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test test D2 A8/S8 (1997 - 2003)
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  • Strange "out of gas" issue
    Posted by: Mortem76 (102530) on 2017-10-31 17:00:23

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    Hi All,

    Today, I had for the second time a weird issue with my D2:
    While I still had about 30L in the tank, the engine stopped as if the tank was empty and restarted only after a refilled it 20L.
    This occurred to we about 6 months ago when I had 20L left in the tank.
    I feel the issue might be either due to the full pump or maybe the Suction Jet Pump.
    Any idea which part can be the root cause of this issue as it is quite annoying as I must always ensure to have the tank almost half full to avoid trouble of walking to the gas station to fill a Jerrycan.

    Thank you
    • Thanks for all your helpful replies
      Posted by: Mortem76 (102530) on 2017-11-01 00:26:42

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      You all confirmed what I was suspecting but I did not have the background knowledge to confirm that.
      How long you think before my fuel pump will die and will it die all at once or little by little? Do you think I can wait 2-4 weeks (meaning 200-500km as I use the car only during weekends) before bringing it to my mechanic?
      Do you think I should replace the full assembly or just the fuel pump only?
      And could you provide me the Audi part number(s) I can use as well as the brand(s) (unless only OE is recommended)?

      Sorry for all those questions again and thanks again for your useful help :)
      • O rings
        Posted by: AUTO-UNION.US (6318) on 2017-11-07 01:40:44

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        We found that the O rings deteriorated allowing the pump to suck air instead of gas from the remote parts of the tank. The reason it starts working when you fill with gas is there is a cup that holds about a pint-quart of gas that fills up first when you put gas in. The pump sits in it and the car works normally until that cup is empty. Until you get it fixed you can carry a little gas with you and when it dies, fill the cup up. Any more than a quart will drop into the tank and cant be utilized until the O rings are replaced.It is such a pain in the *** to get that pump out and back in the correct position that I would replace the pump at the same time.
        • My car is with my mechanic now
          Posted by: Mortem76 (102530) on 2017-11-27 21:19:59

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          He is taking care of it. When I will have the car back, I will carry my 20L emergency gas jerrycan and drive the car until out of gas to check if it is working fine.
          • This is not the fuel pump but the Fuel Level Sensors that are defective
            Posted by: Mortem76 (102530) on 2017-12-09 05:57:28

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            Hi Guys,

            I know it's been some time but my car was with the mechanic for 3 weeks (a few other things to do also) and today he told me the fuel pump is good. The issue is coming from the "Fuel Level Sensor" in each of the 2 gas tank that are defective and need to be replaced. Basically, when you fill the tank, they stop refilling when you have only 60-65 Liters in the tank and when the tank is empty, the reading says you still have 1/3 of the tank.
            He will have them next month.
            In case anybody may need them, please find below the VAG reference for both of them:
            Left one: 4D0919051J
            Right one: 4D0919051C
            • Sorry, I disagree with this diagnosis
              Posted by: AUTO-UNION.US (6318) on 2017-12-10 02:06:29

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              First, I have to say it has been 10 years since I have replaced an A8 pump and/or the seals, but the level sensor[s] have nothing to do with the delivery of the fuel.
              First, as succinct of an explanation as I can about the A8 fuel delivery system which has never changed from 1994 to 2018.
              There is a two stage fuel pump that sits about 1/2 way up in the main chamber of the fuel tank. [N.B. You must get the two stage pump. We pulled cars apart that were worked on by mechanics unfamiliar with Audi and they had installed single stage pumps!!!! which of course did not work.] It sits in a kind of cup that probably holds between a pint and quart of fuel. The pump has two stages. The first stage is to send pressurized fuel to the 2 or 3 different chambers of the fuel tank. [They arent separate chambers; they just lie on either side of the center drive shaft and need separate pumps to get the fuel to the bowl in the main chamber] This is done by a kind of venturi action where the pressurized fuel is pushed through a nozel with a hole to the fuel cell on the low pressure side where it picks up surrounding fuel and carries it back to the bowl in the main cell where the fuel pump is located. It keeps that bowl full so that the secondary stage of the fuel pump can pump fuel to the injectors.
              So one of two or both happens. The seals between the fuel pump assy. and the lines get displaced and the first stage fuel pressure never makes it to the respective chambers and the jet nozzels don't pick up any fuel.
              Or, the first stage of the fuel pump goes out and again, the jet nozzels don't pick up any fuel.
              When you put gas in your vehicle, it dumps directly into that bowl, which overflows and the gas is dispersed into the different chambers. That is why if you put a quart of gas in your tank it will start up and run until the secondary part of the pump has sucked all the gas out to go to the injectors.
              The reason the gas gage says the car is 1/2 full is because the gas level in the main chamber where the float is located IS HALF full while the other chambers are completely full because nothing is pulling the fuel out of them. And when the main chamber is less than half full, the secondary stage inlet is above the fuel level so it doesn't pump to the injectors.
              When working properly the jet pumps in the bottom of the chambers are continually collecting fuel and dumping it into the fuel cup where if not needed overflows the top of the cup into the main chamber. So the chambers on the opposite side of the drive shaft can be empty and the main chamber can still have gas in it and the bowl keeps full from the jet pump in the bottom of that chamber.
              I still argue that either the pump or the seals are your issue, not the fuel sender units which have nothing to do with the actual collection and distribution of the fuel. Just my $.02. Best, /peter
      • They often jam up from the commutator falling apart. Who knows when they die?
        Posted by: JFrahm (4193) on 2017-11-01 11:26:07

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        Stock (for the V8, maybe all) is a VDO, 4D0906091C which is around $150USD. There was a UK seller we used to get these from but now they are available in the US and other locations.

        Most of us have just replaced the pump, careful to align and seat everything and not to damage the blind nut in the tank the long bolt threads into. It was not difficult for me when I did it on my A8, just had to be careful.

        Where are you located? Some of us are trying out a pump from Poland that is under $50 USD, I bought one and it appears to be the right pump but I have not installed it yet. The seller has a good rep and has been around for a long time.

        https://www.ebay.com/itm/Fuel-pump-BENZINPUMPE-KRAFTSTOFFPUMPE-AUDI-A8-S8-D2-2-8-4-2L-1994-02-4D0906091C/281599154855

        Careful of the ones from China, HK, etc. it looks like many of them lack the extra bottom outlet that drives the scavenge pumps.
        • I am in Thailand so my access to parts is more difficult ;)
          Posted by: Mortem76 (102530) on 2017-11-01 11:36:38

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          So this is why I search for good parts and it is sometime a bit challenging ;)
          I also read on another forum that sometime this issue is related to the lines "sucking" the gas from the other side of the tank that are not well connected. Do you think it is more likely that or the fuel pump that start getting old?

          Thanks
          • Since the fuel pump wears out it's good to replace unless you are sure it's been replaced before...
            Posted by: JFrahm (4193) on 2017-11-01 12:06:21

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            I have heard of bad fuel pump installations that caused problems with the scavenge pumps (poor sealing of the pump assembly to the tank-side coupling) but I do not think I have ever heard of that happening with a factory pump.

            If the problem comes and goes it's much more likely the pump is jamming/dying or you have an electrical problem rather than some sort of intermittent in-tank issue that is blocking the fuel flow.

            If you want to goof with it, make a rig to connect a headlight bulb or something to the fuel pump connector, go drive until it poops out, hook up this test lamp in place of the pump and see if it'll light up when you crank the car.

            The lamp needs to be something like a 35 watt headlight or auxiliary light to be sure you have enough current back there to drive the pump. Make the wires long enough so you can see the lamp from the driver's seat.

            If the test lamp shows you have good current to the pump, try reversing the power to the pump for a second to unjam it and see if you can get home without a flatbed. Adding fuel might unjam it also.
      • Did my research for the part numbers of the fuel pump
        Posted by: Mortem76 (102530) on 2017-11-01 11:14:37

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        Hi All,

        So it seems the OE and Manufacturer P/N are as below:
        OEM: 4D0906091C
        MPN: 405-052-002-001Z

        The MPN seems to be the one of VDO / Siemens VDO / Continental VDO (depending of year of production of the part)

        I found alternative fuel pumps with the correct OE part number from different supplier such as Bosch, WAJ, etc...
        Should I stick with the VDO one of some alternative ones (Bosch?) can be of the same quality? I just want ensure that I can source the right part quality :)

        Thank you
    • Probably the fuel pump or the connection between the pump and the scavenge pumps.
      Posted by: JFrahm (4193) on 2017-10-31 17:31:00

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      If you have not replaced the fuel pump why not do that?

      Have you been able to run down to a few liters of fuel without stalling?
      • I'd guess the cross-tank connection.
        Posted by: hyperlogos (20841) on 2017-10-31 18:11:56

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        I think the easiest thing to do if it happens is a current test. Instead of testing at the fuel pump connection, you could pull the fuel pump fuse (passenger kick panel, carrier ST4, fuse 1 - fourth carrier down, rearmost) and supply power to its bottom terminal directly from the 60A fuse at the top of the fuse panel. The pump should draw 8 amps when pumping fuel, or only 4 amps if it's sucking air.

        I don't know about yours, but my meter has a 10A ammeter mode which would let me do this on the side of the road if I set up a jumper wire to get from my meter into the fuse block ahead of time. Then you can press the other lead of the meter into the exposed contact of the 60A fuse while you turn the key. 8A means your problem is after the fuel pump (bad line, clogged filter, fuel pressure regulator, ignition/injection system) while 4A means your problem is probably that cross-tank line and 0A means your problem is with the pump, relay, or a wiring connection.
      • Or maybe the relay? Have someone test the circuit when it's stalled and you are cranking.
        Posted by: JFrahm (4193) on 2017-10-31 17:47:56

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        Maybe while you are going to get gas the flakey relay cools down and works again.
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