We got a new-to-us 2019 E300 with 83k miles in November.

Went to check the coolant today and it seems like the last person to replace the coolant cap over-tightened it past the detent bump, and the cap is stuck on.

What is the proper way to remove this cap without damaging the plastic reservoir bottle? I don’t want to introduce any hairline cracks (or worse) by prying.

 

Could you destroy the threaded area of the cap with a sharp blade, or just cut the tabs, and remove the cap? If so, replace with new cap.

 

I've been in the same situation before I started to DIY everything on my cars. An Indy mechanic that I used to go to did the exact thing to my coolant reservoir cap after he added coolant in my car after he replaced the radiator.

In all honesty just go ahead and twist the cap off while the engine is cold. Do not try to warm it up, as the pressure from the warm coolant will make it harder to remove, and possibly dangerous as well

I'd buy a new cap directly from Mercedes and install it. Drive it around and get the car up to temp. Monitor the new cap and coolant reservoir for leaks and any dashboard lights that come on. Monitor the coolant temp and level for a while as well.

If you have any issues, replace the reservoir. That's what I did in the end.

 

Have you tried asserting a bit of downward pressure while attempting to twist it off?