate off the tank and re-insert the pipe. Make sure you don't drop any of the grub screws into the tank, they're a bugger to get back out again!!
|