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Fuel tank problem

Discussion in 'Early CJ5 and CJ6 Tech' started by scott milliner, Apr 25, 2014.

  1. Apr 25, 2014
    scott milliner

    scott milliner Master Fabricator

    Seattle Wa.
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    I installed an electic fuel pump in my gas tank with a return line for the TBI. Now the tank is building up a lot of pressure. Any ideas how to vent the under seat tank? I never had this problem with the old motor.
     
  2. Apr 25, 2014
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor

    Medford Mass USA
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    I suspect there has to be something wrong, and the return flow now contains some air. If anything, the pressure above a sealed tank should go down, since some of the tank volume (the consumed gasoline) is being removed from the tank. So air has to be injected into the stream somewhere to get a pressure build up.

    Try directing the return stream into the bottom of a bucket, and see if there are bubbles in the stream.
     
  3. Apr 25, 2014
    Mike S

    Mike S Sponsor

    Cameron Park Ca.
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    Tim has a good suggestion, try chasing it down.

    You could also add a vent line fitting to the filler neck pretty easily. Run a line from the fitting with a couple circles/loops in it, and dump the open end overboard.
     
  4. Apr 25, 2014
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor

    Medford Mass USA
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    Also, do you have two tanks? You're not pulling fuel from one and draining it into the other?
     
  5. Apr 25, 2014
    scott milliner

    scott milliner Master Fabricator

    Seattle Wa.
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    Only one tank. I was thinking of running a vent line into a carbon canister. Being most newer tanks are vented that way.
     
  6. Apr 25, 2014
    PeteL

    PeteL If it wasn't for physics, and law enforcement... 2024 Sponsor 2023 Sponsor 2022 Sponsor

    Hills of NH
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    Sounds to me like the constant flow would saturate a canister. They are only meant to be a reservoir for overflow, as I understand them, which I don't much.
     
  7. Apr 25, 2014
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor

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    The carbon canister needs to be (nearly) constantly purged by engine vacuum, otherwise it will become saturated with fumes and potentially drip liquid gasoline. Its main purpose is to trap fumes when the engine is off, so they can be captured by engine vacuum when the engine is running. When the engine is running, any vapors can be consumed directly by the engine vacuum. Seems unlikely that a constant pressurized stream of vapor could be handled by the canister. And realize that gasoline is being carried away with any fumes that you vent to the atmosphere.

    I've read a lot of threads about EFI conversions, and this is the first I've read about the return pressurizing the fuel tank. There has to be something else going on.
     
  8. Apr 25, 2014
    Waggs

    Waggs Member

    Peyton, CO
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    This may be a silly question/comment, but are you sure it's pressure and not vacuum?

    When the tank on my 4-Runner gets low and I crack the gas cap, it's sucks air.

    What are the indications of pressure?

    Waggs
     
  9. Apr 25, 2014
    Warloch

    Warloch Did you say Flattie??? Staff Member

    Falcon, CO
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    Just as a note - all of the FI systems I have worked with installing require a vent for just such a reason. FI runs at higher pressure and as such seems to inherently introduce some air into the system as it moves through. With 1/8" steel tanks, I didn't get swelling, but a blocked vent sure caused flow problems and thus hesitation and wouldn't keep running. Once vent was cleaned, problem went away.

    Vent to charcoal canister is what is recommended per install instructions.
     
  10. Apr 26, 2014
    45es

    45es Active Member 2024 Sponsor 2023 Sponsor 2022 Sponsor

    Naches, WA
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    I am probably off base but I think the pressure build up is being caused by the fuel being warmed by the engine and returned to the tank. If I remember correctly, you had a carbureted 3.8L v6 previously and no return line to tank. When I install my fuel injected v6 I also experienced a pressure build up in the tank. It is no greater than some cars I have seen in the past and has not been or created a problem.
     
  11. Apr 26, 2014
    Phil P

    Phil P Member

    Ottawa Canada
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    My gas cap had a vent that was painted over when I got it. I just cleaned the vent and have had no problems sense. I replace a carb engine with a 4.3 TBI
     
  12. Apr 26, 2014
    timgr

    timgr We stand on the shoulders of giants. 2022 Sponsor

    Medford Mass USA
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    Not off base. That's plausible. When the fuel temperature rises, the partial pressure from the fuel vapors also increases. So pressure in the tank would rise, if the tank were sealed.
     
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