@WCBlueSky meh not really, a handful of people saying they have that issue and others stating with Gas boiling. Heat from exhaust and T-Case/ trans. One person report that they had a bad fan shroud and a new one fixed it, talk avout the fan clutch function, cooling aspect. My 80 runs cool and does not get extremely hot (coolant wise) that would trigger an problem... Nothing definitive.
I ordered a new VC120 (will have it tomorrow)
Checked to make sure return line was clear.
Checked operation of the purge valve, appears to be working properly.
I really don't want to have to re-pipe my exhaust (again) but may move some stuff if I can't seem to get a solution with the CC/ Vent components.
Thoughts @GeoRoss ?
The 1990 Clean Air Act requires summer gas blends to have a lower level volatility and that goes for ethanol containing blends also. In theory, our mid-90's Cruisers should be able to handle these levels of summer gas blend volatilities. Maybe the 1FZ-FE design team didn't have enough time to fully address the Clean Air Act. I don't know if ethanol is to blame or our EVAP systems are just designed on the edge. All I know is that different parts of the gas vaporize before others and maybe our system can't handle the modern blends or our EVAP systems are just getting old and need refurbishing.
I haven't had the problem in 12 years and only coupe of really bad ones. It was after a long highway run coming from Tucson to Flagstaff (2300-7000ft) in the summer and the next day going across the four corners region headed to Dolores, CO. This may have been because I had filled up a week earlier and the pump shutoff didn't work and I overfilled the tank. Maybe I saturated the charcoal canister. I have replaced all the easily accessible rubber lines going to the charcoal canister, vacuum lines in the engine bay, check valve under the intake and a new gas cap. I haven't had this problem since and now just have the normal "whoosh". I do have a new OE charcoal canister that I bought back in 2009 if the one in there ever fails. Fingers crossed as I'm headed up to the White Mountains in a few days and there is a long sustained climb that should test the system.
Bottom line is I think we are just running into PV=nRT and unless our EVAP system is working really well it just can't handle whatever blends they use in the summer when there are dramatic elevation changes.
The times it has happened to me were where I ran the tank down to 1/4 tank in the summer with significant highway miles and elevation change. I have always been curious what the temperature of the gas is coming back to the tank from the fuel pressure regulator. Is it heated enough to affect tank gas temperatures enough to start boiling when you remove the gas cap (lower the pressure dramatically).
There is the fuel cut off valve (check valve) that is a part of the fuel tank breather tube that goes the charcoal canister. I guess if this isn't working it would build up pressure in the tank too.