Lots of air bubbles in my fuel

[A6V6qs]

Dear fellows, I'm asking for your help in solving a problem regarding my beloved Audi. Actually, I'm begging for your help, because it's driving me crazy. The car (A6 2.5 V6 AFB quattro tiptronic) suffered from power loss and excessive fuel consumption. Obviously, no error codes were stored. After 3 painfull months and a bucket of cash spent on new MAF, MAP, TCRV and a few others, I'm still there. Also cleaned the intake and the VNT. I'm about to start shooting mechanics. Or myself. Today I found by accident something that might explain it. With the engine idling, everything seems fine. But when it's revd, i can see that in the clear tube between filter and pump air bubbles form. The higher it's revd, the more air she drinks. when it reaches 3000 it's basically running on air. When it gets back to idle, the bubbles disappear. Given the fact that I've tried so many solutions by now, I would like to have the members opinion on what would be the cause for this and, if possible - the solution. I followed the fuel lines to the tank - no leaks. In the engine compartiment - no leaks. One of the rubber hoses coming back from an injector seemed a little tired so I replaced it. It wasn't that because the bubbles are still dancing in my fuel line. What could it be and where should I start?

Your input is higly appreciated. Thank you.

 

[enda1]

Is it that it is air or just that there isn't enough fuel getting through and what you are seeing is vacuum bubbles (not enough fuel to fill the pipe entirely). Then this would suggest a blockage in the fuel lines or in the tank soemwhere

 

[A6V6qs]

Thank you for your input, enda1. Must admit this hasn't crossed my mind. It makes sense but .. how could I check it? I visually inspected the lines to the tank and everything seems fine. They're not ripped, squashed, nothing. In the mean time I found two other peculiar things: 1.the fuel filter empties over night and 2.I inspected the top of the tank (because from under the car everything is perfect) and I found a coating of dust stuck on old diesel allover the connectors. It's not much, but I imagine sometimes in the past there was a leak or, more probably (because of the fine and uniform disposal) vapours of diesel there. With the addition of the filter emptying (the "leak" at the tank is definately old) can you think of a way to diagnose it further?
When should the transfer pump be runing? Is it supposed to keep the fuel level equal in all sections of the tank or higher in the final one?
Should ALL fuel system be pressurized (including the tank)?
Thank you.

 

[enda1]

I'm not that familiar with the fuel systems on these engines. Is the filter in the engine bay or back at the tank. if the filter is at a higher level than the tank and is emptying it would suggest that air is getting in somewhere and this causes the diesel to drank back into the tank overnight.
The access to the tank should be under the carpet in the boot on the right hand side near the back of the seats. The balck plastic cover accesses the tank whre you could check if there is crap in the engine blocking the pipes

 

[klogz]

Did you solve this problem? thanks

 

[TYphoon GB]

Might help...

http://www.audi-sport.net/vb/a6-s6-allroad-forum-c5-chassis/171256-temperemental-starting.html