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  • electric fuel pump ?

    whats up guys, have a quick question about my fuel pump. hopefully you can shed some light.

    i purchased this holley fuel pump from our host.
    as i am still putting my cj back together from the frame off, the engine(360 with holley 670 truck avenger) i custom bent my own 3/8ths fuel line and have the pump piped in close to the tank of course. it is currently just pumping straight to the carb. my question is do i need to make it a loop type system where it pumps to the carb and also returns back to the tank? it runs fine but i dont want to burn up the pump or screw up the carb. thanks in advance. jeff

  • #2
    vapor lock...

    Jeff, I'm not familiar at all with the electric pumps, but I've read on numerous occasions that one of the reasons for the 'loop' is to help avoid vapor lock in the heat. Might want to consider it for that reason alone if the electric pump can handle the flow.


    • #3
      Since you're running a carb, no need for a return feed.


      • #4

        Originally posted by NOCHEEPGAS View Post
        Since you're running a carb, no need for a return feed.

        NOCHEEP, not trying to start an argument on this one, but if you plug 'vapor lock' into the advanced search on the CJ forum you'll see a right good bit of 'discussion' on the return and how it affects vapor lock potential.

        However...there is always a 'however'... If you're in Arizona and have experienced no trouble with this I'm a little stumped on the 'vapor lock theories'...

        Would you elaborate on this one a little bit?


        • #5
          Just never had a problem with it before. What I have had happen was the vent on my fuel filler cap got plugged and as the day went on pressure in the tank built, pushing fuel past my carb seat assy. Flooded the whole engine, from the oil pan up! Talk about hydraulic lock! As far as fuel pump longevity goes, if it's constantly working (for no reason) it's life expectancy is shortened accordingly.


          • #6
            Trouble is most electric fuel pumps run the whole time the motor is running anyway. I have seen very very few that bother with a pressure shutoff switch.

            The theory behind the return line is simple enough...

            When you put mechanical energy into the fuel with a pump or whatever, the fuel has two choices, flow or heat up. If it can't flow it has to heat up because of the laws of physics and thermodynamics. Now if you stop the bulk flow of the fuel (with a closed float valve for example) the pump continues to put energy into the system and the fuel has to heat up. Add the fuel pump heat to the already warm under-hood temperatures and eventually the fuel will vaporize (boil) inside the lines. Fuel pumps are not designed to pump fuel vapor, so if that vapor bubble happens to occur inside the fuel pump, then you get vapor lock since the pump can't move the fuel any more.

            Adding a return line helps in two ways, first the fuel can continue to move so most of the kinetic energy that the fuel pump puts into the fuel doesn't change into heat. Second, the fuel tank is usually cooler than the lines inside the engine compartment (and has a lot more surface area to dissipate heat). Cool fuel is pulled out of the tank, then heated by the engine temperature and the fuel pump (some heating is inevitable) then returned to the cooler fuel tank if it is not needed by the engine.

            So in short, if the electric pump you have does not have a pressure shutoff switch built into it, and it has enough flow to maintain pressure with a fuel return, then it is a good idea to put one in.