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DIY - Double to Single Fuel Pump Conversion

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    DIY - Double to Single Fuel Pump Conversion

    First, a word of warning:
    I did this on a pre-9/1987 325i convertible and everything I post is applicable to how I did it on MY car. Your car may be different. It is YOUR responsibility to look up the Electrical Troubleshooting Manual (ETM) for YOUR car so YOU can verify how it was wired.
    BMW ETMs

    Next, you are working with gasoline and electricity. It can be very dangerous if youíre not careful. Although there should be too much fuel to not enough oxygen for a spark in the gas tank to ignite fuel vapors, this is still a possibility because you are modifying electrical connections.

    Finally, do not attempt this if you feel uncomfortable performing this modification. Read the entire write-up before starting and make sure you have the appropriate tools.

    Parts required:
    -TRE-340 (comes with fuel pump, fuel filter, fuel line, 2 hose clamps, long fuel line, electrical plug, insulation, and rubber cap)
    -3/8 in barbed fuel nipple (a slightly larger size would be better, preferably 1/2 inch since the larger line is 13mm ID)
    -5/16 in barbed fuel nipple
    -Union to connect the two nipples
    -Teflon tape (for pipe threads)
    -Heat shrink cable wrap
    -extra hose clamps as needed

    Tools required:
    -Philips head screwdriver
    -Flat head screwdriver
    -10 mm socket
    -8 mm socket
    -socket wrench
    -wire cutter
    -wire stripper
    -multi-meter
    -lighter
    -soldering gun and solder
    -sand paper

    Initial steps not covered:
    Remove the in-tank pump, the fuel filter, the external pump, and the mounting bracket for the external pump/fuel filter.

    How to convert:
    Here is an image showing the fuel system from the ETM for my car. It shows that both fuel pumps are fed by a common wire. It also shows that the wire to the external pump has a diameter 2.5 times larger than the diameter of the wire going to the in-tank pump. I am assuming that the wire going to the single in-tank pump will be capable of providing enough electricity without overloading and/or shorting. I intend to cap off the wires for the external pump and use the in-tank pump wires to power the new TRE pump.


    Once you have the in-tank pump out, bring it to your work bench. Notice that the BLACK wire goes to the positive terminal on the pump and the negative terminal grounds to the part holding the pump. The unit will look all rusty like this:


    Iíve cleaned the holder and set the pump next to the holder in the approximate spot where it would be attached. Notice that the strainer is roughly 0.5 cm below the bottom of the metal return fuel line. This is the location you want the new pump filter so you donít experience fuel starvation in corners or ďrun out of gasĒ when there is still fuel in the tank.


    Here are the contents of the TRE-340 fuel pump kit:


    The fuel line supplied with the pump easily wonít fit over solid line, so I re-used the old reducer. Here is pump mocked up with the old reducer. Notice that the pump extends roughly 2.5 cm past the return hardline. It is too long when positioned like this and will hit the bottom of the fuel tank before you can tighten the pump unit. Make sure you visually inspect the reducer for cracks. My reducer split down the side one day as I got to work. It was a huge pain in the ass to limp it home, but I eventually made it. I spent about 10 minutes looking for a part number for the reducer, but it looks like it only comes as part of the fuel pump assembly. It is possible to heat and stretch the opaque plastic tube that comes with the TRE pump to make it fit on both the metal hardline and the pump. (It's just not the best option.)


    I have cut the solid line so that the top of the old reducer is right at the bend. You can see that the bottom of the fuel pump is now in roughly the same location as the stock strainer:


    Now slip a piece of heat shrink cable insulation over the ling RED wire for the fuel pump plug. Then solder the end of the RED wire to the BLACK wire on the pump holder.


    Pull the heat shrink over it and shrink it:


    Now solder the BLACK wire to the pump holder return hard line. This is a good time to test for continuity. First test the positive wire from the car side plug to the pump side plug, then repeat for the negative wire. Next, plug in the pump and test for continuity from the positive to negative terminals on the car side plug. If everything is soldered correctly you should be good to move on to the next step.


    I zip-tied the black wire into place so it couldnít move and break the solder joint. Then I put the insulation over the pump and slid the black rubber bottom piece on. Iím not sure either of these are needed, but the insulation makes it easier to hold the pump to the metal unit. Then I used the supplied hose clamps to tighten the old fuel hose reducer to both the metal unit and the pump. At this point the pump should not be able to fall off, however, it would still shake around. So I used metal zip-ties to secure the pump to the return line. I tried using a hose clamp, but the hole in the gas tank is too small to accommodate the pump and a hose clamp. I considered using plastic hose clamps for this, but was concerned about them degrading in the fuel. Finally, I plugged in the pump and zip-tied the extra wire length to the metal holder. Make sure you do this so it will be out of the way of the fuel level sending unit!.




    Once the in-tank pump is installed, itís time to by-pass the external pump. Take some heat shrink cable wrap and cover the wires that power the pump. Then push them up into their protective covering. This step ensures that you wonít have a short because unused connectors are dangling against the car. (I would suggest not cutting them in case you or the next owner wants to revert back to the original setup.)



    These are the pieces I got from Advance Auto. The 3/8 in fuel line nipple is a little smaller than the stock 13 mm diameter large fuel line by roughly 3.475 mm, which is why I would suggest finding a 1/2 in nipple (1/2 in = 12.7 mm). The 5/16 in fuel line nipple is basically a perfect fit. Make sure you wrap the threads with Teflon tape and then tighten them together.



    Next, take the mount that held the external fuel pump bolt it back into the car. Run the large fuel line through the opening for the external fuel pump and connect the line that will go to the fuel filter. I suggest zip-ting the electrical connectors from the external pump to the mount that held the external fuel pump so they donít flop around.



    Make sure the large hose clamps are in over the fuel line so you can hang the fuel filter, then connect the fuel lines to both sides of the fuel filter. Finally, hang the fuel filter and replace any trim bits.

    Last edited by McGyver; 11-11-2015, 12:33 PM.
    sigpic
    1987 - 325i Convertible Delphin Auto [SOLD], 325i Convertible Delphin Manual [SOLD]
    1989 - 325i Convertible Bronzit m30b35 swapped [SCRAPPED], 325i Sedan Alpine Auto[DD]
    1991 - 325i Coupe Laguna Manual [Project], 535i Sedan Alpine [SCRAPPED]

    #2
    Any idea if this would work on the Eta, as it has the dual pump set up as well?
    Estoguy
    1986 BMW 325, Alpenweiss ~ "Elsa"

    Need a photographer, come visit my site: http://estoguy.wix.com/unique-perspectives

    Comment


      #3
      Originally posted by estoguy View Post
      Any idea if this would work on the Eta, as it has the dual pump set up as well?
      You need to check the ETM for your m20b27 car to make sure you know how the electrical system works. I updated the DIY to include a link to the BMW ETM directory. You also need to use realoem to look at how the fuel system works for your car to make sure it is similar to an m20b25 car. I assume everything is roughly the same, but BMW was just coming off their '70s cocaine binge and anything was possible.
      sigpic
      1987 - 325i Convertible Delphin Auto [SOLD], 325i Convertible Delphin Manual [SOLD]
      1989 - 325i Convertible Bronzit m30b35 swapped [SCRAPPED], 325i Sedan Alpine Auto[DD]
      1991 - 325i Coupe Laguna Manual [Project], 535i Sedan Alpine [SCRAPPED]

      Comment


        #4
        I can't believe that you left all that rust on the holder. You can clean all that off then coat it with plastidip to make sure that it will never rust again.

        Comment


          #5
          Should be the same for all cars with the dual fuel pump setup ie 84-87 325e and 88-91 M3's as well. Still not sure what the vert cutoff is...was it when they facelifted it in 91 or before?

          Anyway, many years back I dumped about $300 + to get the 2 OEM pumps but if I had to do it again, I'd probably go this route. What concerns me though is the part about the wiring being much smaller than the main pump wiring. This is because the original intank pump is only meant to be a transfer pump. The transfer pump's smaller wiring may be sufficient but it depends on how much current the aftermarket pump draws for your needs. I would not just assume it is with something as critical as this.



          Does anyone know the gauge of the transfer pump wiring?

          If it's the same diameter of the wiring at the pump, then it appears to be 16 gauge and at around 5-6 ft it should be okay for at least 200lph which is more than the stock main pump. The tre 340 is actually 255lph at just under 13A.
          Last edited by reelizmpro; 10-01-2015, 02:41 PM.
          "I'd probably take the E30 M3 in this case just because I love that little car, and how tanky that inline 6 is." - thecj

          85 323i M TECH 1 S52 - ALPINEWEISS/SCHWARZE
          88 M3 - LACHSSILBER/SCHWARZE
          89 M3 - ALPINEWEISS II/M TECH CLOTH-ALCANTARA
          91 M TECHNIC CABRIO TURBO - MACAOBLAU/M TECH CLOTH-LEATHER

          Comment


            #6
            Originally posted by reelizmpro View Post
            Does anyone know the gauge of the transfer pump wiring?

            If it's the same diameter of the wiring at the pump, then it appears to be 16 gauge and at around 5-6 ft it should be okay for at least 200lph which is more than the stock main pump. The tre 340 is actually 255lph at just under 13A.
            According to the ETM I have in the in the OP, the wiring from the fuse box all the way back to the external pump has a cross-sectional area of 2.5 mm^2, so thats ~13 gauge. The wire for the in-tank pump splits off somewhere between the fuse box and the external pump and has a cross-sectional area of 1 mm^2, so that ~17 gauge.
            http://www.engineeringtoolbox.com/aw...uge-d_731.html
            sigpic
            1987 - 325i Convertible Delphin Auto [SOLD], 325i Convertible Delphin Manual [SOLD]
            1989 - 325i Convertible Bronzit m30b35 swapped [SCRAPPED], 325i Sedan Alpine Auto[DD]
            1991 - 325i Coupe Laguna Manual [Project], 535i Sedan Alpine [SCRAPPED]

            Comment


              #7
              17 gauge is smaller than I was hoping and I assumed it split where the main pump is for the shortest possible length. From what I've gathered that's going to be marginal. I would probably run the 13 gauge from the main pump to the tank to be on the safe side especially if you have any mods that require more fuel. It wouldn't be too hard to do either. Is your engine stock? running well after the conversion?
              "I'd probably take the E30 M3 in this case just because I love that little car, and how tanky that inline 6 is." - thecj

              85 323i M TECH 1 S52 - ALPINEWEISS/SCHWARZE
              88 M3 - LACHSSILBER/SCHWARZE
              89 M3 - ALPINEWEISS II/M TECH CLOTH-ALCANTARA
              91 M TECHNIC CABRIO TURBO - MACAOBLAU/M TECH CLOTH-LEATHER

              Comment


                #8
                Originally posted by reelizmpro View Post
                17 gauge is smaller than I was hoping and I assumed it split where the main pump is for the shortest possible length. From what I've gathered that's going to be marginal. I would probably run the 13 gauge from the main pump to the tank to be on the safe side especially if you have any mods that require more fuel. It wouldn't be too hard to do either. Is your engine stock? running well after the conversion?
                I've done this on both of my 1987 verts with bone stock engines. The manual has been running great and seems to have the correct amount of power. Other than than the reducer splitting (as I said in the edited OP), it's been running great. The auto is running a little rough, but that's probably because the gas is more than 2 years old at this point.

                The TRE pump makes perceptible noise at idle at a stoplight, but isn't that noticeable when driving or with music on.

                I agree, it would probably be a good idea to run a thicker wire to the in-tank pump (especially if you have any engine upgrades), but so far it seems to not be necessary on a bone stock engine.
                sigpic
                1987 - 325i Convertible Delphin Auto [SOLD], 325i Convertible Delphin Manual [SOLD]
                1989 - 325i Convertible Bronzit m30b35 swapped [SCRAPPED], 325i Sedan Alpine Auto[DD]
                1991 - 325i Coupe Laguna Manual [Project], 535i Sedan Alpine [SCRAPPED]

                Comment


                  #9
                  This is more or less how I did mine except I used a 86-93 moostang pump from the parts store, which is basically what most after market pumps are modeled after. I also moved the fuel filter to the back as mine was up under the brake booster and about impossible to get to.

                  It is possible to use a standard hose clamp to mount the pump, you just need to grind a a bevel on the head of the screw so it fits in the hole of the tank.

                  Comment


                    #10
                    Just did this today and 3/8" is definitely too small especially if the hose has swelled a bit. Hopefully it doesn't leak in the meantime. I'd say go 1/2" or 5/8".
                    "I'd probably take the E30 M3 in this case just because I love that little car, and how tanky that inline 6 is." - thecj

                    85 323i M TECH 1 S52 - ALPINEWEISS/SCHWARZE
                    88 M3 - LACHSSILBER/SCHWARZE
                    89 M3 - ALPINEWEISS II/M TECH CLOTH-ALCANTARA
                    91 M TECHNIC CABRIO TURBO - MACAOBLAU/M TECH CLOTH-LEATHER

                    Comment


                      #11
                      After a few days it doesn't appear to be leaking. All is well. I measured current draw at the pump as 6.47 amps. Using online calculators, 17 gauge can support 6.5 A upto 6ft or so. We just have to determine exactly how long that wiring is.

                      Edit, interestingly the ETM for 89 M3 shows that the main pump and transfer pump use the same 1.5mm square wiring.
                      Last edited by reelizmpro; 10-14-2015, 03:50 PM.
                      "I'd probably take the E30 M3 in this case just because I love that little car, and how tanky that inline 6 is." - thecj

                      85 323i M TECH 1 S52 - ALPINEWEISS/SCHWARZE
                      88 M3 - LACHSSILBER/SCHWARZE
                      89 M3 - ALPINEWEISS II/M TECH CLOTH-ALCANTARA
                      91 M TECHNIC CABRIO TURBO - MACAOBLAU/M TECH CLOTH-LEATHER

                      Comment


                        #12
                        Originally posted by reelizmpro View Post
                        Just did this today and 3/8" is definitely too small especially if the hose has swelled a bit. Hopefully it doesn't leak in the meantime. I'd say go 1/2" or 5/8".
                        I did notice that, but wasn't able to get a better size from local parts stores. I'll add info to the OP so people know to look around a bit more if they want a better fit. However, I've been DDing the car since the write-up with no issues or obvious leaks.
                        sigpic
                        1987 - 325i Convertible Delphin Auto [SOLD], 325i Convertible Delphin Manual [SOLD]
                        1989 - 325i Convertible Bronzit m30b35 swapped [SCRAPPED], 325i Sedan Alpine Auto[DD]
                        1991 - 325i Coupe Laguna Manual [Project], 535i Sedan Alpine [SCRAPPED]

                        Comment


                          #13
                          Yeah, it seems be working just fine. It just worries me possibly having a fuel leak in front of the rear muffler. I've been keeping an eye on it and it seems okay. Another option is to use a 1/2" to 3/8" barbed reducer and 3/8" to 5/16" barbed reducer to step the hoses down. A 1/2" to 1/4" barbed reducer would work too...if you can find one for fuel. This is another option that will get the job done and cost about the same. If I had to do it again, I'd probably do it this way just to be safe.

                          I have not found a step down submersible hose that will work for the intank pump but I cut my stock reducer as short as possible and clamped them. Hopefully, it won't split. Still looking for the perfect solution.
                          Last edited by reelizmpro; 11-11-2015, 01:20 PM.
                          "I'd probably take the E30 M3 in this case just because I love that little car, and how tanky that inline 6 is." - thecj

                          85 323i M TECH 1 S52 - ALPINEWEISS/SCHWARZE
                          88 M3 - LACHSSILBER/SCHWARZE
                          89 M3 - ALPINEWEISS II/M TECH CLOTH-ALCANTARA
                          91 M TECHNIC CABRIO TURBO - MACAOBLAU/M TECH CLOTH-LEATHER

                          Comment


                            #14
                            I shortened the tube on the hanger and brazed a smaller nipple on it so I can use the special hose that comes with most pumps, I didn't know how the factory reducer would have liked the higher pressure of the newer pump.

                            Comment


                              #15
                              Bit of a bump, but I'd like to say it's important that you do not cut the original wires for the external pump. I did so and it turns out that positive side is made of 2 wires connected at the pump, one of which is the only power lead that goes to the internal pump. I soldered both positive wires back together and my internal pump started functioning again

                              Comment

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