I am not familiar with the location of the overflow tank, and any provisions the system has for venting when the pressure rating of the cap is exceeded. I figure it is possible to lift that cap just on temperature alone if you see a rapid increase in temperature indication that goes near the red zone.
The indicator on the dash has a lag time and it not sensing the highest temperature in the engine. So, it is feasible that the pressure in the system is near or at the cap's relief set point.
Overall the cooling system is pretty complex. Our interactions with it, as owners and maintaining it, are pretty limited, by design. Checking the cap is good idea though as the loss of pressure will eventually lead to loss of coolant and then serious overheating. Jim
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)