From my experience I have learned the following lesson.

If I would have to remove again the cover I would insert a tool on the temp sensor on the back and the temp valve on front before tightening the cover in order to have enough space for the tool in case of later change of sensors.

On my engine, the head gasket was changed for the first time at 200000km (12 years ago!!!) and I believe the mecanics did not take care of that. That is why I had to release the cover.

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Keep us posted especially if your problem is solved
Present cars:
My car: E-class 420CDI, 2008, W211, V8, 180 000km

Wife's one: C-class 220CDI Sport Coupé, Euro, 2002, W203. 345 000km

Son's one: GLK class, 220CDI, 2009, W204

Sold: E-class 260E, 1988, W124, Wonderfull car sold after 489 000 kilometres of reliable services (engine M103, clutch, brake system and 5 speed manual gear box all original).

Sold: E-class, 320CDI, W210, 2000, 225 000km