Try adding air pressure to push the tank back out. Just be gentle about it. ie: dont let loose 100+psi all at once. I would imagine that if you attach a line to the fuel lines you wouldnt need much air pressure to push the tank back to shape. Even the cheap 12V tire inflators should do the trick. You just have to figure out how to attach to the fuel lines and plug the vent line. Once pushed back out, clear the vent.

Note to all: There should almost never be a significant rush of air when removing the fuel cap.

I have not seen a plugged vent on an MBZ diesel to date, but it is good to know to look for it.

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85 300D 3 pedal. Current project.
83 300TD (need rear wiper assembly dead or alive)
84 300SD Daily driver
85 300TD almost 400k miles and driven daily.
98 E300D *sold
86 300SDL *sold and made flawless 10 hour journey to new home.