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How do I tell which pump failed?

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Old 10-14-2011, 01:26 PM   #1
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Default How do I tell which pump failed?

So I'm driving home the other day and the car starts bogging when I give it gas. I've still got a 1/4 tank so I'm thinking maybe the MAF isn't metering air properly and is flooding the engine but then I remember that I could hear the fuel pump the night before.

By now the car is in my garage and I'm planning on looking at it. First things first, I pull it out and take it to the gas station to fill it up. No go. It stalls at the end of the driveway. It starts back up, runs for 30 seconds and then stalls again. Unfortunately I'm half way around the block so I need to drive it home now. Each time I start it up it runs for a shorter amount of time until it eventually stalls 3 houses from mine and won't restart.

So I'm talking to my buddy (Audi mechanic) and he tells me they have two pumps and it's possible one failed. He says to put some gas in it and see what it does. Low and behold, I put gas in it and it's running fine.

Today my buddy says he wonders if the gauge is wrong on the dash and told me to watch the mileage but now that I think of it, I heard the pump whining about 100 miles before any problems arose so either way, it had plenty of fuel in the tank when I heard it. I'm sold on one of the pumps failing.

Ok, so I have a bad pump. How do I tell which pump is bad? Does one pump sit lower than the other?

S4master, I'm looking at you.
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Old 10-14-2011, 05:48 PM   #2
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Default Pump

Easiest way is to unplug the driver's side pump while the engine is running.
if no change, that pump has failed, if the engine dies plug it back on and try the other one.
I bet you is the driver's side, so far I have not seen the passenger side fail in any Cayenne, Q7, Touareg, Phaeton, Bentley or A8.
by the symptoms I agree on the fuel pump, at least is easy to check before you spend any money on it.
always replace them both at the same time.
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Old 10-15-2011, 08:34 AM   #3
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Does the driver's side feed the passenger side or does the passenger side feed the driver's side? I'm curious if you've never seen the passenger side fail why do you recommend replacing them both?
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Old 10-15-2011, 01:20 PM   #4
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Default Failed

Quote:
Originally Posted by Mike_k View Post
Does the driver's side feed the passenger side or does the passenger side feed the driver's side? I'm curious if you've never seen the passenger side fail why do you recommend replacing them both?
My mistake, I never seen the passenger side die, but in reality it also already failed.
when both pumps are good the car will run with just one, that's why you never noticed that one is dead,
after one dies the second one is already weak and can't pump enough fuel to keep up, replacing just one will cause more stress on the new one and will fail earlier.
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Old 10-15-2011, 03:02 PM   #5
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Ah, that makes perfect sense.
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Old 10-18-2011, 05:40 AM   #6
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I cannot 100% agree with the concept both fuel pumps have to be running all the time. Here is how it used to work on older 4.2 engines (in A6 and A8).
They used to have 2 pumps. One it was always working and the second one used to be kicked only when the engine had 3000 and higher RPM to supply enough fuel. I still think it’s the same concept.
But there is a catch! The passenger pump is always running the driver pump is not. It will run only when engine needs it. The test case proposed above will indicate the driver pump is failing when in reality it is not!
Based on the symptoms you described I would guess it is the issue with the valve\pipe connecting lower sides of the tank to each other. Think about it. If you have low amount of fuel it suppose to be distributed equally in both lover sides of tank. The passenger pump starts. Engine is working. While it is working the excess fuel will be deposited in Driver side! The connection pipe supposes to move the fuel from one side to another. If it does not happened fast enough you’ll be running out of fuel on passenger side!
I do not recall if any additional pumps helping that process. To validate that concept you need to confirm if your car is running fine when tank is almost full. If so, then it is definitely not the pumps.
But keep in mind. The pumps do not like to be running on very low fuel level and they should not produce any noise. The noise is not a good sign. I would say it is a sign of wear and possible even death.
Also, I can see the nose happening if the pump will start catching the air.

Anyway, that is my 5 cents.
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Old 10-18-2011, 10:12 AM   #7
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Default I don't think there is a (non-pump) transfer pipe in that set up...

I have a C5 4.2. To work that way would indeed require a transfer hose working by gravity and principle of fluids seeking own level. But, I don't ever recall seeing one, nor does it strike me as safe. The tank is described as a "saddle." What is in between the two sides of the "saddle" of course are the exhaust and the driveshaft. Running a transfer hose below those to let gravity do its thing would have the hose hang well down (especially to avoid the hot exhaust that is lowest), and then put it in harm's way from everyday road debris, piled up snow on winter roads, etc. Elegantly simple, but the practicalities of road debris frustrate that approach.

Long story short, you may be on the general right track on an older design but I think it involves a transfer pump to get up and over the "saddle," with the transfer line internal to the tank. Been a long time since I looked at the documentation though.

To OP: on the main subject, avoiding fuel pump change outs (hard and $$) is a big reason I change the fuel filter once in a while. With the timing belt cars I would do it alongside the belt change interval. Not in the service list for most cars for a long time now, but cheap and can reduce flow resistance that the pump has to work against.
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Prior (each modded): 2000 C5 A6 4.2 & '96 C4 A6 2.8Q, both still w/ family; '85 C3 5000S 5 sp FWD; '73 C1 100LS

helpful cross reference to C6 MMI and various other TSB's that also cover D3 A8's.


Last edited by MP4.2+6.0; 10-18-2011 at 10:19 AM.
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Old 10-18-2011, 10:21 AM   #8
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The job actually doesn't look difficult at all, just awkward.

I agree, I doubt there's an equalizer tube to keep levels the same on both sides. It would need to run under the driveshaft/ exhaust and that doesn't sound practical.

More likely is one pump feeds the other which explains why I have symptoms of running out of fuel despite the gauge reading that I'm 1/4 full.
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Old 10-18-2011, 12:54 PM   #9
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Let me research how it works now and I'll let you know! I'll try to do it by tomorrow!
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Old 10-18-2011, 01:31 PM   #10
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So fare I confirmed few things:
1. 4.2 will kick the driver side pump only when it will reach 3000 RPM and under load
2. transfer hose does exist! It is not replaceable part and it is part of the tank. It is actually going UP and repeats the shape of the tank. I also confirm it is outside of the tank!!!
3. I confirm the return house “dumps” the excess fuel to driver side of the tank

To check the driver side pump you need to try to run engine more than 3000 RPM under the load. If it does not going up it is 100% the driver side pump. If it does going up easy it is not a driver side pump.

I do not understand yet the principal how driver side of the tank transfers the fuel to passenger side. The places where pipe connected are quite far from the pumps!!! The natural flow possible, but I question the ability to flow if it will catch the air in the pipe. I may get more info tomorrow.

Anyway, for some reasons I’m not buying the solution to replace the pump\pumps. More research needs to be done.

BTW. C5 4.2 works exactly as I’ve described earlier. 2 pumps and only 1 is always working. The second one will be kicked only when needed.

That absolutely makes sense to me. If engine needs only so much the pump can produce only so much. If engine needs MUCH MORE the second pump helps the first one. Technically they could create just one BIG pump, but then most of the time the energy will be directed to circulate the fuel, heating it and etc. 2 pumps with one always working is make sense to me.
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Old 10-18-2011, 02:13 PM   #11
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Default Jet pumps handle transfer function; both main pumps have overlapping function

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.

-------------------

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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Current #1: '06 D3 A8L W12 brilliant black w/ amaretto; 4 seat setup, CPO; modestly lowered, H&R 20mm rear and 12mm front DRs with 275/35 on OEM polished + forged 20's; S8 sway bars (see: rear bar and gen'l D3 bar info and front bar); tweaked 385mm front factory brake mod and matching rear brake mod; matching C6 A6 back headrests (better rear view: headrest tweak); owner installed AMI retrofit with part number details here. [pics now only in picture poster] Upper control arm reference (+ more info in replies) here. Lower cost D3 OES sway bar links here.
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helpful cross reference to C6 MMI and various other TSB's that also cover D3 A8's.

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Old 10-18-2011, 03:20 PM   #12
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Default Pump

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.
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Old 10-18-2011, 04:42 PM   #13
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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.
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Old 12-08-2011, 07:42 AM   #14
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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.
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Old 12-08-2011, 08:48 AM   #15
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Default Raw fuel smell where?

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?
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Current #1: '06 D3 A8L W12 brilliant black w/ amaretto; 4 seat setup, CPO; modestly lowered, H&R 20mm rear and 12mm front DRs with 275/35 on OEM polished + forged 20's; S8 sway bars (see: rear bar and gen'l D3 bar info and front bar); tweaked 385mm front factory brake mod and matching rear brake mod; matching C6 A6 back headrests (better rear view: headrest tweak); owner installed AMI retrofit with part number details here. [pics now only in picture poster] Upper control arm reference (+ more info in replies) here. Lower cost D3 OES sway bar links here.
Current #2: 2013 Q5 2.0T hybrid; Scuba blue w/ chestnut sport interior; Euro delivery 7/2013 (pictured at Ingolstadt); RS5 front brakes
Prior (each modded): 2000 C5 A6 4.2 & '96 C4 A6 2.8Q, both still w/ family; '85 C3 5000S 5 sp FWD; '73 C1 100LS

helpful cross reference to C6 MMI and various other TSB's that also cover D3 A8's.

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Old 12-08-2011, 08:55 AM   #16
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It can be clogged fuel filter and few drops there that don't reach garage floor.
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Old 12-08-2011, 09:03 AM   #17
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Default Run codes first ! (?)

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.
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Current #1: '06 D3 A8L W12 brilliant black w/ amaretto; 4 seat setup, CPO; modestly lowered, H&R 20mm rear and 12mm front DRs with 275/35 on OEM polished + forged 20's; S8 sway bars (see: rear bar and gen'l D3 bar info and front bar); tweaked 385mm front factory brake mod and matching rear brake mod; matching C6 A6 back headrests (better rear view: headrest tweak); owner installed AMI retrofit with part number details here. [pics now only in picture poster] Upper control arm reference (+ more info in replies) here. Lower cost D3 OES sway bar links here.
Current #2: 2013 Q5 2.0T hybrid; Scuba blue w/ chestnut sport interior; Euro delivery 7/2013 (pictured at Ingolstadt); RS5 front brakes
Prior (each modded): 2000 C5 A6 4.2 & '96 C4 A6 2.8Q, both still w/ family; '85 C3 5000S 5 sp FWD; '73 C1 100LS

helpful cross reference to C6 MMI and various other TSB's that also cover D3 A8's.

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Old 12-08-2011, 12:30 PM   #18
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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!
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Old 12-08-2011, 01:45 PM   #19
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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.
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Old 12-08-2011, 04:33 PM   #20
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Quote:
Originally Posted by jmhjgh View Post
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.
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.
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