My '87 190D OM602 has a valve in the inlet. It is normally open. It closes under certain conditions to create sufficient vacuum to allow drawing exhaust through the EGR valve. It is controlled by one of those rectangular black modules, two temperature switches, and a few switchover valves. I'd expect the 300D to have a similar system, but I don't know that for sure. If you disconnect and plug the vacuum line, the valve should stay open and allow you to use the car. The EGR system will not function properly, and I like to have it functioning (trying to do my part....) You may not care, in which case there is no detriment to the car by disabling it.

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1987 W201 190D