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One common issue on this model was the catalytic converters either cracking or clogging up.Mercedes had even released a recall for dealers to check the cats. for cracking issues with them needing replaced under warranty at no cost.I would have the cats. checked for any cracks to see if any are found.It is the welds that crack on them.If any are found I would speak with a dealer to see if they can still warranty this as sometimes they can get the recall re opened to have the affected catalytic converter replaced.
If no cracks are found then the exhaust back pressure needs tested to see if there is excessive back pressure.If there is excessive back pressure this indicates the affected catalytic converter tested needs replaced.I would have both tested.
One other issue with this engine is the mass air flow sensor.In allot of cases it will not set any codes when it acts up but it can send an incorrect air flow signal and cause rough running needing replaced.
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It is possible it is a sticking valve but I havent seen this issue on this model here myself unless the engine is really gummed up with engine oil build up.Usually this can be checked with a valve cover removed to see if this is a possibility here.
I would have the cats. checked and a back pressure test done as well.
Let me know if you need anything else and feel free to rate my help here.