The EGR could be stuck open. I suggest taking a quarter and some rvt sealant and sealing up the EGR on the intake manifold.

DO NOT REPLACE THE TIMING CHAIN as "maintenance". No where in the FSM does it say that they EVER have to be replaced and if you read around here they are the number one source of failures after such "maintenance" is done. Before you do anything else pull all the injectors have them pop tested and rebuild as needed. While you are in there check your PC balls. Rough idle has nothing to do with the turbo what so ever.

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#1994 E320 Cabriolet
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OBK #42