Originally Posted by 97 SL320

Is the roughness rhythmic or random?

If rhythmic the problem is related to a single cylinder , if random it could be a erratic single cylinder or a global problem affecting multiple cylinders.

Injectors with poor spray patterns will give rough idling.

Do you have access to the mixture adjuster located between the air flap and fuel distributor? If so, insert the proper Allen and gently push down and see if things smooth out. Then rotate just enough to grab the adjuster and slightly pull up. This will tell you if the mixture is too lean or too rich. With a down flow CIS flap, pushing down richnes the mixture and pulling up leans it. An up flow operates the opposite.

If leaning the mix makes it run better, you are not chasing a vac leak.

If the fuel distributor plunger is sticking, subtle changes in air flow will not result in fuel volume changes and a rough idle will occur. My 91 300SL M103 / M104 suffered from old fuel, removing the fuel distributor and spraying in and out with carb cleaner while cycling the plunger made a world of difference.

Does this engine use a plastic "dog dish" under the distributor cap? While sorting out the SL I came across some information that the dish can carbon track causing roughness.

Does this engine have a EGR valve? If it is leaking you will get a cold rough idle.