I have also experienced that noted symptoms of high tank pressure and boiling sound on occasion at elevation in warm temperatures in my 1999 EVC with no fault codes. Being from Minnesota it happens very rarely and only when traveling west into the mountains. However, following is my latest hypothesis on the cause.
It is my understanding from viewing limited photos of a carbon canister that was cut open, there are vertical chambers in the canister, connected in series, to provide an effective/longer treatment path of vapors flowing through the carbon. For whatever reason, say overfilling or a component issue (e.g., evap purge valve or vent valve problem), the carbon canister becomes saturated with liquid fuel essentially creating a liquid trap in the bottom of the canister thereby restricting or blocking vapor flow during tank venting and carbon purge/recharge events. In moderate elevation or temp changes there is not enough pressure differential to create any notable symptoms of an issue. In higher temperatures and/or elevation changes, pressure builds up to the point that it overcomes the liquid trap in the canister creating the boiling/bubbling sound and high vapor levels/smell that is likely being emitted from the vent line when the engine is off. Moreover, because of this liquid trap or super saturation, recharge of the carbon canister can't occur effectively because the liquid trap blocks or restricts the flow of fresh air during purge/recharge events potentially causing further saturation and/or liquid accumulation in the canister.?
Again, this is only a hypothesis but seems quite plausible to me as a starting point to further evaluate if and when this reoccurs in my case. Sure would love to see better detailed photos on the interior of the carbon canister to confirm configuration and vapor flow pattern along with confirmation that the boiling/bubbling sound is coming from the base of the carbon canister and not the fuel tank.
What could be the fix? Remove the carbon canister and manually purge any liquid fuel/saturation, repair or confirm evap purge and vent systems are working as designed, and try not to overfill the gas tank during refueling. Cheers, Bob
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