If I'm causing tension with the tone of my response it isn't intended. It's common and appreciated practice to include the year and model of your car every time you post a question. There are thousands of forum members. Even the techs can't remember who has what kind/s of car/s.
Maximus, I see from previous posts that you have an 89 300E. Is there a VIN tag on the driver side windshield pillar? If so, does it have a number like WDBEA30 or WDB124? If it has a VIN of the WDBEA30 variety then you have a US car (possibly a Canadian car) and it should have an EGR system. The EGR valve will be towards the rear of the forward exhaust manifold.
I hope the attached photo is clear enough. I had to trim it to keep it under the attachment size limit.
From what I've read in this forum and others, the EGR system is not a common cause of rough idling. More common causes are tired oxygen sensor, leaking electrohydraulic actuator, vacuum leak, etc. These problems may or may not have other symptoms.
Not having an EGR valve whether your car was originally equipped with one or not eliminates a possible cause of your rough idle.
I hope I don't sound condescending because I really don't mean to. I've rough idle problems with my SE for over a year so I'm by no means an expert on these engines. 'Rough idling M103' ranks among the top 10 post topics along with 'synthetic vs petroleum oil' and 'MB vs other brands of coolant.'
81 300SD (on ebay)