Have experienced issues with very high fuel tank pressure. Started out on the first few trips as fuel vapors hissing from the charcoal canister that we could smell inside the cab. It was worrying, but in a "hm, that's odd" sort of way. On the last trip, pressure in the fuel tank broke the seal on the fuel cap. I actually noticed because I could see fuel vapors exiting the closed fuel lid in the side mirror. When I stopped, the smell was really bad and there was fuel was leaking down the side of the truck. I decided it was probably better to crack the cap and vent while I could monitor the situation then to roll down the rest of the trail with fuel bubbling out of the cap. Took maybe 30 minutes of slow venting to release most of the pressure, all the while I was thinking of the land cruiser that recently burnt to a crisp from what I can imagine was some sort of similar issue with excess fuel tank pressure.
I followed diagnostic flow chart in the FSM and although I didn't have all of the expensive tools to properly diagnose I am pretty sure it is a failed evap (purge) VSV which didn't allow the pressure in the charcoal canister to purge to the engine. I am assuming that after several trips the pressure finally found the weakest link and broke the seal to the fuel cap. Unfortunately when the evap VSV fails, the FSM also recommends replacing the charcoal canister which includes a new vapor pressure VSV. Not a bad idea in my case, the walls of the old charcoal canister were visibly ballooned and I worry that both the vapor pressure VSV and some of the internal valves in the canister could have been damaged by the pressure. Ordered both evap VSV and charcoal canister along with a new gas cap. My cruiser always smelled like gas when I parked in the garage, so I assume my charcoal was shot and hoping this solves both problems. Major ouch to the tune of nearly $700.
FWIW I never had a CEL... Fingers crossed that the new parts fix the issue... the only other culprits in the FSM are either the vapor pressure sensor or the ECM, both of which are unlikely and even more unnecessarily expensive.
However, if made to be easily removed prior to smog (with the original under spare canister back in action), then reinstalled after smog, might eliminate the overpressure/altitude/temp problem.
