I have to recommend making sure your gauge is reading correctly.
-Verify the engine temperature apart from the gauge-
I did all sorts of cooling sytem work on the 300E to help its not getting warm enough problem.
It turned out it was a bad connection to the temperature sending unit.
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)