A8 D2 runs out of petrol with 2/3 tank
 

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.

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.

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.

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.

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.

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.

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.

I am sure I have posted this before but there is fuel delivery removal and installation procedure pdf attached again.

What happens is that the pipe from fuel delivery unit does not go in the hole on baffle housing as shown in picture below. This is where return fuel goes to suction jet pumps.

http://forum.a8parts.co.uk/attachmen...1&d=1408744527

Thank you very much +++

Picture greatly illustrates that the pipe needs to be connected in the area which is not visible through the opening.

D2 Fuel pump Issue remedied
 

Right, thank you for all the replies I have received to this and now prepare for an epic tale to rival the feats of Alexander the Great.

I have two A8 4.2 Quattros; a 1995 I have been driving around in for a couple of years and a 1998 I just bought to replace it. The replacement was bought cheaply because it ran out of fuel as described below. The diagnosis of my fellow A8 parts forum member harrybosche echoed what I had researched on the web and I narrowed it down to two options. The first was that the correct pump had been incorrectly fitted, the second was that an incorrect pump had been correctly fitted.

Day 1: I removed the pump, stripped down the subassembly and checked the pump. Diagnosis, a brand new pump of the wrong type (no secondary inlet) had been fitted. However, as my 95 car is now at a yard some 30 miles away being stripped for parts I then had to reinstall the pump into the subassembly and fit the unit back into the tank to drive the new car to get the pump out of the old car. This took a few hours, the bulk of which was fiddling with the subassembly and trying to get it to slot back into position. This time it took about an hour of fiddling. The process is well described here:

https://www.youtube.com/watch?v=7TVwYspbFC4

The tip about using a pliers and mole grips to avoid buying a special part is genius, as is the tip about securing the inner wiring.

Day 2: was spent stripping all various bits off the 95 car.

Day 3: Today I removed the subassembly from the 98 car, dismantled it and replaced the fuel pump with the one from the 95 car after checking it and confirming it has indeed got the dual inputs. I used the existiing subassembly as the filter and gaskets were almost new and because I knew it had been fitted twice and should fit again! I started jogglin it around at 11am and it eventually slotted into place at 4pm.

I now know the zen secret and will pass it on.

Get the subassembly in position, centred but with no rubber gasket in the tank opening and level with the tank opening but raised about 40mm. It will not want to go down straight. Now for the golden secret. apply some anticlockwise pressure keeping the top of the subassembly level. It will creak a little as the plastic takes the stress, but it should then drop down with both the relevant pipes and spiggots mating perfectly. It should not need any more downard pressure. It is the anti-clockwise turn which mates the various apertures.

Pausing only for a lap of honour and a cup of tea while imagining booting the Audi technician who designed this in the bollocks for 10 hours, I then put the long rod in place and gave it a couple of turns to make sure the whole thing did not come out again, then lilfted it an inch (24.5 mm to you purists) and fitted the rubber gasket to the tank opening. This is another absolute pain in the arse because it seems to be too big. However, you need to prise the "skirt" into the tank opening and keep the "shoulder" around the edge, with the top of the sub assembly centred. It seems too big, but it will go.

I am betting they made it this big because the tank opening has to be big enough to accept the tilted sub assembly. This is like designing a barrel which leaks and then making the lid incorporate a non leaking inner barrel. In other words if they had done the job right in the first place they would not have needed to bugger about with other bits to fix it.

This is way below the standard of engineering which I have come to accept and love from Audi. However, now I can drive it I am starting to feel the love again.

If anyone is in the same situation I will accept commissions at £50 per hour for joggling, plus travel. This is way cheaper than a £200 pump, £150 gasket kit or £700 sub assembly. Strewth!

Thank you very much for this writeup. Very very helpful.

+++