How to get a smooth idle on 300d?
 

I just had the dampner, tensioner, idler pulley, and serpentine belt replaced. Maybe it's my imagination, but it seems like the car does not idle as smooth as it did before fixing that stuff. It actually idles fairly well, but I once saw a '92 300d that was super quiet, I couldn't believe it, and very smooth at idle. What can I do to make my '91 300d quieter and smoother? It only has 90k miles on it. Engine mounts have been replaced previously.

anyone know if their is form of rack dampening pin on this engine.. i know you can fiddle with that on the om617 but what about his engine.. what about your engine mounts.. how are those holding up

How many miles on it?

Valve adjustment, diesel purge, injection nozzles, pump timing, timing chain stretch test/adjust.

These are some of the items that will contribute to a rough or less than smooth idle. The Mercedes does not have lots of extra power to spare and it several of these items is out of adjustment then you will get a poor idle.

My 300d idle
 

I just edited my original post with the miles, etc., on it. How hard is a valve adjustment to do? I hear they're somewhat easy to do.

you dont have to adjust the valves on your engine

Valve adjustment
 

I thought the valves needed adjusting from time to time. DieselGiant also wrote that it is needed.

'91 300d 90k

Try an injector cleaner and cetane booster like Red Line DFC. The valves are self-adjusting on 1986 and later models.