Sounds familiar. I found the same thing when I did my GPs a month or so ago. Fix it fast! You are losing so much Diesel and money. A search will give you all the info needed to buy and install the return lines. It is very easy. I found I had to go to a MB Indy for the parts, or get it from Fast Lane. AutoZone and such do not have the right fuel lines in my experience. You will not believe how your mpg improves.

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1982 Mercedes-Benz 300SD 210k miles