I think the 5 cylinder design makes the rough idle unique. both of my 91 300D and 83 300SD are more shacky compare to those 87 300SDL and 87 E300D with 6 cylinders. I guess 6 cylinder design always has 1 cylinder to cancel the other one travelling in opposite direction.

Yes, the 5-cylinder motor is a bit strange -- it does have vibration not seen in the others due to the "un-natural" number of cylinders. MB was rightly proud of it when it was first produced!

The main cause of vibration at idle on the later (77-85) W123/W126 5 cylinder engines, though, is the design of the rack on the injection pump -- this is the part that rotates the injection sleeves to control fuel output. It is located with a spring, and at idle will oscillate in harmony with the engine when it gets a bit worn. The real fix is a complete rebuild of the IP (at least $1500 plus R&R) to replace the spring. The "normal" fix is to replace the rack bolt with the adjustable one and set it to eliminate the vibration.

I checked with my indy mechanic today while getting the third door stop so far for the 300TE (!) -- to set the rack bolt, one runs the car long enough to get it good and hot so as to obtain maximum idle vibration. One then sets the parking brake and puts the car in gear, then loosens the lock nut with a 14 mm wrench and slowly turns the 12 adjusting bolt in until the vibration just goes away. Run engine up off idle a couple times and re-check. Lock 14mm nut down while holding 12mm bolt.

It is important to do this adjustment hot and to not over-do it -- too much adjustment will give cold start problems.

Peter

Peter,

Do the non turbo 300D's also have the rack bolt?

Have a great day,

Larry

According to my books, the 300 non turbo (from 77 to 81) do not have the adjustable rack pin damper.

Joe

Thanks.

Have a great day,