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I'd first check all the high pressure hoses post turbo/s for any split hoses or cracked metal pipes, also check the ends of the intercooler as its not unheard of to pop the end caps off under high boost.
a burst or loose pipe will result in over fuelling and black smoke as you describe
If this Ok then it could be a major engine sensor such as the airflow meter or cam position sensor has failed so that the ECU is now overfuelling or injecting at the wrong point and resulting in poor combustion.
The quickest route to repair is to have the car plugged into a diagnostic machine which will read off the fault code stored in the ECU and indicate which sensor has failed or whether the fault is more serious.
Another possibility is that the fuel filter is blocked so resulting in low fuel pressure and again poor combustion - replace the filter in any case as diesels are very sensitive to this
also check how much oil is going through the breather by popping off the intake hose in front of the turbo and checking for oil deposits. Diesels will run off engine oil so this could also be making it rich
If its been overfilled with oil it will also produce surging as the oil finds its way back into the engine
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