Fuel Gauge issue

A6 All/ S6 All
#1

Having a fuel gauge issue where the fuel gauge won’t come off full until I’ve driven at least 150 miles, where it used to start to move at half that. Then it will only get to about ¾ regardless of how much more fuel is used.
Some background- took car for service, ran great, filled it up a few days later, and when I got home, fuel was leaking everywhere. Back to shop and they found bad fuel pump gasket, so it was replaced. Also replaced the lines going to the filter. Fast forward a few days, and the fuel gauge isn’t working. Bring it back again, and after almost 3 hours, the shop finds nothing, all sensors and the cluster are working as they should. Apparently, the issue might be with the siphon pump, which I’m not finding much information on.
Can someone help me understand how the system works, and if there is a way to test the siphon pump? Seems the fuel tank straddles the driveshaft and has to pump fuel from one side to the other. Any chance it could be the new fuel filter?
I also did notice a change in the sound of the fuel pump when I first picked it up, but I thought it was because of the new fuel filter.

Any help greatly appreciated

#2

Not leaking before service, leaking after? Strange. Why were they messing with the fuel pump (or were they). If not why would you have a leak. Something isn’t right there.

Anyway this video explains some things:

https://www.youtube.com/watch?v=J-bwQaUpq88

Ed

#3

It is very strange, they didn’t touch the pump initially, only the filter. I started to think that the leak wasn’t from the seal, but from one of the hoses that might have been damaged during the filter change. Hoses do get brittle, so if it was a hose, I would have accepted that. It’s obvious they did something to affect the fuel gauge while replacing the gasket. I would like to troubleshoot myself, i just don’t know what to look for. I don’t know how it would be related, but I may try replacing the filter and see if it has any affect. I did watch the video, thank you for that. The idea that the fuel has to be pumped from one side of the tank to the other is quite unusual.

#4

I would have them reseat the fuel pump assembly in the tank. I think they may have it rotated so the float is hanging up on the side of the tank.

Ed

#5

When i asked them if that was a possibility, they said the pump is keyed so that it can only go in one way. Is that true? They seem to think there is an issue with the siphon jet pump, which I cannot find detailed information on. And again, how did that just happen to fail when they replaced the gasket.

#6

No other Audi I have is keyed, but I have not pulled this one.

Here is something from the Bentley:

08_S6_Fuel_Delivery_Unit
08_S6_Fuel_Delivery_Unit363×761 57.5 KB

You can pop your rear seat out, remove the 3 screws and verify it looks like the picture.

Ed

#7

It looks almost exactly the same as the diagram, maybe a few mm clockwise

S6 Fuel Pump
S6 Fuel Pump2016×1512 1.02 MB

#8

That might be just enough to hang up the float on the front side of the tank.

Ed

#9

I’m going to loosen the ring and move it to match the diagram exactly, looks like that tab just to the right of the screw hole should be aligned with the screw hole at 12:00. thank you for the help here, the car is running great, and it’s annoying that this even happened.

#10

Almost in every case, the answer to the question, “What changed?” is where the solution lies.

It is MUCH more likely than some other unrelated cause creating the problem.

Ed

#11

I was able to move the pump very little, there is a locating tab on the pump that allows a very small amount of play. No change in fuel gauge behavior. The only things fuel related that were touched during the maintenance were the fuel pump and filter. i did notice the pump sounded different when i first drove it home, but i attributed that to the new filter. I can live with it for a few weeks until i can get it back to the shop. I’m thinking now they did something to the pump related to the driver side sensor.

#12

I wish it could have been something simple.

I wish you luck convincing them they caused the issue. To me it’s obvious, but since we live in the age of “I’m not responsible for anything” that may be more difficult than it should be.

Ed

#13

Thanks again for the help, not sure how this is going to go, but the shop has been very good about backing up their work so far. This is also the worst case, something changed, but something indirectly related is broken.