No announcement yet.

difference in between in-tank vs. external fuel pump

  • Filter
  • Time
  • Show
Clear All
new posts

  • difference in between in-tank vs. external fuel pump

    subject pretty much explains in. I was just wondering what the difference was. Trying to figure out which one to change?

  • #2
    Usually the intank pump goes bad first and will then result in the external pump going bad.

    What are the symptoms you are chasing?
    "Never lift"


    • #3
      The external pump is the main pump. The in tank pump is just an auxillary pump to assist the main.


      • #4
        As adam says. the in-tank unit is a "lift " pump to pull the fuels up against gravity to a level above the tank. It has less pressure than the main pump.
        The main pump is a pusher pump, and does not pull fuel up against gravity easily. It sounds loud and strained and fails quickly if the in-tank unit is inoperative.

        There is RUMOUR about a ford mustang Cobra pump which is part of an in-tank single pump, able to be used in the BMW tank, with the elimination of both BMW pumps.


        • #5
          i changed out the external pump, but still no go. I do have the in-tank pump, but i dont want to intall it untill i find out more.

          is it the external pump that makes the buzzing noise? if it is, then there must be no power, because the new pump doesnt make any noise.

          after looking in the engine bay....i found some wires burnt through that connected to the long plastic thing that runs parallel to the fuel rail.

          where do these wire go? can this be the culprit?

          here's a picture (fuel rail is just above):

          Last edited by KauzE30M3; 04-28-2004, 02:21 PM.


          • #6
            try this, if no pic


            • #7
              I have just changed my external and internal pump. My old external pump buzzez as well as the new one but not as noisy as the new one. The reason I changed both was that I was getting a low fuel pressure reading and of course I was worried with the buzzing sound of the external pump. After changing both pumps, I still had the same situation (low fuel pressure). Again I suspected my adjsutable FPR is faulty, also changed to the old one to check. After spending $$$ for the pumps, I found out my fuel pressure gauge faulty and giving false reading! There was nothing wrong with my fuel system but a faulty fuel pressure gauge. I am learning everyday on this car! Anybody needs a fuel pump?



              • #8
                that is the power supply circuit to the injectors.
                you'll need to repair the harness and hope that the fuel pump relay is ok.
                You'll also need to check the fuses for the electrical components of the fuel injection.
                If you have no main pump after the check and repairs, you'll need to find out if the fuel pump relay has failed.
                Last edited by Mmark; 04-29-2004, 02:57 AM.


                • #9
                  The first thing I replaced was the relay. I even check the plug where the relay sits in, and i got a reading, so that should be fine.
                  I'm gonna repair those wire this weekend before I put that in-tank pump in. Hopefully that's it.
                  I've also checked the fuses and they look good. I just want to double check that fuse #11 is 7.5a. Because the one in there right now is 15a.

                  There was nothing wrong with my fuel system but a faulty fuel pressure gauge
                  It sucks after changing stuff up you come to find out nothing was wrong, or it was something simple. I just take it as a learing experience.


                  • #10
                    fuse 11 is 7.5A . A 15 was put in there because a failing component had excessive current draw and was popping the 7.5A.

                    I would slit a small rubber hose and cover the repaired area for more protection too.


                    • #11

                      Would the exposed wires, in any way keep power from going to the fuel pumps? Where else can I look?



                      • #12
                        Yes, They were shorting out and likely blowing the fuses 9, 10,11 that protect the injection electronics, and possibly damaging the DME ECU.
                        there's also the possibilty that the injectors were damaged by the short circuit.
                        If the main pump is not running, you might want to check if it and/or its wiring supply are ok by jumping/bypassing the fuel pump relay.
                        After you jump the relay, If the pump runs, the relay is bad. If the pump still won't run, the wiring to the pump may be bad, the pump may be bad, or the DME ECU has failed.
                        To double check the pump you can put a 12v battery by the pump and connect a couple of wires with alligator clips from the batt to the pump.


                        • #13
                          I just got it towed to the shop....keep ya'll posted. Schools almost done....too much to do.