They call them "capless" only because you don't have the gas cap. They still need to seal the fill tube or the evap system won't work. On the inside there is a door that is held shut by a spring. When you insert the gas nozzle it pushes that door open so that you can fill it. When the nozzle is pulled out, the door seals shut again. Only thing I've ever read that goes wrong with them is that if you do get an EVAP code, the first thing you should do is to clean the inside edge of the fill tube that the door seals up against. No other issues that I have seen/read. I've had it on my last 2 trucks and I have never had an evap code thrown and thus never had to do the cleaning.