Why is my gas tank under so much Pressure all the time?!!??

scouthead · 02-19-07, 01:34 AM

O.k., my '76 FJ55 keeps building pressure in the fuel tank. Everytime I take the filler cap off, I have to crack it a little and let all the pressure hiss out slowly. I have had it projectile vomit several pints of gas onto me once or twice, when I was trying to check my milage (the tank was not overfilled, and the truck had been driven far enough to burn a gallon or two...and no changes of elevation). The less gas there is in the tank, the less severe this condition is, but even when it's not belching gas onto me, it's bad enough to worry me about creating leaks( or explosions?).
The vapor seperator valve, attached to the rear vapor recovery deal, was making a high pitch noise like a pitch fork, so I replaced it. The noise is louder now. I know it was directional, and it was clearly marked, so I'm pretty sure it is installed correctly. But even if it wasn't, I don't think it would create pressure in my tank. I think the sound is a bi-product of all the pressure forcing the valve to open the wrong way...So where is all this pressure coming from??!!
Anyone with ideas chime in here...It's got me a little concerned, and if it gets any worse I might have to park the poor pig until I can figure it out..

Herb · 02-19-07, 05:57 AM

Sounds like you may need a vented gas cap...

workingdog · 02-19-07, 08:40 AM

It's pressure and not a vacumm that has built up? You can feel air rushing out? That's unusual. What's your set for tank and fuel pump.

rjcruiser · 02-19-07, 09:35 AM

Does the rig run the same whether or not the gas cap is on or off? If it does, just get a vented cap like Herb stated.

tucker74 · 02-19-07, 10:21 AM

Sounds like your vapor recovery/charcoal filter is clogged up, you can try finding a used one (unobtainium new) or vent your tank. This is very common with poly tank swaps where these stock vents aren't used, you can drill a small hole under your fill cap (use magnet to catch shavings inside

) or buy a vented cap.

Tucker

Hobbs · 02-19-07, 07:56 PM

scouthead, is your truck still equiped with all of the factory evaporative emission control components?? Any modifications...?

cruiser88 · 02-19-07, 08:02 PM

Quote:

Originally Posted by tucker74

Sounds like your vapor recovery/charcoal filter is clogged up, you can try finding a used one (unobtainium new) or vent your tank. This is very common with poly tank swaps where these stock vents aren't used, you can drill a small hole under your fill cap (use magnet to catch shavings inside

) or buy a vented cap.

Tucker

yep but you could also try blowing it out with air while pluging the other outlets.but im sure its done for now

1973Guppie · 02-19-07, 10:37 PM

sounds like the tank isn't vented correctly, do you have all of the stock venting equipment setup? Any pressure should be easily released via the stock setup and out to the charcoal canister. I would check all lines. Also, I have had several of those fuel filter type blowback valves go bad on me, I finally just replaced it with a length of metal fuel line, no problems, A way to test the system, as I did, is to disconnect right after that fuel seperator unit and blow into the tank to cause pressure, the pressure should come right back out through the system, if you can't get air in or it doesn't come back out you have blockage somewhere.

Noah

Noah

scouthead · 02-20-07, 12:18 AM

I should have mentioned that EVERYTHING on this truck is stock. None of the evap stuff has been tinkered with. I'll check on the canister, and remove the noisy fuel filter looking valve, and check for blockages.
The vented cap never even entered my mind...way to easy. I may put one on even if I find out what's going on.
Thanks for the help guys. I'll let you know if I find anything interesting.