Direct Injection carbon cure
 

Disconnect the PCV valve!
Been to the Auto show and like the new ML but scared of intake valve carbon build-up resulting from the lack of cleansing gasoline from the missing port injector.
New DI Engines -- Excessive Carbon Buildup ?? - MBWorld.org Forums
Where does that carbon come from with all the air cleaned by the filter prior to induction? Only one place and that would be crankcase vapors admitted by the PCV system. Legal issues aside, why not just disable PCV and vent the crankcase to the atmosphere through a generic PCV filter. Probably want to hook it back up before you see the dealer or the emissions authority but this should be a relatively easy hack. Do this to a new ML and the valves should not get dirty and I can be happy with the increased power and mileage of DI. Any problems occur to anyone beside environmentalism, legality and a CEL if you don't do it right?

or install water injection

Most cars now have air oil separators that have vacuum diaphragms in them that drain back to the oil pan. They can't be vented quite as simply as old cars. Also since they are separators they don't load the intake up quite as much. A lot of your carbon build up is from your EGR. And with modern ecus they don't like having that deleted.

Road draft tube:
 

I'm thinking the problem is condensation of crankcase oil vapors on cold intake valves. EGR should help to warm the valves and carry no oil (unless the rings are worn out) and should not contribute to carbon build up. The 2015 ML quick warm-up mods (very rich to heat the cats and electric thermostats) are probably the MB response as they may warm the intake valves sooner to avoid condensation.
PCV was mandated in 1961. According to Wikipedia, older cars and modern off-road vehicles use a road draft tube which would have to be added after removing the PCV:
"The first refinement in crankcase ventilation was the road draft tube, which is a pipe running from a high location contiguous to the crankcase (such as the side of the engine block, or the valve cover on an overhead valve engine) down to an open end facing down and located in the vehicle's slipstream. When the vehicle is moving, airflow across the open end of the tube creates a draft that pulls gases out of the crankcase. The high location of the engine end of the pipe minimizes liquid oil loss. An air inlet path to the crankcase, called the breather and often incorporated into the oil filler cap, meant that when a draft was generated at the tube, fresh air swept through the crankcase to clear out the blow-by gases." I remember those and could fabricate and install one or two in a couple hours.

Crankcase blow-by thru PCV sticks to cool intake valves
 

This Audi owning retired Ford engineer has made an extensive study of carbon build up on intake valves. He agrees that the PCV valve is the primary culprit and offers cures such as dumping blow-by into the exhaust . He bought a pricey book that concludes the valves are no longer susceptible to carbon build-up once they reach 280 C.
http://www.audiworld.com/forums/a6-s6-c6-platform-discussion-58/carbon-buildup-cel-tsb-update-2777956/

Seems to me that, if blow-by is the culprit, every direct injection engine w/o extra port injectors or Ford's overlap system is doomed to disassembly and cleaning at increasingly frequent intervals as blow-by always increases with age. Wonder how expensive that would be for a ML.

I spoke with my tech specialist this morning regarding this issue, or should I say non-issue. He hasn't heard any reports of carbon fouling on MB direct injection engines.

A road draft tube on a new ML sounds like a cheap way to create thousands of dollars of damage solving a problem you haven't experienced.