Dismiss Notice

Welcome To CK5!

Registering is free and easy! Hope to see you on the forums soon.

Score a FREE t-shirt and membership sticker when you sign up for a Premium Membership and choose the recurring plan.

FUEL TANK VENTING

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by jackedjimmy, Jul 10, 2002.

  1. jackedjimmy

    jackedjimmy 1/2 ton status

    Joined:
    May 18, 2002
    Posts:
    195
    Likes Received:
    0
    Location:
    Redmond WA
    On my 81 jimmy I have removed all of the factory gas ad brake lines and am going to replace them with new ones. The truck orrininally had a 305 in it and when i got it, it had a fried 350 in it. Any way, I wanted to know what I should do with all ofthe remaining leads that come out of the 31gallon tank.

    There are atleast three venting leads that come out of the tank and I was wondering if I should just run rubber lines up into the fenders and use a diff breather to cap em off.

    The reason I'm doing this is becaus I'm switching to an electric fuel pump and a fuel filter in the frame rail ad i don't see the need for the extra lines.
     
  2. dyeager535

    dyeager535 1 ton status Premium Member

    Joined:
    Dec 13, 2000
    Posts:
    26,975
    Likes Received:
    189
    Location:
    Roy WA
    One of the lines is fuel return from the mach. pump...aftermarket electrics don't have those do they? (could come from an inline filter as well I believe) The third line should be for the EVAP cannister.

    You have to run a pre-EVAP gas cap if you plug the vent line. Later (like yours) gas caps vented at a relatively high PSI, the earlier ones either vented at very low PSI (2PSI maybe?) or ran at atmospheric pressure. If you don't run an early one, you'll end up with the gas cap hissing after driving it, and gas will come flying out everytime you take the cap off, drenching you if you make the mistake of standing in front of it.
     
  3. jackedjimmy

    jackedjimmy 1/2 ton status

    Joined:
    May 18, 2002
    Posts:
    195
    Likes Received:
    0
    Location:
    Redmond WA
    That sounds good but i am not going to run a charclo canister because of hte new electric pump. so what should i do about the return line.

    I don't think that there is a lot of gas actually coming back, it's probably just vapor right?
     
  4. dyeager535

    dyeager535 1 ton status Premium Member

    Joined:
    Dec 13, 2000
    Posts:
    26,975
    Likes Received:
    189
    Location:
    Roy WA
    I have a feeling there is a LOT of gas going back to the tank when cruising. High RPM's with low fuel demand on a mechanical pump would mean to me that it would be sending a fair amount back to the tank. I don't see why this is necessary on an electric pump though, really, although IIRC, even the GM electric fuel pump setups use a return line. Maybe to keep gas temperature as low as possible to prevent vapor lock? It wouldn't be significant, but I guess it could be worth it....pure speculation there though.

    Charcoal cannister really doesn't have a lot to do with what pump you use, both mech and electric pump equipped trucks had the cannister. You could cap the line or put an axle breather cap on the line, but personally, I'd rather cap the EVAP line and vent at the cap.

    Then again, I've got the EVAP line where its supposed to be, with no cannister. I was going to fix that, (extra gas vapor in the engine bay isn't my idea of a good idea) but I always forget. So far it hasn't blown my hood off : )

    I might go back to the evap eventually, it was kind of pointless to remove it, not like anything else needs to go where the cannister is, and if I ran it with 2 hoses, it wouldn't be complex or "cluttered". With a carb it is pretty easy, I had a 2 line EVAP cannister, maybe 3 with one line plugged, and I just had the tank line into it, and the purge line straight into the carb. Not the way GM set it up, but the lesser of two evils I guess : ) Might as well burn that gas since you paid for it!
     
  5. jackedjimmy

    jackedjimmy 1/2 ton status

    Joined:
    May 18, 2002
    Posts:
    195
    Likes Received:
    0
    Location:
    Redmond WA
    so just run my single line from the tank tot he pump and filter?
    i might have miss read your info but i still don't know wht to do with the return and I dont know where the return would be coming from in my setup of the electric pump, canister style fuel filter and up to probably a demo carb.

    any/all help apreciated
    thanks
     
  6. dyeager535

    dyeager535 1 ton status Premium Member

    Joined:
    Dec 13, 2000
    Posts:
    26,975
    Likes Received:
    189
    Location:
    Roy WA
    The fuel "out" line is the one to the mechanical pump. That is what would go to the pump, then the filter.

    I really am not sure HOW an aftermarket pump should be setup return line wise. I only know that GM runs a return line whether the setup is mech or electric. The electric pump return though comes off of the fuel filter, since the pump is in the tank.

    I have a feeling if the aftermarket setup is designed for a fuel return line, there would already be the fitting on the filter for it.

    FWIW, I'm personally not a big fan of aftermarket electric pumps, they are noisy, a pain to mount correctly, you have to cut the stock hard fuel lines, and you have to be careful on their flow rating, and on top of that, add in a fuel pressure regulator thats normally needed, which failed (yes, given, that is unusual) in my dads truck. I guess without an injected setup, you'd still have to add in a pressure regulator though.

    Dad went through all of this on his 454 with electric pump, and is now back to running a mechanical pump, which works just as GM intended.
     
  7. TopOff

    TopOff 1/2 ton status

    Joined:
    Jan 16, 2002
    Posts:
    1,760
    Likes Received:
    0
    Location:
    Folsom, ca
    What kind of aftermarket electric fuel pump is it? If it is the low pressure fuel pumps you probably don't need a return line.

    On the FUEL-Injected fuel pumps they need to move the same amount of fuel all the time to cool themselves. Hence, these need a return line.

    Check with the fuel pump manufacturer.
     
  8. jackedjimmy

    jackedjimmy 1/2 ton status

    Joined:
    May 18, 2002
    Posts:
    195
    Likes Received:
    0
    Location:
    Redmond WA
    its a higher volume pumpdesigned specifically for aftermarket for carb trucks only. I got it through my parts guy at NAPA. I'll askhim about it next time I'm there.
     

Share This Page