I opened my trunk for the first time since putting fuel in a week or so ago and noticed a diesel smell for the first time. I pulled the carpets and did some troubleshooting. I know it is not due to a bad filler neck gasket (that's brand new) and it is not due to a clogged overflow tube in the filler door (tested that by pouring water down the tube and it all went straight to the garage floor.

It appears that there is a wet area under and around the gasket at the fuel return line and vent line end. There's a part on EPC online 123 471 21 80 for this gasket. What does this gasket actually do? Is it simply a seal for the gap between the tank and the trunk floor and doesn't actually come into contact with the fuel?

I replaced all my fuel lines recently and they're not wet on the underside of the car. However, the rear axles looked suspiciously like the ones in this thread when I bought the car, so I think there may have been a leak at some point in the past. Bad Fuel Leak

My car currently does not look like this, but it did when I bought it, only the wet part was limited to the inner spider housing on the axle and did not flow onto the differential.

Does anyone have a good pic of a removed tank showing how the return line connects to the tank itself? My best guess at the moment is that I broke/cracked a connection between the hard lines and the tank when I changed those lines a few weeks ago and this is causing a leak inside the trunk. Does the vent line actually go back into the tank at the bottom, or does it just come through the trunk next to the return line and then wrap around the tank and go through the top?

'83 300CD 283K +

New viton vent line (dry)
New viton return line (dry)
New viton supply line (dry)
New filler neck gasket (intact, good fit)
Overflow tube connected and drains to outside of car

I think I solved the problem. When I replaced the lines I pushed the viton return line snugly up against the rubber gasket that wraps around the metal line. This created a good seal. I also apparently did not tighten the clamp on this line enough. This did not create a good seal. So fuel seems to have leaked between the metal line and the rubber hose, but then did not leak between the butted rubber/rubber connection between the gasket and the viton line. It went back into the gasket and pooled in the shallow section in there. So it leaked (or was pushed) from outside the car up into the trunk and into the gasket.

Because the lines were dry on the underside of the car I could not see any of this, so I pulled the tank. When I replaced it, I made sure to leave a gap between the tank/trunk gasket and the viton return hose.

Info on the gaskets and the lines:

There are two gaskets. These are the rubber parts you see under the car. The tank sort of sits on them since they're both at least 1/2 inch thick at the edges. The return line gasket is very compressible. They do not touch any fuel. The vent line does not re-enter the tank. It sits in a small channel in the gasket, then wraps around the tank. The return line fitting is like a small elbow macaroni noodle that goes back into the tank. The vent lines are flared at the end so you shouldn't confuse them with the return lines that are not flared.