Change fuel filters if you don't know when they were done last. Black smoke is normally a sign of overfueling, but paradoxically, partial fuel starvation can result in intermittent overfueling and black smoke.

If that doesn't clear up the problem, temporarily rig up a boost gauge and check for proper turbo operation. I'm not sure of the specs for the OM603, but boost under max load and high rpm should be on the order of 10 psi or more. I'm sure exact specs are available here if you search. In the likely event that you aren't getting proper turbo operation, there are some things to check in order to see if the wastegate is getting told when to open and close. Switchover valve mentioned above is one of them. Again, search function is your friend. It's rarely a failure of the turbo itself, rather the control system failing to tell the turbo to provide boost.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 401,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 26,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.
99 Mazda Miata 183,xxx miles.