Pressurized FJ55 gas tank

[PabloCruise]

I could still theorize a way for the fuel to come out even with a working vent but only if there's limited or no airspace above the fuel when the tank is full. Sounds like that is probably not the case.

I think I get what you are saying here.
The 4 vent line design looks robust in terms of giving the tank every opportunity to get vapor out.
Somewhere in her we have pics of a tank someone cut open. I think the vapor lines go to each corner, so in the course of driving, one of those corners will get uncovered, even if briefly, and the vapor can escape. If the operator went crazy and overfilled the tank, now there is the volume of 4 vent lines to fill w/ liquid before gasoline enters the separator, and then the charcoal can. But I am sure it has happened...

 

[PabloCruise]

Ambient temperature increase can add pressure, which is why this issue always seems to increase during the summer.
There is also the fuel return which is returning warmer fuel back to the tank while running.
Since gasoline evaporates (not sure if that’s the proper term) very rapidly and produces vapor, it’s constantly expanding and looking for the easiest path out.

This ^, the return line provides a big temp increase to the fuel in the tank. Plus the fact that the tank is under the Pig, getting warmed by the asphalt underneath.

We should take some temp readings on the bottom side of the tank after driving for a few minutes. Heat rises, so...

I think what you are after is the boiling point for gasoline, or the point at which it goes from liquid phase to vapor phase. Temp is a factor, and so is the atmospheric pressure. I am sure people remember the demonstration where they boil water at room temp by applying a vacuum? This is why fuel system issues can get worse when you go up into the high country.

Refiners will make different blends for different seasons, adjusting the boiling point as part of the blend.

 

[PabloCruise]

Yeah, I’ve not forgotten pV = nRT, not yet.



No fuel return on this 74. Yes there are two lines but the second is capped. Idk why, but according to the diagram, it’s stock that way.

View attachment 3681502

I think the 77277 tube in the diagram is your return line. The line above it - 77241 is shown connecting to a fuel filter in the engine compartment.

My '74 does have a return line, and the fact that it was clogged was the bane of my existence as I tried to make my 3FE swap work. Now it very much returns!

 

[PabloCruise]

Yeah, I’ve not forgotten pV = nRT, not yet.

It is ideal that you remember this...

 

[PabloCruise]

This was happening to me. Replaced the check valve inside the quarter panel with one from cityracer. (Didn't work) Made sure every vent line could be blown through with around 10 psi of air including the one that goes into the front cab mount. (Completely plugged) Then replaced the charcoal canister with the one that the fj60 guys were using and replaced the fuel cap with a new one. It's never done it since. Not once. I can't say for certain which of these things made the difference, but I'd venture a guess that the charcoal canister was the ticket. The check valve I replaced first by itself and it overflowed the next fill up. I replaced it again with a cheap plastic fuel filter and made a loop high up in the quarter panel to keep fuel out of it. I live in Redding and the climate is similar to Las Vegas and Phoenix. It was 119 degrees here the other day.

Nice work!
And good to hear that once you open up that complete path - problem solved!

 

[Half Wacked aka Split Pea]

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