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Out and about in LFF yesterday in some very hot weather and after sitting in the sun for a time, I was getting a good whiff of fuel. Given that there's already an open breather pipe venting out underneath the boot floor, should the fuel cap also be vented? I've also had fuel spills from the filler cap in the past, thinking that may have been the seals, but having repositioned the tank it seems that fuel can still sometimes spill out and I presume this is from the vents.

So, what's right, and why? Vented or Non-Vented?

If the tank has the vent tube to under the boot it shouldn’t need a vented cap (as long as the vent tube or hose isn’t blocked). You can tell if there’s a problem if you get a pressure build up on a hot day that releases when you open the cap.