My apologies if this has been brought up before, can't seem to find an exact match to the scenario without reading through each line of several threads, so I'll make this as short as possible.

2012 E550 W278 twin turbo, just got it about 3-4 months ago, cooling system hit over the 120°C mark about a month ago, looked into the reservoir, and it was empty. Thought at the time that perhaps my mistake at the time of purchase was believing the sales guy, but in any case, my mistake. I topped it up with 50/50 mix of coolant from the MB dealership and distilled water. It held about 90°C moving forward, for the past month now.

Suddenly yesterday, parked to pick something up, left the car running with AC on for literally 2 mins, came back to the temperature back up close to the red line 120°C mark - a month later. Checked the reservoir, but it was full. Happen to have my extraction equipment as I was about to do a full overhaul of fluids, oil, coolant, spark plugs, brake bleed - so I suctioned the reservoir, about 90%, and filled it back to the mark with fresh 50/50 mix. Watched the temp sit at 90°C, then creep up, and it stopped at 105°C. Luckily not far from home! So it's been sitting there hood up, ready to get my hands dirty and start pulling this thing apart. And oddly I can report as I moved through traffic, the temp dropped slightly and hovered between 102°C to 105°C from stop light/sign to the next, which makes me wonder - thermostat going?

Visually don't see any signs of leaks, it all looks completely dry. Was going to test the coolant for pH/acidity level with a simple test strip, also have U/V leak detection dye I was going to put in there ... question now is, as I'm doing the serpentine belt for simple maintenance reasons (don't know when it was ever done), do I want to take the extra hour or two and replace the thermostat assembly as a preventative maintenance step as well? Not sure the pH kit will do much now that I've replaced all the reservoir coolant. Do want to flush the coolant from below, and run IronTite flush through, distilled water and all that, before re-filling the whole system with new/fresh coolant.

Thoughts on the process? What approach would you all take on this? Air/oil temp sensors too? ... process of elimination? ... I'm open to thoughts ... just don't want to miss anything while I'm in there.

Thank you all in advance!

 

Replacing coolant is straight forward.
Replacing a new Tstat is a good idea.
Replacing serpentine kit is good maintenance

Is that gonna help lower M278 extreme heat... I don't think so....


Extreme heat is a built-in feature to "save-gas". The engine is kept near overheating temperature.

No1 offenders are dry-pistons accumulating heat from limited oiling. Coolant can not circulate heat away from pistons, only oil can.

Beware of "oil in harness" supplied by 4x Cam sensors, particularly exhaust side ($10k repair).

 

If mileage is already 100,000 or more and engine is 10 years old or more, replace the thermostat assy and coolant mechanical pump.
However since m278 has an inherent disease of breaking its turbo cooant cooling circuit plastic pipe combo with aluminum ( poor design ), these will be best to be renewed too.
Search W212 forum on common M278 issue.