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How does fuel make it from the LH side of the tank make it to the RH side?

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    How does fuel make it from the LH side of the tank make it to the RH side?

    My came to spluttering halt with half a tank of fuel.

    I think the fuel pump might be at fault but how does fuel make it from the left hand side of the tank make it to the right hand side?

    What is the sucking jet for?

    I've just checked hidden function 6.0 and i get 103208

    So i guess i have 10.3 litres in the LH tank and 20.8 litres in the RH tank.

    The pump has 12 volts going to it, so i guess it must be at fault.
    Last edited by ac427; 08-13-2020, 08:57 AM.

    #2
    I've never looked in the tank in much detail before but I think the tank is just at an angle so that when the right side of the tank is low the fuel settles on the left side.

    BW makes a fuel starvation kit that well help alleviate this issue in hard cornering and physically pumping the fuel from the left to the right side.

    If you turn your key in the ignition and don't hear the fuel pump prime and you are measuring 12v at the pump connector then the pump is bad and must be replaced.


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      #3
      The siphon pump (it’s a passive pump) siphons fuel form left to right as the level drops on the right side.

      What happens is the pump comes off of the housing. Probably need to just put it back on or replace the sucking jet pump.

      Could use a fuel resistant adhesive to make sure it stays on.


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        #4
        Originally posted by bigjae46 View Post
        The siphon pump (it’s a passive pump) siphons fuel form left to right as the level drops on the right side.

        What happens is the pump comes off of the housing. Probably need to just put it back on or replace the sucking jet pump.

        Could use a fuel resistant adhesive to make sure it stays on.


        Sent from my iPhone using Tapatalk
        Learn something new every day. Ignore what I said op.


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          #5
          As per WDS...Sucking jet pump (ejector) in fuel tank

          The sucking jet pump is a part of the fuel tank and pumps fuel from the left-hand tank half into the right-hand half. Since the sucking jet pump delivers the fuel directly into the fuel baffle, it is always filled first, thus ensuring that the electrical fuel pump always has a maximum of fuel available.
          The sucking jet pump is driven by the fuel in the return flow line. It is routed via a jet nozzle and accelerated through a narrow cross-section. The fuel flowing at high speed produces a vacuum which pumps the fuel from the left-hand tank half into the right-hand half.
          To ensure that the fuel pressure produced by the constricted nozzle in the sucking jet pump cannot rise too high, a pressure relief valve is installed to limit this pressure. Too high a pressure would result in too high a fuel flow rate, thus restricting or cancelling fuel delivery.
          A displaced nozzle of the sucking jet pump or a defective pressure relief valve result in the vehicle breaking down due to a fuel deficiency, although fuel is still indicated by the fuel gauge. In both cases fuel is no longer pumped from the left-hand tank half into the right-hand half.

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            #6
            Originally posted by bigjae46 View Post
            The siphon pump (it’s a passive pump) siphons fuel form left to right as the level drops on the right side.
            There's no such thing as siphon pump for this case; siphon is created by gravity and it is free; this is why human have no need to make a siphon pump.



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              #7
              Originally posted by Pnick View Post
              The sucking jet pump is driven by the fuel in the return flow line. It is routed via a jet nozzle and accelerated through a narrow cross-section. The fuel flowing at high speed produces a vacuum which pumps the fuel from the left-hand tank half into the right-hand half.
              +1. It is similar pulling action as fuel is pulling by high speed air flowing in the carburetor Venturi.

              The OP issue of empty RH but plenty of fuel LH tank is that the main pump ($300) worn and weak and so the volume of fuel return is less than spec., and so it has lower flow speed and weaker suction to pull fuel from LH tank.



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                #8
                Thank you all.

                I have 12 volts at the pump connector. So I connected a rubber hose to the outlet of the pump and the other end in to a fuel can. I then turned the ignition to position 2 but heard nothing from the pump and no petrol came out of the pipe.

                With 20.8 litres in the RH tank would fuel flow into the baffle anyway?

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                  #9
                  If the "sucking jet pump" hose is disconnected or the "pressure relief valve" is faulty...it won't...make sure the sucking jet pump hose isn't disconnected first. If it isn't, then you might have a faulty pressure relief valve.

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                    #10
                    There is a small clip that holds the jet pump on the driver's side level sender that is plastic and can break over time. It can cause the siphon jet to fall off and not send fuel to the passenger side.

                    Click image for larger version  Name:	Screen Shot 2020-08-17 at 8.05.08 PM.png Views:	0 Size:	555.4 KB ID:	51001

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                      #11
                      Originally posted by Slideways View Post
                      There is a small clip that holds the jet pump on the driver's side level sender that is plastic and can break over time. It can cause the siphon jet to fall off and not send fuel to the passenger side.

                      Click image for larger version  Name:	Screen Shot 2020-08-17 at 8.05.08 PM.png Views:	0 Size:	555.4 KB ID:	51001

                      What happened is the siphon jet or whatever thingy on the left side came off. Need to put back on. Pretty easy to figure out. Takes 15 min. Maybe 30 if you can’t get the screw ring off.


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                        #12
                        Originally posted by bigjae46 View Post


                        What happened is the siphon jet or whatever thingy on the left side came off. Need to put back on. Pretty easy to figure out. Takes 15 min. Maybe 30 if you can’t get the screw ring off.


                        Sent from my iPhone using Tapatalk
                        That might not work if the tab is broken. It will fall off again after a short drive. Easy inspection though as you stated jae.

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                          #13
                          Originally posted by Slideways View Post
                          There is a small clip that holds the jet pump on the driver's side level sender that is plastic and can break over time. It can cause the siphon jet to fall off and not send fuel to the passenger side.

                          Click image for larger version Name:	Screen Shot 2020-08-17 at 8.05.08 PM.png Views:	0 Size:	555.4 KB ID:	51001
                          This was my exact issue when I bought my m3. Was very well documented on m3forum. If you run out of gas at a quarter tank, this is the likely cause. My tab was broken so I replaced the sender.

                          The hidden cluster menu has a display showing the left and right tanks. Before replacement, left tank was as full or less full than the right. After fix, the left tank should be empty when the car is running and gauge is half or less. Being an eductor pump fed by the fuel return, it will always suck from the left and go to the right when the engine is running, only going back left if above the driveshaft saddle.

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