Injection pump return line hose.
 

When replacing the injection pump return line hose on a W123 diesel is it absolutely necessary to replace it with the same kind of cigar shaped hose?
http://i8.ebayimg.com/01/i/00/f6/0f/4c_1.JPG

While the car will work without the cigar hose and an ordinary fuel line in its place, you will understand the need for the special hose if you wrap your hand around the hose while the car is idling. You can feel the pulsation and the how cigar-shape works like a shock absorber to dampens this. Otherwise, the pulsation can get transmitted through the body of the car and somehow into the cabin. It can also cause your fuel lines to move with each pulsation, even if ever so slightely, and can wear out some line that are rubbing over time.
I'm sure there is more reason to it, hopefully someone else will chime in....
Cya, Adam

Before I saw this part listed on eBay, I just thought that the hose was swollen from age. I never realized that the hose had such a special function. It's just some of the amazing things you learn while being a member of this forum.