This was happening on my GTI 1.8t. The problem was that the fuel pump connector had a crack in it so there was fuel leaking onto the top part of the gas tank. I had my vw shop replace the fuel pump and it was free since there was a fuel pump recall that affected my model. A new pump is around $100.00 US dollars and the repair is fairly easy to do so a shop should only charge you about 1hr of labor maximum.