pulinsh

Hi!
I've got some problems with gas tank as you can guess from topic name. I have A8 4.2 40v 2000 with one pump.
So I fill gas tank up - and drive some time, when it show almost 3/5 of tank car dies, I fill it up with 10l I can go only till I hit that 3/5 tank. So I took out pump and compared it to diagrams and found out that there was a problem, that 2 tubes was switched places - so I switched them back - pumped out all gas from both sides and it was around 60l of fuel. After that I put everything in place - filled up 15l of fuel, car started - few minutes was ok, than I turned it off. filled all fuel back in - and went on a test drive - after 15km car died ... could not start up again - waited for 5 min car started for 2 sec and died ... filled in 5l of fuel and could drive for 40km with no problem.
so where is the problem? can anyone help?

audinutt

fuel pump is dying/dead, happened the same way to my a8.

pulinsh

and why it is working when I fill it up full, I can drive for 300km - than fill up again and drive again? just want to know how

pulinsh

when I bought it - car died at 1/4 of a tank and it slowly rised till now ...

Adjuster

Could be a leak in the siphon system.

The stock fuel pump has two discharge ports on it. The large one feeds the fuel rail, the smaller one goes over, plugs into the siphon cup housing, and feeds three siphon pumps that then in turn, flow fuel into the cup where the fuel pump sits. (Actually above the lowest point in the tank, and fuel can pour into the cup, as long as it's at a level high enough to do that, but once the fuel level drops below the cup rim, the siphon system has to supply fuel, and if it's not hooked in right, or has a leak, it will not work, and you run out of fuel.)

There are hoses that "snap" into place the siphon cup fixture, and those hoses also hold it into place, so it can move around, and make getting it all hooked up right difficult.

If the siphon hose that sticks off the side of the fuel pump housing is not inserted into the siphon cup port, the fuel just sprays out into the tank, and does not supply the pressure/flow needed to suck fuel back into the pump housing cup.

Or if ANY of the hoses are loose, the pressure is lost there, and you will suck the cup dry, and run out of fuel. Or if the SEAL is bad between the pump and the cup, you lose fuel there, and again, the cup runs dry, and you run out of gas.

I have a brand new, OEM VDO/Siemens fuel pump for this car sitting on my desk, that I can't use, because on my car, the captured nut that the long bolt goes into, is just spinning in the housing. I can't pull the fuel pump housing out without destroying it, and the bolt/siphon cup in the process.

So, I'm looking at ways to create my own external pump system, using the stock fuel tank.

My siphon system is working perfectly, but the pump is failing, and will not allow my car to go over about 70mph before it flashes the CEL, and drops injectors making the engine run like crap until I shut off the engine, and re-start, and then it's fine again around town under 70mph.

I'm going to put an external Walbro 255lph pump just after the stock fuel filter, and it will supply all the pressure/volume this engine needs easy. (And any excess will just go back into the tank after the fuel pressure regulator.)

Let me know if you need a stock pump, I paid 229.97 shipped to me for this pump from England. (They get it out of Germany.) If you want it, I'll sell it for that, and 6.00 for shipping via USPS flat rate box to anywhere in the USA.

And NOBODY sells the right pump in the USA. I bought one from AIP, they advertise it as the right pump, but IT IS NOT. None of the Ebay USA sellers have the right pump. (And they will tell you that it is, and then charge you a 15% restocking fee when you return it.) Sucks, so don't even go there.

Shoot me a PM if you need a pump. But it might just be that your siphon system seal is leaking, or the hose port is not correctly alligned.

Adjuster

LOL, just see now that you are in Europe, so you can get the right pump locally. Good luck with your A8, these are great cars, but can be a real challenge when they have problems.

pulinsh

thanks for big info ... I took put the pump, and tested it - it sounded ok, tomorrow I will test in fuel to see how much pressure it can give.
I was thinking about walbro 225lph too, but if there is leak in that system? or that system will accidentally suck air of smaller pocet? will walbro survive that - or just die after few times?

audinutt

The fuel pump is a two stage pump from my understanding.
My guess which may NOT be accurate is that one stage becomes weak and the system won't pump the fuel from the suction system. This happened on my a8 and after a few thousand more miles it failed completely.
If it does you will know, the car won't run.

-Richard

PS. I would stick with the stock pump and testing it in fuel won't test the second stage.

pulinsh

thanks, will test that tomorrow at fuel pump shop ... but still that idea with walbro ... I like it, just - how to connect all that is in gas tank to outgoing and incomming?

Adjuster

When you look at the stock fuel pump, you will see it has two ports on the bottem.
The large one feeds the fuel rail.
The smaller one feeds the line that goes and plugs into the siphon system. (It has a small o-ring, and slips into the siphon cup deal that's down in the tank, and hooked up to hoses that feed fuel from lower areas in the tank, at three pick up points.)

So, if you want to replicate that, you will need either the VDO pump #405-052-002-001Z, that is the OEM factory pump, or two fuel pumps, and custom make your own setup that uses the stock housing for the fuel pump, but not the lowest part that holds the pump, and has the pickup screen on it.

Both pumps would need to sit down in the "cup" of fuel that's supplied by the siphon system.

One pump feeds the fuel rail. (This is the fuel line with the hose that runs up out of the fuel pump housing.)

The other pump would do nothing but feed the line going to the siphon port.(This is the hose that has the metal bend fitting, and runs to the side of the fuel pump housing, with the o-ring on it.)

Insert both down into the tank, fill will some gas, the filler neck dumps fuel right into the "cup" and it's why the fuel tank will work if you just put a gallon into it, the gas is dumped directly into the cup, regardless of how much fuel is in the tank or not.

But since the fuel cup is HIGHER than the lowest point of the tank, it's also why if there are leaks in the seals or system, the car runs ouf of fuel before the tank is empty, the cup where the fuel pump sits runs dry, and your engine dies.

If the siphon system fails, the cup runs dry as soon as the fuel level falls lower than the open upper edge of the "cup" where the pump sits. (If fuel is higher, it just runs into the cup, and your car runs fine.)

Problem is, about 10 gallons is all the tank will supply this way. So you can keep adding fuel, and carry some extra in a can with you all the time just in case it drops below the cup rim, but a working system is nice since you can run the fuel down to nearly empty, and never harm the pump at all, since it sits in the "Cup" that's full of fuel all the time until the tank is absolutely dry.

If I had access to my stock fuel pump housing, I'd go with the stock VDO pump. It works well, and it's the easy solution.

But, since my fuel pump housing will not come out without major cost and work, and the stock pump seems to be filling the cup fine with the siphon system, I'm going to augment the supply to the engine, and take the load off the stock pump by adding the external pump.

If it works, this might be the best preventive idea for the A8 fuel system ever.

When you change out your fuel filter, just add the in-line Walbro, and your stock fuel pump in the tank should last much longer, perhaps the life of the car. With nearly no load, the stock pump should wear much less quickly.