I am looking at adding some fuel capacity (small additional tank tied into main fuel system) to my 07 FX HO Curiser.

Can anybody tell me if/how the fuel system is vented? I notice that the tank sometimes has pressure in it on opening up the filler cap, so that indicates to me a non-vented system, but was thinking that maybe there is an over pressure relief valve in the system. If so where is it?.

What would happen if we modded the system to just carry atmospheric pressure (open venting to exterior).? Does the FX use a fuel pump system or does it need to have the pressure in the tank?