Mercedes Repair Questions? Ask a Mechanic for Answers ASAP
If you haven't already then 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.
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
Also check that the vacuum hose running to the dump /or turbo bypass valve is intact with no leaks as if the dump valve is opening prematurely you'll be losing all your boost
The dump valve is on the turbo boost air (post compressor) side its there to blow off or 'dump' the excess boost pressure you get when you close the throttle ( like just before a gear change) the important thing with a dump valve is that it seals properly when its not being operated if the valve has a torn seal or a broken spring then it'll blow off before it should and you'll lose boost pressure
this video shows how you can test ithttps://www.youtube.com/watch?v=kXRaGg06m7s
If this is OK then check for air leaks after the mass airflow meter - The best way to locate a leak is to have the engine running and warm and then spray lighter gas /propane around each joint in turn. If the engine rev's up you've found your leak. Work your way through each possible joint one at a time and you should find it. I use a slightly flattened piece of brake pipe and some rubber hose from the can of lighter gas to provide a spraying 'wand' and allow a direct blast of gas into each area, especially those difficult to reach with large implements.
in your 1st answer you said the activator ( actuator) had been replaced already?
If this in not the case then yes its possible that its faulty what can also happen is the vanes on the turbo can seize up
so I'd suggest to disconnect the actuator motor from the turbo and then check the the turbo mechanism moves freely under finger pressure
if not then the turbo will need to be stripped and cleaned and the actuator should move in and out when the ignition is 1st turned on
that servomotor part no. is shown as for the inlet manifold?
so it controls the variable inlet flaps
the turbo actuator motor controls the position of the turbocharger internal vanes that regulate the boost pressure
its the rectangular box on the bottom of the turbo as shown in this image
if it was faulty it would certainly result in a lack of boost pressure
not a huge influence
it changes the shape of the torque curve but shouldn't have any effect on the turbo
can you tell me if you've had any fault codes read off and what they are?
that code does point to the inlet servomotor
as you say it can get damaged by oil dripping onto it
if you want a cheap fix then you could just unplug the motor and fit a 4700 ohm resistor to satisfy the ECU open circuit monitoring
also it looks like Euro car parts keep the motors too a lot cheaper than MB at about £85 quote part number###-##-####/p>
glad to have helped