Models Affected: S4 2.7T, A6 2.7T, allroad 2.7T (these added for search hits in future)
As the 2.7T engines age, a common issue that pops up is a small leak where the fuel supply line connects to the fuel rail. The leak is typically either at the actual fitting to fuel rail connection or where the rubber hose attaches to the fitting. Typically the leak first occurs only at very cold temps, and the symptom will be the smell of fuel in the cabin shortly after the car is started up.
Because the fuel is leaking onto the engine above the exhaust manifolds, I recommend taking care of the issue ASAP. You can imagine what might happen if the fuel leaked out at a rate where it was able to make it to the hot exhaust manifolds before it evaporates.
If the fitting is leaking, I recommend buying a NEW fuel supply line (part number 4B0201541C ). The hose fitting at the fuel rail end is a ball type seat. When tightened onto the fuel rail, it deforms slightly creating a seal. (kind of like a crush washer). Because of this, simply tightening the old fitting tighter on the fuel rail frequently results in continued leaking days later. Replacing with a used line is not recommended due to the same reason. Again, it *can* work, but this isn't something I'd recommend playing around with.
Buying the supply line locally at the dealer will cost just over $70 with tax, and you can find them online for around $55 shipped. If the car is being driven, I highly recommend buying it locally and fixing it asap, as saving $15-$20 isn't worth the risk of fuel leaking in your engine bay. The best/safest advice is don't drive the car with fuel leaking at all.
In order to change this line, you will need to depressurize the fuel rail before disconnecting the supply line (so you don't spray fuel all over the engine bay / your face / your garage). To do this, remove the fuel pump fuse and start the car. The car will die after a few seconds if you pulled the right fuse. Start the car up again, and let it die. Continue until it won't start anymore.
If you aren't able to locate the fuel pump fuse, please stop and schedule some time at a good shop to have this done.
Also worth mentioning to do this in a well ventilated area, as there will be fuel fumes present when changing the line.
When you disconnect the fuel supply line, there will still be some pressure in the line, so make sure to wrap the connection with rag/towel before loosening, and go slow to control rate at which the remaining pressure can be relieved.
You may have a "fun" time getting the clip off the other end of the fuel hose where it attaches to the hard line near the firewall. It's important when attaching the new hose that this connection is secure. Failure to secure this well can result in the hose popping off from the fuel pressure and dousing your engine bay with raw fuel.. near the turbo and exhaust. I like non-perforated rolled edge screw clamps where turning the screw has good resistance so it does not back itself out. Feel free to toss two clamps at that connection right next to each other.
When putting the new hose on, attach the fuel rail end first. This prevents the hose from twisting the wrong way if you attach the other end first. If the hose is twisted, it will have a natural tendency to back the fitting out. Do not over-torque the 17mm nut, but it does need to be nice and secure. I don't know what the official torque spec is for this connector, but I'll try and find out and include in this post.
Hope this helps someone when they run into this issue!