Hey guys, new to the forum here. I have a 2008 GL 450, I recently changed the radiator and assumed the issue was fixed. I also changed the radiator cap for the coolant reservoir, sure enough it turned out it was a defective cap because I still had to put more coolant each week.

Radiator/Coolant Cap:
The cap was later replaced, I figured that was the issue because the coolant reservoir cap would split into two after I twisted it open, leaving the center piece of the cap in the reservoir and me with only the top portion of the cap. How far do I turn the cap to close it? Until it stops? There was a click the recently when I tried to close it, but it may have been twisted too far, I usually close it fully but twisted it even tighter to make sure nothing leaks out. Now I'm realizing that may be the reason why the cap damaged the first place, because it might of twisted the part that goes into the reservoir off the center cap by twisting it too tight. Thoughts?

Coolant:
Recently, after 3 months I get the coolant message again, and the cap is still good, it makes a pressurized noise (opening it 30 minutes of the engine being off) and doesn't separate into two like the prior cap did. Could there be a leak in my tubes slowly going on? I thought I was good to go after replacing the cap a third time because the coolant always remained the same level until recent (3 months later). How does coolant disappear if there is no leak? Could it be a bad mix of coolant/water (e.g. not exact 50/50 mix)? I know my mechanic sometimes puts more water than coolant from what I see. It doesn't snow where I'm at but not sure if that matters. Thanks, feedback appreciated.

 

UPDATE: I finally fixed the issue (hopefully but pretty confident), looks like it was due to a non-OEM cap the whole time. The place that was selling them was listing them as OEM, anyways I went to Mercedes and theres a huge difference how it sits onto the coolant tank, its more aligned and there is an indent where the stop is suppose to be. Photos below:



OEM, marker sticking out on cap does not go past the lifted indent; circled the indent


non-OEM, this indent the marker sticking out the cap goes passed it, not sure if that makes a difference but I got a feeling it does.


OEM cap

non-OEM cap, split up.

The split cap does actually pressurize still (when put back together as one) if it is put on a twisted properly but I don't want to risk it as after 3 months it told me to add coolant. I'm sure the OEM is the fix now. Hope this info will help others on here.