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Fuel Tank Sending Unit Tech Question

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by ike, Oct 4, 2005.

  1. ike

    ike Registered Member

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    I'm replacing my rusty 31 gal. fuel tank and old sending unit in my 76 Blazer with a new tank and sending unit from Classic Industries.

    My question is in regard to the sending unit.The old sending unit had two fuel lines going to the fuel pump,one a little larger the the other maybe 3/8 and the smaller 5/16 ? I'm thinking the larger line is a delivery line and the smaller line is some kind of return line for smog ? Do I need to , or should I run the smaller line?

    I'm replacing the fuel pump and the carburator at this time as well. I'm pretty sure I can get a fuel pump with one inlet. I think I'll run a Edelbrock 600cfm ( I don't think thats a factor )

    The only other thing to add is , I don't need this thing to pass smog...It's titled as a 75 ( Thats why we bought it )

    Whats that small line for ???

    Thanks
     
  2. ike

    ike Registered Member

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    More sending unit questions

    Looks like there are more then a few different types of sending units GM used on the same year trucks...

    My old sending unit as well as the new one has four pipes for hose hookups coming out of the top..From left to right facing away from me,the smallest pipe
    (1/4 ?) I think this is a hookup for a return line from the fuelpump ( This is the hookup I'm thinking can be plugged and not used . Next #2.......

    (3/8 ?) This has to be the delivery hookup with the long pipe going in the tank. Next #3....

    (5/16 ?) This hookup looks like a vent..It has a bell shaped thing that would be in the tank after mounting....I think my old unit has a hose with a breather of some sort on it.......Next #4...

    ( ? 1/2) A hose runs from this hookup to the fuel neck for venting when fueling.....

    The question is should I run the return line (If thats what it is ) I don't want to run it if it's not needed.......I would rather keep it simple..
     
  3. dyeager535

    dyeager535 1 ton status Premium Member

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    Probably more personal preference than anything. If the line doesn't exist on the frame already, I wouldn't run it. But it is useful to help prevent vapor lock.

    The vent is more important, if you don't vent the tank somehow, you'll end up with pressure in the tank which can cause problems.
     
  4. btolenti

    btolenti 1/2 ton status

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    I would think the smaller one would be for the evap canister in the engine bay....especially if the truck is an original california model...
     
  5. ike

    ike Registered Member

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    evap canister in the engine bay

    There is a evap. canister in the engine bay but I think the inline goes to the fuel pump.....(1/4) line on the far left of the pickup...Im going to take a real good look at it today....
    Is that the line your are refering to...Or are you talking about the real small line...the vent ?
     
  6. btolenti

    btolenti 1/2 ton status

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    the 5/16" is the EVAP line
    the 1/4" is th fuel return from the pump
    the 3/8" is the fuel supply line to the pump
    THe large one is the vent for the fill tube.
     
  7. ike

    ike Registered Member

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    1/4 fuel return line

    Can I cap the hookup for the return line at the pickup and run a fuel pump with one inlet? Why do I need or want this return line??
     
  8. mofugly13

    mofugly13 1 ton bucket of rust Premium Member

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    I swapped a 31 gallon into my K5 and the swapped in sending unit had 4 lines, whereas the old one had only three. I have only a two port (in/out) fuel pump so I just capped the return line. I did put on about 18" of hose first and then capped it. My spare pump has the return port on it, so I figured if I ever put that one in, I might hook the line from the tank to it, but prolly not.
     

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