The length of time says to me that it is a continuing process, like maybe the gas is boiling in tank and creating vapor
Even if you built up 20-30 psi in the tank, it is not that much volume. And the fact that the pressure keeps buildup after turning off engine
You can get an infrared temperature gun and measure the outside of the tank
I would love to get a temp probe in the tank, but not possible
How close does your exhaust pipe run to the tank?
BTW, ?I remember this issue from several years from a guy going to Yellowstone?
Stephen?
