Excessive fuel tank pressure

[vegasfj40]

I've had several vehicles do this. I had a 79 FJ40 that would boil going from 4 to 8k feet when temps were over 80 degrees outside. My Dad's Ranchero did this quite often when he would drive out west in the summer. Just had my first experience with this in my 95 FZJ80 a couple of weeks ago. Ambient temps weren't too bad at around 80 but we slow climbed from 8k to 10k feet and when we reached the top the tank was boiling and fumes were pretty strong. As soon as we got back down to 8k feet or so the smell went away and all was fine. I think the problem very well could be the ethanol as my system is functioning fine. The idea of heat building under the vehicle when slow crawling might have some merit. Maybe the heat from the engine, trans and Tcase is warming the fuel tank while moving slow.

 

[WCBlueSky]

I live in Arizona and I have no boiling or any fuel pressure issues since fixing the problem.

I drilled out the check balls and still have the boiling/fuel pressure issue. From a physics point of view, eliminating the ~1 psi of back pressure from the check balls makes it more likely to boil, just like opening the cap does. Now if the check valves are completely stuck closed so the tank can't vent at all, that is a different issue than boiling. As said before, the boiling must be coming from excessive heat, whether from the engine or exhaust... My rig is running a little warm in engine temp.

 

[wer1952]

Happened in my 3fe all the time as well until I rerouted the exhaust away from the fuel tank and fuel lines. Post exhaust reroute I have 0 boiling/excess fume pressure at any elevation.

Another option could be additional heat shielding.

I like the rerouting idea. Do you have any pictures of that? Thanks.

 

[Heelerdude]

Ok so I know this tread is old and likely beaten to death....But I am in the same boat with my 91 FJ80 with the 3fe. All factory, not desmogged, and bone stock. Fuel tank has excessive pressure...I have only had it a couple months and have been working out the bugs...First time I ran it on the road for an hour and went to the gas station when I unscrewed the (New OEM) gas cap it damn near shot it across the parking lot and boiled for about 60 seconds before it would let me add fuel. I left the cap loose for the ride home. (FYI Truck is not overheating and I'm in Florida Sea level, 70 Deg) So I started looking at the Charcoal can and VSV and performing the tests per the FSM. VSV checks out good, but the Charcoal can confuses me. If I look at the test procedure and images in the FSM it shows the Tank Line and the bottom air inlet/outlet positioned differently than they were plumbed on my truck IE: The FSM shows the air nipple on the bottom facing rear and the Tank line at the front of the can and the purge line at the rear. On my truck the air nipple on the bottom points to the rear, but the tank line was connected at the rear and the purge was at the front of the can, When I performed the test I could not force air into the nipple that the tank line was connected to and I could hear the check ball engaging as I pulsed the air. So per the way it is installed on my truck it failed the test....However when I reversed the purge and tank lines, the Can performed the way the FSM states it should IE: low pressure into the tank line with free flow through the purge and air line. Also by plugging the purge I was able to perform the cleaning procedure and did not get any material out of the air line....Crazy, but is is possible some PO got them mixed up? I have seen other pics of 3FE engine bays and they look the same as mine did...Tank line on the rearward side and the short Purge line on the forward side going directly over to the VSV. If I switch them to the way it looks in the FSM and the way it tests properly they cross over each other....Any help or advice would be appreciated. Pics to follow.

 

[Heelerdude]

Ok so I know this tread is old and likely beaten to death....But I am in the same boat with my 91 FJ80 with the 3fe. All factory, not desmogged, and bone stock. Fuel tank has excessive pressure...I have only had it a couple months and have been working out the bugs...First time I ran it on the road for an hour and went to the gas station when I unscrewed the (New OEM) gas cap it damn near shot it across the parking lot and boiled for about 60 seconds before it would let me add fuel. I left the cap loose for the ride home. (FYI Truck is not overheating and I'm in Florida Sea level, 70 Deg) So I started looking at the Charcoal can and VSV and performing the tests per the FSM. VSV checks out good, but the Charcoal can confuses me. If I look at the test procedure and images in the FSM it shows the Tank Line and the bottom air inlet/outlet positioned differently than they were plumbed on my truck IE: The FSM shows the air nipple on the bottom facing rear and the Tank line at the front of the can and the purge line at the rear. On my truck the air nipple on the bottom points to the rear, but the tank line was connected at the rear and the purge was at the front of the can, When I performed the test I could not force air into the nipple that the tank line was connected to and I could hear the check ball engaging as I pulsed the air. So per the way it is installed on my truck it failed the test....However when I reversed the purge and tank lines, the Can performed the way the FSM states it should IE: low pressure into the tank line with free flow through the purge and air line. Also by plugging the purge I was able to perform the cleaning procedure and did not get any material out of the air line....Crazy, but is is possible some PO got them mixed up? I have seen other pics of 3FE engine bays and they look the same as mine did...Tank line on the rearward side and the short Purge line on the forward side going directly over to the VSV. If I switch them to the way it looks in the FSM and the way it tests properly they cross over each other....Any help or advice would be appreciated. Pics to follow.View attachment 2961465

Also to add info, I have a brand new OEM fuel tank, fuel shut off valve, pump, and sock. All rubber lines are new and the hard lines were blown clear. I also noticed while Testing with a vacuum tester that activating the VSV by applying vacuum had no effect on running but when I switched the can lines when I activated the VSV rpms picked up and smoothed out as it is pulling fuel vapor in through the purge line…