Be certain the filler cap gasket is BLUE .

If not, go to the Classic Center and order one in, install it on the original cap, they're different and better than the black replacement caps .

Park the car so you can SAFELY scrooch underneath it and look at the floor just behind the driver's side rear foot well kickup ~ there will be a pipe leading down from the fuel tank area that has a funny shaped rubber bulb on it, *gently* squeeze the bulb, it has 'lips' that will open and perhaps allow liquid / leaves / debris / mud dauber wasps nests / etc. to come out of it .

This is the tank's vent and sometimes it simply sticks closed .

BE GENTLE because it's no longer available if you crumble it by squeezing too roughly .

I've had this same issue on most of my W123's (5 and counting), the blue filler gasket was the solution and it works well on the oil filler cap on the rocker box too .

__________________
-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

I baby sat a 240d of about that year for two years while a friend who owned the car worked outside the country. The car leaked diesel fuel when I filled the tank but not when the tank was less than full. I just never filled it more than 3/4 full and that worked.

Prolly a bad connecting hose. It didn't come out the filler door though did it?

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Excellent advice, this fits the symptoms perfectly.

Thank you

__________________
1981 240D Four on the floor, Orient Red over Parchment, bought with 154,000 but it's a daily driver and up to 180,000 miles, mostly original paint and all original interior.

It's coming out through the filler door.

__________________
1981 240D Four on the floor, Orient Red over Parchment, bought with 154,000 but it's a daily driver and up to 180,000 miles, mostly original paint and all original interior.

then it has to be the cap, or a hole in the pipe below the lid and above the floor of the gas filler area.

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

I will look at that as well, thank you.

__________________
1981 240D Four on the floor, Orient Red over Parchment, bought with 154,000 but it's a daily driver and up to 180,000 miles, mostly original paint and all original interior.

. Is this it?

Attached Thumbnails

 

__________________
1981 240D Four on the floor, Orient Red over Parchment, bought with 154,000 but it's a daily driver and up to 180,000 miles, mostly original paint and all original interior.

Yes .

I guess the older models had a rubber bulb to prevent mud dauber wasps from using that open hole as a nesting point .

__________________
-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

The opening or vent pipe is bone dry and it doesn’t look original as in it does not appear to be nearly 40 years old.

__________________
1981 240D Four on the floor, Orient Red over Parchment, bought with 154,000 but it's a daily driver and up to 180,000 miles, mostly original paint and all original interior.