It might be a long shot, but I’m hoping someone has insight. I have a 90 chassis with all of the engine-bay EVAP equipment removed, and the remaining line is routed to the fitting on the front passenger side of the subframe. The fuel system consists of a Walbro 450 pump, a Fuelab FPR, and a Flyin’ Miata single-pass fuel rail, with all AN6 lines and fittings in the engine bay.



Any time I open the fuel system, either by removing the gas cap or accessing the tank to swap a fuel pump the tank is under positive pressure. This happens whether the car has been sealed up for just a day or even for several months. I’ve been told that having pressure like this isn’t normal and that I should address it. What would be the best place to start looking?

 

The check valve that's known to get stuck, mounted next to the fuel pump on the top of the tank.

https://revlimiter.net/blog/2019/02/...-fumes-solved/

 

Thanks for linking this Curly, this is a great write up by revlimiter. I haven't run into this one before when perusing his site.