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  • Fuel Management Dual Tanks

    I have 2014 Prado 150 series D4D. I am trying to get an understanding of how fuel is managed to, from and between the fitted dual fuel tanks, main and secondary. I'm aware that fuel is transferred (pumped) from main to engine and return (for cooling purposes) at 60.0L/hour. How fuel is distributed and sequenced between tanks is the mystery for me. My questions therefore are (1) When refueling, does main fill first then overflow to secondary, or do they fill simultaneously and equally? (2) During consumption, in what sequence do the tanks drain e.g. (a) simultaneously and equally (b) main first followed by transfer, secondary to main (c) main, to a certain residual volume then transfer, secondary to main (d) other? Also, how is fuel transferred between secondary and main, pumped, siphoned or drained? And further, what is the role of the so called "transfer pump", inter-tank fuel transfer or transfer between tank and engine, or both? When considering the possibility of water contamination, these would seem important questions to understand? Cheers.

  • #2
    the transfer "Pump" between the two tanks is just a venturi effect.

    the return fuel goes into a nozzle in the main tank which draws fuel across from the secondary tank.

    There is only 1 fuel pump in the main tank.

    So your engine is always drawing fuel from the main tank. but its constantly being topped up by the 2nd tank as the engine runs.

    when you fill up its dropping the fuel into the 2nd tank and it flows across to the primary.
    so say both tank are empty. you put 50L in, it will evenly fill both tanks, as soon as the engine starts it transfers everything it can into the primary tank and keeps transferring fuel as quickly as it's used.

    Good Points of the Venturi System;
    Never have stale fuel in tank
    no 2nd pump to fail, very simple system.

    Bad Points
    If you get contaminated fuel its mixed very quickly across both tanks.
    if you get a puncture in your main tank there is no isolating it and unless resolved you will drain all the fuel from both tanks.

    HugoStiglitz
    Member
    Last edited by HugoStiglitz; 11-01-2017, 08:26 AM.

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    • #3
      Originally posted by HugoStiglitz View Post

      when you fill up its dropping the fuel into the 2nd tank and it flows across to the primary.
      so say both tank are empty. you put 50L in, it will evenly fill both tanks, as soon as the engine starts it transfers everything it can into the primary tank and keeps transferring fuel as quickly as it's used.
      I believe it's a common issue getting completely full tanks at the bowser, would you be able to get the job done quicker and more successfully if the engine is running?

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      • #4
        Deleted
        Poppy Joe
        Member
        Last edited by Poppy Joe; 13-01-2017, 06:01 PM.
        2016 ISUZU DMAX TWIN CAB.

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        • #5
          Originally posted by Poppy Joe View Post
          What is the job of the pump in the main tank.
          Regards Joe
          The pump in the main tank is to get the fuel to the engine.....

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          • #6
            Deleted
            Poppy Joe
            Member
            Last edited by Poppy Joe; 13-01-2017, 06:00 PM.
            2016 ISUZU DMAX TWIN CAB.

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            • #7
              Many thanks for your advice Hugo. Sorry re my tardy reply. I did read your post on the day, just before heading off for a vehicle inspection under warranty. My suspicion is that I have incurred water contamination during a 150L fill. Current evidence is, from a fuel sample taken a some point tank side of the filter, no water evident, but sparse deposits of what appeared to me to be fine metal fragments. I'm assuming contamination has not passed the filter. Faulty injectors have been replaced (car has only travelled 29,000 kms) and I suspect another injector is faulty (knock) but diagnostics suggest compliance (Toyota dealer's assessment). It still could be that residual water (if that form of contamination has occurred) is somewhere in the fuel management system - in my view.
              As a general observation, looking at the diagram accompanying your notes, that vacuum intake point could be problematic for contamination, depending on where it is placed and what filtering of air, if any, takes place. I also wonder about water condensation in those tanks as fuel depletes and is replaced by air? The observation game goes on. Thanks again for your input Hugo. Gerundivus.

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              • #8
                Originally posted by diesel69 View Post

                The pump in the main tank is to get the fuel to the engine.....
                This is completely incorrect.

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                • #9
                  Originally posted by lukea777 View Post

                  This is completely incorrect.
                  Actually it is correct, the fuel pump in main tank sends fuel to the engine where it is either then pressurised further by the diesel pump to higher pressures or in the V6 case, sent to the fuel rail. Surprised you waited 3 years to post incorrect info....

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                  • #10
                    Originally posted by Piggy View Post

                    Actually it is correct, the fuel pump in main tank sends fuel to the engine where it is either then pressurised further by the diesel pump to higher pressures or in the V6 case, sent to the fuel rail. Surprised you waited 3 years to post incorrect info....
                    OP was discussing 1KD. The in tank pump circulates fuel from the main tank, through pipework in the aux tank to cool and then back to the main tank through the filter above the tailshaft back to the jet pump, to create negative pressure which draws fuel into the main. The electric pump does not supply fuel to the high pressure pump on the engine.


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                    • #11
                      Think you will still find there is a lift pump that sends fuel forward to engine.

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                      • #12
                        I understand the fuel transfer and supply to the engine bay are explained well here, but what happens with the fuel returned from the engine?
                        Where does that sit with the above diagram?

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                        • #13
                          The info above is mostly correct but would appear to be for a 120.
                          And Guest is incorrect, there is no electric lift pump pressure feeding the diesel fuel pump.
                          Return fuel would go to main tank via the Venturi as shown in the left image of the main description. This jet pump also pumps fuel from the reserve tank to the main tank. The pipe work is extremely complicated, not as simple as shown in the diagram.
                          It’s referred to as the octopus for good reason.
                          I believe the electric in tank pump only operates from a certain fuel level in the main tank, the Venturi jet pump doing the majority of work.
                          This is why you can still drive the car with a non working in tank pump until you see 1/4 of fuel on the gauge when the car will starve for fuel then stop.
                          If you routinely maintain high fuel levels in your car, you may never know if your in tank pump is dead or alive….until you do a big trip and get her down to 1/4 tank.

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                          • #14
                            I have done a fair amount of research into this issue for Prados with this tank configuration.

                            1. For petrol powered vehicles, the fuel pump in the main tank supplies petrol to the engine bay. The return line petrol operated the venturi pump to transfer fuel from the rear tank to the main fuel tank.
                            This is because the petrol engines have a high return line flow, which will operate the venturi pump reliably.
                            It is unlikely that a petrol powered Prado will suffer from the inability to use the reserve tank contents

                            2. For diesel powered vehicles, the return line fuel flow is insufficient to reliably operate the fuel transfer venturi pump, so the electric pump in these vehicles is used to power the venturi fuel transfer pump. In Diesel vehicles, there is also an additional fuel filter in between the electric in-tank fuel pump and the venturi pump, to help prevent venturi blockages due to contaminated fuel.

                            Should this filter become blocked, the car will operate as if it has a 90L fuel tank and the fuel in the back tank (up to 90L) will not be available and the vehicle will run out of fuel when the main tank empties.

                            3. The return line in both petrol and diesel Prados passes via a pipe in the back tank, which acts as a heat exchanger, minimising return fuel temperatures, before being sent to the main fuel tank.

                            I believe that this is the correct interpretation.

                            If you disagree, please explain why you feel this is incorrect.

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                            • #15
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