Had some inspiration after reading this W212 thread, so thought I'd try replacing the PCV Valve on the ML for some additional preventative maintenance (part # 2780100431)

Arguably it's a more difficult job as the engine is slightly recessed behind the bulkhead, unlike the W212. And if you've got a RHD like me you have the steering shaft to reduce the available space even further.


The M157 sits quite far back in the W166. At this point I've removed air intake parts and cylinder head insulation


I followed the WIS (thanks to @Valvestud ) instructions to a degree, no lift at home so I tackled everything from up top.

For the ML/GL we need to remove the firewall partition directly above the HPFP's. I also removed the windscreen drainage bridge (might only be on RHD cars on this side) to give better access. Make sure the hood/bonnet is in the service position, you'll need all the leverage you can get for this job.


Firewall partition removed.. you can see the windscreen drainage bridge is in the way, so I removed this to.


That's better, time to start wrenching!


Removing the HPFP crash plate was a massive PITA, definitely the hardest part of the job, everything else is relatively straight forward. You need to do this though, I think it would be impossible to get your hands behind the PCV cover to remove the necessary bolts and disconnect the vacuum line from the intake.

Once the crash plate is clear there is just enough room to wrench in the space available, far from ideal but doable with some patience.


Crash plate and insulation gone, now to remove the vent valve bolts and detach the vacuum line


To help guide my hands to where they needed to navigate I set up my iPad with a photo of the area I was working in. This was a huge help, and I also used an endoscope to take a closer look at particular parts.




The centrifuge was in a good condition, I could not feel any grooves or wear on the shaft, so I left it.




Here is the old vs new (67k miles)



As mentioned in the other thread there is no chance to get a torque wrench in the space available. So I just snugged up the bolts, they certainly weren't torqued down very tight when I removed each one. Otherwise reinstallation was a piece of cake, just don't be in a rush to get the job done!

Finally, I replaced the partial load oil separator (part # 2780180035) on the left cylinder head cover, it's a cheap part so I thought why not. It's super simple, about as difficult as replacing a cam sensor. You might have to move a few lines to remove/replace the part though. Torque to 10Nm and it's job done.

 

When does this needs to be replaced? At around what mileage? What were the symptoms?

 

Honestly, my engine was assembled in 2014 and has over 65k miles now, that's a lot of heat cycles for 10 year old check valves and rubber parts to start failing. This job was preventative maintenance only, I had no proof it was absolutely necessary.

Others on this forum have suggested a faulty PCV can lead to increased pressure in the crankcase which contributes towards the CPS and cam magnets leaking oil into the wiring harness - no idea if this stacks up, but seems logical.

 

As I understand when the seal goes bad in the oil separator cap you will get an excessive vaccum in the crankcase which can be seen as a suction on the oil filler cap.

 

Everytime you do an oil change, pull the hose off the rebreather valve (the large black plastic part) and look for oil in the tube. That means it's leaking oil into your inlet manifold and is usually the cause of broken inlet actuators as the internals become sticky with oil residue.

Im not sure that MB has this part as a maintenance item, but many on these forums suggest replacing it at 100k miles.

see this video

 

Replaced the PCV on my, 2015 ML-350 3.5L V6 M276 W166, because there was what I thought was a lot of vacuum in the crankcase as evidenced by suction when removing the oil filler cap. There was a lot of oil in the intake manifold.
I did not replace the spinning drum they call the "oil pan" because it was clean. There is still vacuum on the oil filler cap but does not seem as strong. Below is a picture of the PCV taken apart. Mine was leaking at the green o-ring and dripping on the O2 sensor electrical connector.