rghollis

2005 Audi A8L 4.2L runs good when gas meter is in the red (low fuel). Any amount of gas put in where the meter exceeds the red markings of the gas meter and the car will not run for more than 40 seconds. Right fuel pump (passenger G23) was just replaced hoping it would fix this problem but no change, car will not run properly with more than about 4 gallons of gas in the tank.

Just recently before the fuel pump was replaced, I now have back pressure on the fuel filler. When removing the gas cap there’s a good force the pushes gas out the filler. Not just a little fuel either, it will shoot gas if I don’t slowly relieve the pressure when unscrewing the cap. Trying to put a little gas in at the gas station also gets backflow, must fill very slowly to get gas in and no more than 3 gallons or the car won’t run properly. Does the Expansion tank have problems on these things or more likely a clogged evap line somewhere? Again, this just started before the right (passenger G23) fuel pump was replaced not afterwards but it has continued to happen. Any ideas to troubleshoot would be greatly appreciated.

I do have vag-com to assist if there’s any procedures to run but it seems a bit limited around the fuel system. This A8L doesn’t even let me check fuel pressure; I can only see whether the Fuel pumps are on or off can’t seem to find the measuring points for the barometric pressure. Car throws no fault codes. Fuel filter was replaced about 2 weeks before the fuel pump which was done yesterday.

I’d really be grateful for any ideas ... can’t find any answers on the internet.

BASARAB

I would say the charcoal canister needs to be replaced

rghollis

Thanks Basarab, appreciate the feedback. Would the bad Charcoal canister support the problem related to the car being able to run perfectly with less than 4 gallons of gas or just the back pressure; or both? Is there any way to test the charcoal canister (below the spare tire), purge valve (engine compartment top right side), and/or fuel pressure regulator (on the injection rail)? vag-com isn't totally useful on the 2005 A8L 4.2L and even less useful if the car won't run for more than 40 seconds.

rghollis

I inspected the Charcoal Canister and found a bent line. Obviously not good and needs repair, but still wondering if this can cause the car to not run with more than 4 gallons of gas in the tank but run perfectly fine with less than 4 gallons of gas in the tank.

BASARAB

I am not an expert, but my Saab recently had a similar problem and it was the canister, not the valve and not as extreme as yours though. The back pressure from the tank was not too strong and the car would not run only when it was over 5/6 full

dvs_dave

Try running/driving it with the fuel filler cap loose to allow air in. You will of course get an EVAP code, but it will tell you if it’s a breather problem or not.

rghollis

Thanks dvs_dave, Problem does not change when trying to run without the fuel cap on or very loose. I will say that now today the back pressure doesn’t seem to be happening. Maybe I can’t keep it running long enough to build the pressure again but it appears to have stopped for the moment.

A previous reply shared this discussion, https://www.audiworld.com/forums/a8-...table-2965889/ and I’ve tried some of the things recommended.

Pulled the MAF connection and no change, car runs for about 40 seconds and just slowly stops like it’s starved for fuel with or without MAF connected. On occasion it will almost bog out ... rough idle for a few seconds ... and then kick back up to a smooth idle, but will eventually die after another 40 seconds (give or take).

I’ve also tried to run the VCDS process to check the fuel pressure but that measuring point is not available. I can only see whether the pumps are on or off, no pressure value. They do appear to run as expected. Both fuel pumps are on during startup for about 30 seconds then the left (G6 driver side) goes off and the right (G23 passenger side) stays on as it should. After left pump stops, within 10-20 seconds the car will rough idle to stall.

dvs_dave

This is classic fuel pump failure symptoms. The fact the problem manifests itself only after the second pump (G6) shuts down means the problem is almost certainly to do with the primary (G23) fuel pump. It’s not working correctly, or something is preventing it from being able to deliver sufficient fuel pressure/flow.

Did you replace the original pump with a new unit or used unit? Could be a bad “new” pump. Also, have you replaced the fuel filter? Again this would tie in with your symptoms. When both pumps are running there’s sufficient pressure to overcome a blocked filter, but not when it’s just the one pump running.

rghollis

I couldn't agree more so I keep looking around the fuel tank and pumps. The replacement primary (G23) pump was brand new from RockAuto (VDO A2C53089989Z). Fuel filter was also replaced about a month prior to the Fuel Pump.

I remembered when I changed the primary fuel pump that there was no fuel in the right side of the fuel tank, so I took the fuel pump covers off again today and looked inside the left and right sides of the tank, and sure enough, the right side of tank has no gas in it that I can see around the fuel pump. I can clearly see the fuel pump mounting screws at the bottom of the tank on the right side (G23) with no fuel covering them but on the left side (G6) the fuel pump mounting screws are covered with fuel.. The Gas Gauge on the Instrument cluster shows just over a 1/4 tank. I'd like to think I should have fuel in both tanks equally. So my question .. what gets the fuel into the right side of the tank? Or, why wouldn't the right side of the tank have the same amount of fuel as the left side?

I'd like to fill the tank up and see what happens, but if that's not the solution, I'm stuck working with a fuel problem and having a full tank of gas to deal with. And if gas isn't transferring into the right side of the tank as it should, eventually the car will stop running again. Appreciate the feedback and ideas?

dvs_dave

There are Venturi “suction jet pumps” driven by the main pumps within the tank that take care of transferring the fuel between the two sides. You may have a problem with the internal piping within the tank itself, although I’ve never heard of this happening before. Something has perhaps become disconnected? Study SSP 282 (p34 onward) which details the fuel system. Maybe you’ll find your answer here.

http://www.volkspage.net/technik/ssp/ssp/SSP_282_d1.pdf

It also shows that fuel pressure regulator is set at 4 bar.