My 190D is a non-turbo, and now I realize that it is likely a very different system. If the 300D has EGR, it needs some mechanism to create intake manifold vacuum. In the NA 190D 602 engine, there is a butterfly valve (like a throttle valve) in the airbox between the air cleaner and intake manifold runners. It makes sense that on a turbo, the valve (if it exists) would be on the inlet to the compressor so the compressor isn't working against the valve.

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1987 W201 190D