Maybe, just maybe this is the problem, but I'm not sure. I need someone to go out on their S5 TurboII and check for me which one is the fuel inlet and which is the return. Here's what I got so far:

There are two fuel lines physically ON the engine. The one closer to the front of the engine is the fuel inlet I believe. That one on my car is hooked up to a line that goes almost straight down to a little canister, then God knows where it goes from there.

The one more towards the rear of the car goes to a metal connection on the firewall, then down along the firewall towards the gas tank. Any help?

 

that little canister is the fuel filter. that is your inlet line from the fuel pump. the other one is the return line comming from the fuel pressure reg. at the rails.

 

that's an awfully big canister for a fuel filter.. holy crap. Ok, if you say so. Thanks.

 

ya

 

If you're comparing it to a carb fuel filter, EFI filters are much bigger because they must filter down to much smaller particles while keeping the same flow, plus the pressure is much higher so they're a bit beefier!