My 76 240d w/ rebuilt om 615 has been running well but when the idle was increasing until I hit 30 mph w/ my foot off the petal, I pulled the pump after setting to 26 degrees on compression and inspected the diaphragm (shot) as well as the bushings on the vacume cam. (shot)
I got the diaphragm from Medford Mercedes and noted it was identical to the original and carefully swapped parts.
The manual says the spring and diaphragm should be replaced together but there was no spring included so I used the original parts. There are three springs in the assembly and I don't know which one they are referring to.
I've since pulled the pump 4 times, did compression check (300lbs +) carefully checked timing -drip test/injectors & valve adjustment. The motor runs like crap w/ governor hunting like I'm blipping the petal and throwing light smoke. I may have the cam in the vacume chamber backwords, but switching still didn't cure rough running. Clues anyone? The photo may help. Thanks - desperate in N.C.

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Perhaps some of the images in this thread will help:
Removing 220D Injection Pump Pneumatic Regulator Diaphragm...

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1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).

Also, this benzworld thread has lots of possibly relevant info:
White Smoke 220D - Mercedes-Benz Forum

Good luck.

__________________
1968 220D, w115, /8, OM615, Automatic transmission.
My 1987 300TD wagon was sold and my 2003 W210 E320 wagon was totaled (sheds tear).

Using your suggested thread, I found that "my careful replacement of diaphragm parts" missed the two shims below the spring.
Probably stuck when I tapped the diaphragm to remove the cam & spring and I overlooked them.
Another reminder not to throw away replaced parts too soon!
Thanks for your time.

Thanks Shortguy!
I see that my "careful parts replacement" missed the two shims below the diaphragm spring. Probably stuck and I didn't look close enough.
Hey, thanks for your time, I'll try & do the same for somebody else here in the future.

There is another small spring inside the poppet cam plunger...you have to remove the snap ring to get it apart. The one in my 220D broke in half and collapsed on itself with the same symptoms you are experiencing. I found an approximate sized spring at hardware store cut it to right length and installed. Its ran great ever since

Its the round thing in your photo...also if that whole apparatus doesnt hold vacuum it wont idle either. Make sure it has a good gasket and replace the O rings on the poppet camshaft...I found identical sized ones in the plumbing section at hardware store

I will check that while I still have the pump out. Great suggestion. Thanks!

It had me scratching my head for awhile. I did the diaphragm and o rings it ran fine for a couple days than that little spring collapsed.I didnt have snap ring tool around when doing it or I would of opened it anyway just to look. I bought a roll of gasket paper and smeared a little oil on the cover assembly to get a good imprint and just made the gasket. It was easy.