troubleshooting help, pressurized fuel tank

[TDS-MN]

Hey All,

OK, so fairly recently I have encountered thsi problem with the '85 pickup.

If I tighten the gas cap fully after re-fueling, and drive the truck, I often get pressure building up in the gas tank. Sometimes to the point that it pushes fuel out the top of the tank somewhere, and it drips on the ground. Sometimes it doesn't get to the point of leaking, but when opening the gas tank cap, there is a 'wooosh' of pressure being released.

I can't recall exactly when this started in relation to the tank replacement, to think if the 2 may be related in any way.

My 'solution' has been to leave the gas cap loose maybe a quarter-turn.

Stock engine/fuelpump. Seems to start and run fine, although I think it may be running kind of rich from the smell of the exhaust.

Any thoughts on where I can look for the cause of this. Hate to waste precious fuel, seeing it puddle on the ground under my truck.