Pics to go along with this thread would be really awesome. Im having the same problem you are with the rocking. Id love to see how you rigged your engine shocks so i can do the same and see what happens with mine.

How do you check to make sure your fuel pressure is correct? Id like to do this but i cant make sense of how to do it from reading this thread. I have an electric fuel pump feeding my engine, but it makes no difference in running when you pull the fuse to it. Its part of an aftermarket biofuel system.

Also, what is the millivolts test?

My friend just bought a total beater 300D rust bucket that does exactly the same thing. (picked it up the other day for 500$, rust holes all over it)

Both cars make a thuddy (thud thud thud thud) sound from the exaust and a steady side to side rock of the engine. On both cars this only happens after the car is fully warmed up and driven a while and only at idle. Give either car a touch of throttle and it runs smooth. Adjusting RD bolt seems to have no effect on either car (both cars have old style)

We think the rocking and thud thud sound from the exaust indicates either low compression on a cylinder or two, OR that our valves are way out of adjustment and are not seating correctly when idling. Neither car has any record of the last adjustment. PO of my car said hes never done it and he had the car for 3 years and unknown miles.

I dont have the extra cash for the valve adjusting wrenches right now and he says he cant and wont justify spending 80$ on a set of wrenches for a 500$ car. I will buy the wrenches soon so we can eliminate this as a possible cause or a cure.

Today we both did diesel purge. His IP was so dirty that the clear liquid turned dark BLACK. Definitely did some cleaning and plugged the filter. Mine didnt change colors whatsoever and didnt plug the filter either.

We also cracked the pressure lines going to each injector while the cars were both rocking and every single line cracked made a very noticeable change in engine note. Does this eliminate injectors causing the rocking?

EDIT: My car is an 83 300D and his is an 82 300D

I really appreciate all of the help and support given here. Thank you