A8 D2 runs out of petrol with 2/3 tank

[jeremyn099]

My 1998 A8 D2 4.2 Quattro runs fine with a full tank, but as soon as the level drops to 2/3 (about £40 used at today's prices) the car coughs and acts just like it has run out of fuel. It will not start until I have put in more fuel.

There is no indication of a faulty fuel pump when it is running and the guage seems to work fine.

I am mystified. Any help or advice members can give to solve the mystery would be gratefully received.

[ainarssems]

Probably had work done on fuel pump or level meter in the past and pump has not been seated properly as it's fairly tricky to get it in correctly. I have re-done it on several D2 8's now and also on couple of A6's where it has not been done properly the by garage.

[notorious]

On D2s the quattro differential and prop shaft effectively separate fuel tank into two separate tanks (chambers) -- right and left.

Pump resides in the right chamber. Left chamber is connected with the right chamber with a pipe. Fuel is pushed through that pipe from the left chamber to the right chamber with the help of the force of unused returning fuel stream from the engine. The effect is similar to what we see in old perfume bottles with the pulveriser.

I don't know how exactly the system is implemented as I have never opened my fuel tank yet, but my hypothesis is that the 'perfume pulveriser' thing that pumps fuel from left chamber to right chamber has failed or got dirty on your car.

There must be an explanation how exactly it works (just google) and how to fix this as this should be common problem on quattro cars, not only on A8.

[ainarssems]

There are actually 3 suction jet pumps in the tank(what you call pulverisers) 2 on the right side and 1 on the left side. They pick up fuel from lowest parts of tank and bring it to fuel delivery unit. As you say they work from returning fuel stream going through them. If fuel delivery unit is not seated properly returning fuel just drops in tank and does not go to suction jet pumps.

The problem is not just for quattro cars as exhaust goes in the middle anyway and even if it is not divided pump is not in the lowest part of tank.

[notorious]

Thanks Ainars for clarification.

>> If fuel delivery unit is not seated properly
>> returning fuel just drops in tank and does not go to suction jet pumps.

Does it happen because they simply forget to connect something or because system should be 'airtight' for Bernoulli's effect to work and people just not assembling the damn thing with enough attention to airtightness? Would be great to know what error they make and how that error materialises.

Also, does the '2/3 tank problem' always appear as a side effect of incompetent repair? Can this problem appear on a car with no work done in the tank since new?

Thanks again.

[ainarssems]

does it happen because they simply forget to connect something or because system should be 'airtight' for Bernoulli's effect to work and people just not assembling the damn thing with enough attention to airtightness?

Generally it's because of limited space, because you do not see anything and because of lack of knowledge and experience from the fitters. You either do it by the feel or you need to feed endoscope through the left side fuel level meter opening to see what you are doing. It's very easy to put it in wrong angle so that return pipe miss right opening in fuel delivery unit. If the o-rings are damaged it could also result in similar issues but in all cases I have seen it just has not been fitted properly.

Also, does the '2/3 tank problem' always appear as a side effect of incompetent repair? Can this problem appear on a car with no work done in the tank since new?

I would say very unlikely and I have not seen any with a problem develop on it's own. It's always been after some work was done in tank.