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Normal for gas tank to be pressurized?

Discussion in 'The Garage' started by mrk5, Jun 29, 2005.

  1. mrk5

    mrk5 The Sticker Guy Moderator Vendor GMOTM Winner Author

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    I've noticed lately when I take my gas cap off there's what I would call a good deal of pressure in the gas tank.

    I have a fuel return line.

    I'm wondering if I missed something when I did the diesel to gas swap. Or is this normal?

    TIA
     
  2. dyeager535

    dyeager535 1 ton status Premium Member

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    Need a *vent* line on the tank, or a cap that vents pressure much lower than stock EVAP equipped applications which is 16PSI IIRC. Haven't personally found a cap that will fit that vents as is needed, even though a non-EVAP application should have used a cap like that.
     
  3. Robert79K5

    Robert79K5 1/2 ton status

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    Yes you really need to get a gas cap spececified for a gas powered truck to put on there so it will vent. I threw a 350 in a c10 that had a diesel in it and didn't think to change the fuel cap.. well it did exactly like yours is doing but it built up so much pressure that after I shut it off the pressure caused gas to seep past the needle valve in the carb and fill the pistons with gas. Had to pull the plugs and change the oil before I could drive it back home. Im just glad it happeded at my fiance's house and not out in the middle of no where.

    Put a cap for a gas powered truck on there and all is well.
     
  4. mrk5

    mrk5 The Sticker Guy Moderator Vendor GMOTM Winner Author

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    Does just any cap work? (aside from it being for a gas application)

    My truck was military, so it has a big opeining without the little door thingy.
     
  5. dyeager535

    dyeager535 1 ton status Premium Member

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    Well, at some point GM changed the threads on the fill neck, for the most part all interchange though.

    From my experience, no, any cap will not work. EVAP caps are designed to vent around 16 PSI, which means a tank that has no vent line, and one of those caps, will build up 16PSI before it vents. That's alot. They allow air in easily, but that doesn't help much obviously. Suck on the bottom of the cap, then try to blow, you'll see.

    I've got a pic in my CK5 gallery showing a 2 valve cap my dad had laying around, which breathes at very low pressure both directions. I have that on my truck now.

    I went to Napa looking for a non-EVAP application cap (like a 1 ton 1970's truck, even my friends 1983 1 ton doesn't have EVAP IIRC) but all the caps crossreferenced to caps that are definitely EVAP applications.

    It could be that non-EVAP applications just used a breather on the sending unit, but that doesn't seem very likely. I haven't been able to inspect a non-EVAP sender to see.
     
  6. Russell

    Russell LB7 Tahoe Status Premium Member GMOTM Winner

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    My suburban tank has three lines coming out of the top, a supply line, return line and a vent line. The line originally had a little vent cap just sitting on it, but since my truck may occasionally get dunked a little bit, I will be relocating both the diff vent line, and the fuel tank line up a bit higher so they do not take on water when I am launching a boat, or playing in a deep mud pit or something.
     
  7. dyeager535

    dyeager535 1 ton status Premium Member

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    Pretty certain that vent was original?

    Is it the same style as is used on the axles for breathers?

    I thought GM had problems with the vented gas caps and rust, (what I've heard anyway) I'm surprised that they would vent the tank up against the underside of the bed/top of the tank.

    For those of us that don't submerge the trucks so far, that would work ok, it puts the breather about the same height as the rear axle breather...
     

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