Blocked fuel vent?

[Kamaloha]

We are on a trip across country in our 1989 Toyota motorhome with the 3VZE engine. We are experiencing a problem that is slowly getting worse, where it appears that we need to fuel more and more frequently. The engine starts to stumble, and at one point died completely when it still had fuel in the tank. I’ve had problems in the past where the fuel pump became disconnected from the pick up tube, and the fuel pump became disconnected from its filter, so fuel is left in the tank unusable. However, we’re noticing a new phenomena with this; there is an audible hiss when we take the cap off the fuel tank. I’m thinking that perhaps there is a blocked fuel vent and air cannot replace burned fuel in the tank. How does the fuel vent connect to the tank, and where does it go? Could this be blocked and causing a problem?

[Ssunrader]

Have a look at this;

https://www.yotatech.com/forums/f116/gas-tank-pressure-260511/
 

It sounds like either the charcoal canister or one of the valves/vacuum controls is at fault.

[Odyssey 4x4]

I had this exact issue, I figured out it was a failed charcoal vapor canister under the hood. There's a vent line that goes from the fuel tank to the vapor canister. The vapor canister has an internal valve controller by engine vacuum. It decides when to vent fuel vapor from the tank to atmosphere or to engine intake depending on temp and throttle conditions. There is a vacuum line going to the vapor canister coming from the "bimetal vacuum switching valve" that's location on the rear side of your intake manifold between the manifold and the firewall. It is a line with 2 blue (very fragile) plastic nipples coming off. This is the vacuum switch that controls the valve inside of the vapor canister.

 

If I remember correctly, the vapor canister has a hose coming from the fuel tank, a hose that vents to "atmosphere", mine runs into the frame rail, a hose that runs to the engine intake, and the vacuum lines that control the valve. 

 

The valve in my vapor canister has rusted and seized shut, not allowing the tank vapor pressure to release and giving the conditions you described, over pressurizing the tank.

 

You can no longer get a 3vze specific replacement canister. You can either find one at a junk yard in unknown condition, or find another works canister from a different vehicle. I went the second route and it seems to be working.