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3100 RWD swap fuel pump(s) and fuel rail

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  • 3100 RWD swap fuel pump(s) and fuel rail

    1. I have a Walbro fuel pump I plan to use for my 3100 RWD swap but haven't decided on where to mount. I did a Ford EFI swap a few years ago and used a low-pressure/high volume pump at the tank to push fuel to the high pressure pump mounted underhood. The system works fine but did require extra wiring and relay to operate the second pump. I can make a similar system for my 3100 swap but would like to simplify if possible. How close to the tank do Walbro gerator type pumps need to pull fuel without cavitating? The fuel line from the tank does exit from the bottom of the tank so grvity will push fuel but not sure yet where I can mount the fuel pump in relation to the bottom of the tank.

    2. On my Ford EFI swap, I plumbed the fuel return line into the fuel line after the low pressure pump and before the high pressure pump. If I use two pumps, I can do the same for this swap or if I use only the one Walbro pump, I can plumb the return line into the fuel line before the pump. That is working well on the Ford EFI swap but does anybody know if that will also work with the GM EFI? What is the pressure range of the return line?

    3. I read somewhere on the net (can't find it again) the fuel rail on the 3100/3400 cannot be reversed. For my swap, the lower and upper intake are reversed (TB and thermostat towards front/pulleys). I have the fuel rail mounted with the fuel lines entering/exiting towards the rear (trans end) of the engine and it appears to fit OK. Does anybody know of any reason the fuel rail can't be reversed? I want to make sure I didn't miss something.

    Thanks again for all the help I've gotten from experts on this site.

  • #2
    Go with a single pump. Mount the pump as close to the tank as possible, and don't worry any more about it. You can have a few feet of feed line between the tank and pump without issue.

    Plumbing the return line back into the feed line defeats the purpose of the return line, you're just deadheading the system then. You will need to plumb the return line back into the tank, otherwise you will have very high fuel pressure. I honestly don't know how you don't have over fueling issues with the Ford, other than the high pressure return will at least allow a little bit of a pressure differential that may be allowing the fuel pressure to be sort of controllable.

    The fuel rail can be turned around with the upper intake and from my experience has to match the direction of the upper intake, due to the crossover and interference with the throttle body neck. In all my RWD swaps I've always pointed the fitting end towards the throttle body (front of engine).
    James: 1985 GMC Jimmy, 3.2L turbocharged, intercooled hybrid 13.873 @ 99.08 218HP & 270FT/lbs @ the wheels
    The Daily: 2000 GMC Yukon
    1973 Datsun 240Z: Turbo intercooled LX9, running on '7749 with $59 and DIS. 12.71 @ 115

    "If you're not living on the edge, You're taking up too much space."

    Still waiting for the 1st or second Ostrich I was promised, from Paul.

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    • #3
      I am leaning towards the single pump - just need to get under the car and find a good spot to mount it.

      The fuel return line on my Bronco is plumbed into the fuel line before the high pressure pump so the return fuel has no impact on the fuel pressure delivered by the high pressure pump. It has worked well for about 6 years.

      I have two 3100 upper intakes - one of them has a cast-in passage on the bottom of the TB neck. That may be what would hit the fuel rail. The other intake (without casting on bottom) seems to fit OK. I'll take a closer look at the crossover to make sure it isn't causing any bind that might prevent a good seal between the upper and lower intake.

      I appreciate the replies- I'd rather figure things out (as much as possible) now instead of after everything is in the car.

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      • #4
        In my RWD set up, to make it easier for me to route the supply/return lines, I pointed the fitting end of the fuel rail to the rear. Obviously, my throttle body points forward. This arrangement does make it difficult to test the fuel pressure at the Schrader valve. There is not a lot of room between the valve and the upper intake.
        Ed

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        • #5
          For my setup, having the fuel lines at the rear of the engine will work better- glad to know someone is running it that way. On my Bronco, I removed the schrader valve from the fuel pressure test fitting and grafted an electric oil pressure sender to the port so I could put a fuel pressure gauge inside the truck. When I turn on the key, I can see the fuel pump pressurize the fuel rail before starting the engine. It would also show if the fuel pump fails. I may try to do the same on the 3100 swap if there is enough room for a 45 or 90 degree elbow before the sending unit.

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          • #6
            The fuel pressure gauge is just an oil pressure gauge converted to read fuel pressure with a label maker. Before modifying the valve, I checked the fuel pressure with a handheld fuel pressure gauge to confirm I got the same pressure reading with the dash pressure gauge and it is spot on.

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