I've been plagued by rough idle since buying my 85 300D last October.
I've run the entire checklist.
1. Tank Screen; (Totally Clogged)
2. Filters
3. Monark Nozzles (Balanced and Pop Tested)
4. Replaced Timing Chain and Timed using Drip Method
5. Cleaned Delivery Valve Pipe Connections and plunger (more on this later, apparently not good enough)
6. 3 Valve Adjustments performed from October to May
7. 2 Diesel Purges.
8. Idle Adjustment
9. Lift Pump Rebuild.
10. Return Spring Stretch... and subsequent replacement ( I understand that the FSM advocates this, but I think it is a bandaid for masking a real problem. My concern was that the pump would have inadequate cooling with the decrease fuel flow through the pump.

Under advice from Cho and Eaton999 I decided to revisit the delivery valves. I got a set of pipe connections, pump plungers and springs from the junk yard, cleaned them up a bit and installed them. I did note they were slightly different than mine, but decided to install them as a set, Pipe Holder, Spring, Element and Plunger. No real difference as the shake was now better with all of the other maintenance done, but still not right. Well when I installed the Delivery Valve parts from the junk yard, i decided to put my originals in a jar with some brake cleaner and some pb blaster. Gave it a shake and swirl every couple of days.

This past weekend, I decided to reinstall the originals. What I noted on the pipe connections is a tiny orifice in the pipe connection of different diameters on the donor compared to the originals. This orifice is small. Didn't realize just how tiny until I set it up on my pop tester and saw the tiny jet stream of fuel. This orifice is easy to clog and will likely present itself under low rpm conditions.

Checked each pipe connection with the pop tester and reinstalled them. Unbelievable. This car now reminds me of the 84 300D I had in the mid 90's.

Add the pipe connection to your maintenance list, especially if the vehicle has sat for extended periods, or has had alternate fuels run through it.

Hope this helps someone else.

Still not sure of all the differences in pump plungers and pipe connections. the pipes have different numbers on them, the plungers have the same numbers even though they are different overall lengths.

Don't play with the delivery valves....that is asking for trouble.

Do a compression check...
Check timing stretch...
Check injection pump timing
Replace mounts with Lemforder or phoenix brand mounts, including trans mount
Look at your rack damper bolt....
Disable the egr, so you know your getting fresh/clean air..

If you have low compression on one cylinder, you will never get rid of the rough idle..

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The OP is not asking for advice, but is providing his experience for others. His rough idle is resolved.

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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).

Resolved by:

"This car now reminds me of the 84 300D I had in the mid 90's." ?

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

The car is running great through all RPM Ranges with smoothe idle.
Compression was checked a while back; Was a little low on #1, but came upto 325-350 after a MMO/PB Blaster soak; 2-5 are all at 400.
Never did a leakdown test since #1 came up to what I consider an acceptable level.

Good for the OP but I wouldn't recommend others try this to cure a rough idle....There's a reason why an IP rebuild cost 3 grand....

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Okay; Shame on me for using the wrong terminology. The delivery holder was never removed, and I agree that it should not be removed. The parts that I'm referring to is the plunger, spring, and pipe connection. Not the delivery valve holder that is secured with the 2 nuts.

It was the pipe connection that apparently had blockage and caused the rough idle. Item 1 in the attached illustration.

Bump for shakes

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1981 240D 4sp manual. Ivory White.