MP4.2+6.0

See below for what the Audi A8 SSP says. Basically, both main pumps appear to have overlapping and full time function, and there are separate jet suction pumps (2) that transfer fuel back and forth across the saddle. SSP 992303 (The 2004 Audi A8L Technical Features. The SSP has lots of pictures and layout diagrams too.

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Fuel Flow
When the ignition is turned on (“terminal
15 on”), the Transfer Fuel Pump G23 forces
a maximum volume of fuel to the pressure
regulator at the fuel rail to achieve short
starting times. The Fuel Pump G6 also
routes fuel to the pressure regulator, as
well as into the pipes for the two suction
jet pumps in the side tank compartments.
Each suction jet pump conveys fuel from
the chamber on one side across the tank to
the reservoir in the pump in the chamber
on the opposite side of the tank.
Pressure Regulator
(58 psi; 400 kPa; 4 bar)
This pipe routing prevents dry running of
a pump in critical driving situations such
as cornering or if the vehicle is at an
extreme angle.
The return pipe is shared by both fuel
pump reservoirs.
If one fuel pump reservoir is full, the pipe is
closed by a check valve and the entire
return volume runs into the second fuel
pump reservoir.
If both reservoirs are full, the check valves
are overridden and the excess return fuel
runs into the tank.

Also, description of pump operation:

Two-Stage Fuel Pumps
Transfer Fuel Pump G23 and Fuel Pump G6
are both of the two-stage flow type. Both
fuel pumps work the same way.
In the first pumping stage, the pre-stage
pump draws in fuel from the bottom of the
tank and conveys it into the pump reservoir.
This ensures that even small residual
quantities can be transported. In the
Return
second pumping stage, the main stage
pump draws fuel from the pump reservoir
and routes it to the fuel pressure regulator
at the fuel rail.
The fuel pump assemblies rest on and are
clipped to the bottom of the tank. Flanged
covers provide access to the components.

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s4master1

Is getting to complicated to simply figure which pump has failed.
bottom line: on a good running car unplug either one of the pumps and it will still run.
when one pump fails the other one will keep it alive.
just for curiosity check if you have fuel on the driver's side. I bet there is as I never see one side going empty when we replaced pumps.
the system is designed for both to work at the same time it also has 2 separate relays, is like having a back up system.
there's no transfer hose outside the tank, those hoses collect fumes and direct them to the charcoal canister.
using one big pump would draw massive amounts of current creating other issues,(radio,nav system)
the previous post explain very well how the fuel is transfer.
Either way you can just replace one if you want.
I just follow the principle of how things were designed.

Bangerla

Check with the D2 guys. This was a common problem on the D2, if the pump is not seated properly, the car will run out of fuel at 1/4 tank. There is a connection at the bottom of the pump, which runs to the suction jet lines. These are what transport the fuel from one side of the tank to the other. From the description on the D3, it looks like only one pump is connected to the suction jet lines. If this pump is loose, the car would run out of fuel before empty.

Mike_k

So the car has just been sitting in my garage for a couple months now. I bought a 6 speed BMW 550i to play with and that's been my main squeeze but it's cold and I'm ready for my A8 again.

I suppose I just don't follow the logic here. I can start the car and regardless of fuel level (it's up to 1/2 tank now) it dies after a couple of minutes. When it dies you can smell raw fuel. Two issues here:

1. Why would a vehicle with a bad fuel pump smell like raw fuel? There's no change in the system other than fuel not being pumped.

2. If S4Master is correct, and I have no reason to assume he's not, then I can run one pump at a time to keep the car running which means if it's shutting off after a couple of minutes that both pumps are simultaneously failing or that one pump isn't working altogether and the other pump is failing after a few minutes.

The raw fuel smell after it stalls is especially disconcerting.

MP4.2+6.0

Out the exhaust (a guess)? In engine area? Under/around passenger compartment?

If it is out the exhaust--as in flooding basically--I would look at other things than the pump. If its a vapor smell but not out the exhaust and no fuel on ground, maybe somewhere in the whole vapor recovery system coupled with what becomes a vacuum leak and kills the motor?; haven't had issues w/ that so no direct experience. Hard to guess remote of course too. BUT, if any of these, is it still under the emissions warranty that might come into play?

mishar

It can be clogged fuel filter and few drops there that don't reach garage floor.

MP4.2+6.0

So when I look back at this string, it seems like it goes right down the fuel pump assumption from the start. The newest report though has me wondering if it's in the vapor system or in the control electronics or various other things.

But unless I'm missing something, it seems like the OP never had it scanned for codes. I would be doing that long before a not insubstantial job to be getting at fuel pumps when it could be many other things, some involving a lot of $ and labor, and others pretty cheap and easy. Plus, per my last reply post in the succession of messages below might be getting into the emissions warranty depending on age and miles on car, and what the issue is.

Mike_k

I went out there and ran it. If I can keep the RPMs up and some load on it, it stays running which leads me to indicate that the previous poster that mentioned the other pump coming on when needed, might have been right.

If you leave it idling though it will stall after about 30 seconds. I ducked down and listened to the pump though. You can hear it shut off before the engine stalls. The pump just sort of fades out and then the engine stalls.

This, combined with the few times that passenger side pump got really loud, leads me to believe it is indeed the pumps. The job is easy enough that I can replace them anyhow and they only cost me $180 for both with a one year warranty.

I do have a code for the EVAP system which has been there pretty much since I've had the car. My buddy said they see a lot of flooded charcoal canisters from people topping off the tank which I do. I didn't realize you couldn't!

jmhjgh

Where did you find both pumps for $180? That sounds like a good deal for even 1 pump. On my D2, the retaining nut (there is a long bolt securing the pump) was spinning inside the tank, and the whole tank had to be replaced. Hopefully, that problem has been addressed.

Mike_k

eBay. I found a seller and lowballed them. You have to imagine they're not selling stock A8 fuel pumps that often and apparently I was right. I got a year warranty too.

I read about the retaining nut and I do believe that issue was resolved on the D3.