Need some help. My 2000 A6 Avant won't start with the fuse in for the fuel pump but It will start briefly but sounds like it backfires thru intake. It lost power going down the road, like the tranny wasn't shifting then the engine shut down. I just changed the fuel filter a few days prior to this happening. I pulled the plugs..cleaned and regapped...they were a little wet, but not dripping. I pulled the vacuum line from the fuel press regulator and didn't see any fuel come out when cranking......could it be the timing belt????

 

If indeed it's backfiring through the intake it could be timing that it too advanced, thus leaving the intake valves slightly open during the detonation. Or a fuel mixture that is too lean. I would lean more towards a too lean condition as you recently did a fuel filter swap and you said you weren't getting fuel when cranking the engine.

After ruling that out I would begin investigating the timing belt.

Do you have a check engine light on?