If you have a CEL and limp mode, you are getting a trouble code. What is the code? Do you have a Xentry/iCarSoft? If not even a $20 cheapie code reader should at least get you pointed into the right system.

These things store massive data in the computer so it should lead you right to the problem.

Check the simple stuff first. If the crankcase breather diaphragm is shot it will suck down oil. Also if the air filters are dirty it will do that too.

How about this for a hypothesis: the breather is shot, it’s leaking oil out the turbo inlet air seal because it’s old, which dripped down on the swirl flap motor, killing it and setting the limp mode. BTDT on my 08 ML 320 (which has the identical engine as the 08 E320 BT), see the oil cooler thread.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech