Mercedes Repair Questions? Ask a Mechanic for Answers ASAP
Ideally measure the pressure at the inlet to the main engine fuel pump and if this is low also consider changing the fuel filter as if this partially blocked this too will reduce pressure.
If OK then also check the connections to the EGR valve, if either the vacuum pipe of electrical connection are damaged / corroded then the EGR can be on all time which certainly will hurt performance. Its also possible that its a fault with the EGR valve so its worth removing the valve and cleaning it out with brake cleaner, if it looks particularly clogged then replace it.
If this is OK then check the small vacuum lines to the turbo and its connected control solenoid on the bulkhead, any cracks or leaks can give turbo issues and its best to have the boost pressure measured actually measured with a boost gauge to check that the turbo is healthy and that the sensor is reading correctly
If its a variable vane turbo (they’ll be an actuator on the turbo body - but not a wastegate) then check its vacuum pipework as above and check the connections to the diaphragm / solenoid valve
In my comments I mentioned that the car did not go into the service with this problem, but came out of the dealership later that day with the problem - does that point to any of the above issues being more likely? or that the dealership could have inadvertently caused the problem? (as stated, the same problem appeared to have happended previously and was somehow solved when the car went in for some other work to be done). I dont see that it could be a cooincidence?
I put the car into a different dealership for a 2nd opinion, with your comments, and they tested everything they could think of and even rebalanced the wheels as newish tyres but hasnt made any difference and they say it points towards torque converter.