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Old 10-14-2011, 02:26 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, 06:48 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, 09:34 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, 02:20 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, 04:02 PM   #5
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Ah, that makes perfect sense.
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Old 10-18-2011, 06:40 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 itís 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 youíll 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, 11:12 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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helpful cross reference to a C6 post with MMI and other TSB's that also cover D3 A8's.


Last edited by MP4.2+6.0; 10-18-2011 at 11:19 AM.
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Old 10-18-2011, 11:21 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, 01:54 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, 02:31 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 ďdumpsĒ 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 Iím not buying the solution to replace the pump\pumps. More research needs to be done.

BTW. C5 4.2 works exactly as Iíve 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, 02:31 PM
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