Gas tank pressure problem
- Thread starter 77jimmy
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- 454 - Turbo 400 - 3.73
Wow, really that much??? So your cap is sealing good obviously, and all your lines and evap lines are good and tight and no leaks, so that leads me to beleive you canister purge device is clogging up, and/or it's not being purged to the intake if it's that bad. Pull your tank line off at the canister. I beleive it's located in behind the core support about behind your left headlights. Follow the line that goes to your tank, or switch valve. And speaking of tank valve, your problem could be a clog right there too. So if pulling the canister line doesn't releive the pressure, then it's at the switch most likely. Do you have a 1 or 2 tank system?
77jimmy
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The one way check vavel on the tank is good. I have no return line from the fuel pump could that be it. Maybe it's making a vapor lock issue. I think I am having that problem.
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That's probably your problem then. The tank has to be vented somewhere. If you plugged the Evap line, then it's not vented.
Swims350
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my old mans 84 does it, sounds like the truck farts when you take the cap off, the valve in the filler flaps back and forth, thus farting, and his had NO charcoal can just open line and has return and such and still does it.
crazy4offroad
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Yep I would check that line, I bet a spider or mud dauber made a nest in it!
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I never could understand why people take off and disconnect canister purge. 
Really hurts that mpg and performance huh? And always in the way when working on the truck, even though it's down there in the fender well behind the headlights. 
Maybe it's the 2lbs lighter in the truck that makes the difference.
Really hurts that mpg and performance huh? And always in the way when working on the truck, even though it's down there in the fender well behind the headlights. Maybe it's the 2lbs lighter in the truck that makes the difference.
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If the tank is building pressure, the breather on the sending unit HAS to be clogged.
I had the same issue with my '77 Blazer, the pressure in the tank actually blew out the rotted fuel hose coming off the sending unit. I had thought that the exhaust was heating up the tank and the sole reason for the pressure buildup (the exhaust ends right before the axle). When I dropped the tank to replace all the rubber hoses, I blew out the little vent cap on the sending unit with compressed air. After that, there were no problems with building up pressure in the tank. Still the same exhaust too.
Swims350
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i tried to blow thru ours with a compressor and it would blow, and make that same fast noise, but you couldn't use high pressure it wouldn't do anything, it had to be slow going in or lower psi. I know that's not right but looking at the lines none were bent in two or anything but for use something sure is plugged. I figured I'd fix it when we drop the tank and put on the new tank brackets.
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The older tanks just have a little breather right on the sending unit, I removed it and blew it out. I didn't blow air into the tank.
John Zarrella
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I have an 85 2500, the charcoal canister is filling with fuel and when i open the tank lots of pressure behind cap, any ideas?
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I'll take a guess and say your tank is not getting vented since the charcoal canister is wet with gas. All that should go up that line is vapor from venting and can't vent to air which is the canister if it's saturatedk and not letting the gas tank get vented.
