Pressurized FJ55 gas tank

[WarDamnEagle]

A new gasket might help, but this was pushing fuel over the top of the filler neck and I’d not filled it that much.

Fuel expansion? Difficult to accept. Gasoline does expand when it gets warm. A 15° F rise in temp will increase volume by about 1%. If we assume the fuel came out of the tank at, say 65° F and then rapidly got to 90° F, that would be about 2% more volume.

A stock fj55 fuel tank is 23.8 gal. Two percent more volume would be another 0.5 gal, max.

Assuming it was full to the bottom of the fuel filler line, 0.5gal is a lot. Certainly more than that line holds.

I had the same issue one hot day when I topped the tanks off on all my vehicles as we were leaving the US for several months. I got home with the pig (3 miles) and tried to get a can of Seafoam in the tank only to be met with gasoline coming out of the neck. Let it idle outside the garage with the cap off for over an hour and I’m guessing a good quart or more of gas came out. Never did get the Seafoam in on that trip.

Edit: Assuming I did this quick calc correctly, a 2” ID line, 18” long holds 1/4 gal.

 

[RUSH55]

I’m gonna go out on a limb and say it’s caused by pressure buildup in the tank.
The fuel inlet goes into the lower half of tank, which means when the tank is full the inlet is well submerged. Any pressure build up is going to push fuel back out the inlet, and it doesn’t have to be very much, just a small amount of positive pressure.
The vent system has to be completely clear in order to relieve the pressure. The vent line between the tank and charcoal canister is almost always rusted out and blocked off by dirt, debris, bugs, whatever. This line runs through the front right cab mount, which is notorious for getting packed full of dirt and rusting out.

 

[WarDamnEagle]

I’m gonna go out on a limb and say it’s caused by pressure buildup in the tank.
The fuel inlet goes into the lower half of tank, which means when the tank is full the inlet is well submerged. Any pressure build up is going to push fuel back out the inlet, and it doesn’t have to be very much, just a small amount of positive pressure.
The vent system has to be completely clear in order to relieve the pressure. The vent line between the tank and charcoal canister is almost always rusted out and blocked off by dirt, debris, bugs, whatever. This line runs through the front right cab mount, which is notorious for getting packed full of dirt and rusting out.

That certainly makes sense but the only way pressure would build up even with a plugged vent is if the contents heat up. A 15F rise in temperature will increase the pressure of the air/fuel vapors and will expand the liquid (thus further increasing the pressure potential in a fixed volume) so both are working together to push fuel up and out. So you need both to happen.

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.

 

[gonzopancho]

That certainly makes sense but the only way pressure would build up even with a plugged vent is if the contents heat up. A 15F rise in temperature will increase the pressure of the air/fuel vapors and will expand the liquid (thus further increasing the pressure potential in a fixed volume) so both are working together to push fuel up and out. So you need both to happen.

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.

My working theory is that in the absence of an external source of pressure, there can’t be any “pressure on the tank” with the fuel cap removed.

This pig “made gas” out the fuel neck for hours, despite being transported down the road 200 miles (last 40 of those over “unimproved roads”) the night before, and despite being driven over trails, small hills snd dirt roads for several more hours in a relatively cool environment.

@DTC72 had a theory that all the lines and the separator were full of fuel. this is likely, as the fuel level in the filler neck (tube) was all the way to the top, and running over the top, and it just kept coming.

Can’t be a siphon, because that requires the outlet to be lower than the inlet.

(Evap/charcoal canister would qualify, but no fuel there… No fuel coming out the evap line though. Likely plugged, so want to fix that too…)

It will be a fun problem to find, understand, and fix. (At least now the decision to move BATPIG’s exhaust to the PS seems valid.)

Most of the Internet is down today because Crowdstrike screwed the pooch with an update that crashed a couple hundred million Windows servers and Azure VMs. Repairing these require each to be rebooted (as many as 15 times) before a file can be edited to remove a single line.

So at least I’m not having to fix that. (They are one of our customers though.)

 

[WarDamnEagle]

My working theory is that in the absence of an external source of pressure, there can’t be any “pressure on the tank” with the fuel cap removed.

Yes and no. The vapor space over the fuel will develop pressure if the temperature increases and that pressure will be in equilibrium with the hydrostatic pressure of the fuel "climbing" the fuel inlet line. Doing a quick calculation and using your 65F to 90F temperature increase, the pressure (with the cap on and assuming 100% sealed) would increase from 14.7 to 15.4 psig. So that's the potential pressure increase.

Although 0.7 psi doesn't sound like a lot, it's enough to push gasoline around 1.7 feet. So if the cap is off, the vent is plugged, and the vapor space above the fuel inlet is not connected to the fuel inlet, gasoline will come out for a long time.

 

[RUSH55]

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.

 

[gonzopancho]

Ambient temperature increase can add pressure, which is why this issue always seems to increase during the summer.

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

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.

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.

 

[scrapdaddy]

@DTC72 had a theory that all the lines and the separator were full of fuel. this is likely, as the fuel level in the filler neck (tube) was all the way to the top, and running over the top, and it just kept coming

I'm going with this. I've seen the "separators" full of fuel and that would cause the fuel to puke out. The early models, like mine have two separators, one in each cargo area. Take it out and drain any fuel so tank can vent proper, if that four beer ramble made any sense.

 

[derail]

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.

 

[55guy]

So where did you find the fj60 canister ? New ? Thank for the info

 

[derail]

So where did you find the fj60 canister ? New ? Thank for the info

It's the vc120 from autozone. It's not an exact fit for replacement, but it wasn't a challenge to figure out a mounting solution.

 

[55guy]

I gotta ask did you some how make all 4 vent hoses go 4 to 1 ? or ?

 

[scrapdaddy]

That's the one I'm using.

It's the vc120 from autozone. I

 

[scrapdaddy]

I gotta ask did you some how make all 4 vent hoses go 4 to 1 ? or ?

Check out the diagram on post #27 above. In the lower right corner you'll see the separator with all four lines coming in the bottom and one line going out the top to the canister.

 

[derail]

That's the one I'm using.

You're probably where I got the initial idea. On mine (1975) I hooked up the line that comes across the firewall to the "tank" fitting. (It comes from the fuel tank all the way to the engine bay) Then I hooked up the "purge" fitting to the ported vacuum on my sniper. The original line that went into the cab mount I left disconnected and tucked it away because it's not needed as this canister is self venting through a covered baffle in the bottom. Blue to the tank line, red to the vacuum port, yellow no longer needed.

 

[PabloCruise]

This is always a fascinating topic, IDK why...

I had to sort through issues with venting to, and purging from a charcoal cannister on a different (Non Land Cruiser) after an engine swap. What I learned is that the manufacturer changed the evap system 3 different times from '92 to '98. Granted, I think ODB1 to OBD2 was part of that, but that could not have been cheap. I think it speaks to the difficulty of properly venting the tank, capturing the vapors, and then burning them in a way that doesn't cause issues (stumbling, missing).

As others have said, you need a clear exit for those vapors. So the separator/check valve/line/charcoal can all have to be open for business. Back in the day when people kept filling the tank after the first time the pump would shut off, that was probably guaranteed to push liquid into the charcoal can, clog the media, and then ruin the can as it could no longer accept vapors.

I wonder what the temp of the fuel coming back to the tank via the return line is? If the fuel from the tank is making the trip under the vehicle, and then goes to the carb, nice and close to the head, that gasoline has to be warm. Then the return line gas hangs out under the hood, getting warmer. That return line fuel has to be toasty.

 

[PabloCruise]

Mine does the same thing, I’ve blew air in all lines and all are open. It has been very hot hear 100’ heat. Iam thinking about putting a new gasket on the gas cap

Is the charcoal can open? You need the entire path to be clear to get the vapors out of the tank.

 

[PabloCruise]

A new gasket might help, but this was pushing fuel over the top of the filler neck and I’d not filled it that much.

Fuel expansion? Difficult to accept. Gasoline does expand when it gets warm. A 15° F rise in temp will increase volume by about 1%. If we assume the fuel came out of the tank at, say 65° F and then rapidly got to 90° F, that would be about 2% more volume.

A stock fj55 fuel tank is 23.8 gal. Two percent more volume would be another 0.5 gal, max.

I think we are talking about fuel expansion plus vapors = mo volume needed.

 

[PabloCruise]

Assuming it was full to the bottom of the fuel filler line, 0.5gal is a lot. Certainly more than that line holds.

I had the same issue one hot day when I topped the tanks off on all my vehicles as we were leaving the US for several months. I got home with the pig (3 miles) and tried to get a can of Seafoam in the tank only to be met with gasoline coming out of the neck. Let it idle outside the garage with the cap off for over an hour and I’m guessing a good quart or more of gas came out. Never did get the Seafoam in on that trip.

Edit: Assuming I did this quick calc correctly, a 2” ID line, 18” long holds 1/4 gal.

And even a little bit of gasoline coming out does not seem good, so it probably feels like a lot.

 

[PabloCruise]

I’m gonna go out on a limb and say it’s caused by pressure buildup in the tank.
The fuel inlet goes into the lower half of tank, which means when the tank is full the inlet is well submerged. Any pressure build up is going to push fuel back out the inlet, and it doesn’t have to be very much, just a small amount of positive pressure.
The vent system has to be completely clear in order to relieve the pressure. The vent line between the tank and charcoal canister is almost always rusted out and blocked off by dirt, debris, bugs, whatever. This line runs through the front right cab mount, which is notorious for getting packed full of dirt and rusting out.

Yes, you need the complete path.