Not exactly a diesel question but it does involve the (diesel) fuel cap on my 1999 E300. The original or what I assume was the original one was lost maybe 8-10 months ago when some fuel jockey forgot to put it back after refueling. The new one left me stuck on the PA TUrnpike when I stopped to refuel while returning from a long trip. Spent nearly an hour trying to get the darned thing off and eventually had to meet up with a MB roadside technician who yanked it off with a wire puller. Got another today at my local MB place. They had to get a tech to yank it off again. He made it clear that they see lots of these. Seems that if you or a fuel jockey turn it a bit too far past its usual quarter turn odds are you'll break something delicate inside and not be able to get it off again (unless you know how). I was astonished to learn that my old cap was guaranteed for a year--but the warranty was voided by breakage, if you can believe it. So much for German engineering in America! It's only $20 but since I didn't know how to fix the problem I ended up stuck on the road and paying an MB tech to fix the problem.
Can't be sure who broke the cap. I live in NJ where you are not allowed to pump your own fuel. Maybe I did it when refueling a few days earlier in a place where I pumped my own.
The solution: Turn the cap so the wing or tab you grab is parallel to the ground. With a rag to protect the cap grab it with a pair of pliers (or a puller) and yank it firmly. Forget tryhing to get it off with a screwdriver.

 

the reason for that happening is actually the filler neck itself...there a 2 small tabs on the inside lip of the filler neck that get bent up from the gas nozzle banging it when filling up. those tabs need to be bent back into place and your cap should be fine after that. and yes the fuel cap does get twisted and you need to turn the tabs back with a strong hand or pliers.

 

Good you are fixed again,
My experience is that most stations I frequent make the exception to fill diesel fuel. I mostly get fuel in Pennsauken, but others in NJ, I have found do the same. I am getting too old to "over torque" the fuel cap.
Take care,

 

Thanks, chassis221. Oddly, the explanations I got at my local dealer (Cherry Hill NJ) did not mention any problem with the tabs in the filler neck, although they could obviously see the results of my efforts to remove the defective cap with a screwdriver. In their view it is simply a matter of overtightening the cap. Caps on most other cars, they explained, cannot be overtightened: you twist them until they start to make a clicking noise. You can keep on turning but the cap will get no tighter. The MB cap, however, can be overtightened by a fuel jockey who thinks he must keep turning until he hears the clicking. He eventually gives up but by then it's too late. The problem is obviously foreseeable and, per my dealer, occurs pretty frequently. A little nylon ring which is part of the cap twists past the point of no return. Once that happens you cannot remove the cap except by force. Of course, the solution is for ME to step out and do the necessary with the cap instead of some lackey at a gas station. But since I had no idea how delicate this cap is or how it is designed until I got the lecture from the MB tech and then from the parts guy, the "owner workaround" works only after getting stuck somewhere! I never had or heard of such a problem with my W124s in the past. Why MB seems uninterested in solving such a simple but embarassing defect--either by a better cap or even a cap with a simple warning--is a puzzle.