I've been dealing with a persistent rocking/shaking idle in my '87 300 td since I bought the car. The roughness remains from idle to approximately 1500-1700 RPM. I've replaced the injectors, glowplugs, and engine mounts, as well as checking and adjusting the chain stretch and IP timing--to no avail. I've performed lots of 'Italian tuneups' so I don't think the pre-chambers are coked at this point. I've come to suspect that the IP requires re-calibration, but lately I've been wondering if the delivery valve seals could be the culprit.

I've heard these seals can leak externally (mine don't ), but does anyone know if they can leak internally and cause a rough idle or mis-fire condition?

Background:

ELR works fine
EGR and ARV are dismantled
Air Cleaner clean and clear
ALDA has been re-sealed and works fine
0-60 time is around 12 secs.

Haven't cracked the inj. lines yet to isolate which cyls. are misfiring.
Compression is unknown at this point.
Cyl. head is known to be cracked (pressurized coolant overnight), but doesn't lose coolant.

Biodiesel seemed to smooth out the roughness, but I'm not currently running the stuff for the duration of Winter. Shaking has gotten worse with the cooler outdoor temps.