When my 2005 C220d cdi gets to 80 degrees temp it idles badly , ok before that and when temp comes down again , still drives ok with no loss of power ???,
or cracked head, which would expand when hot opening the crack but cooling down would close it again, but if the oil is ok, coolant ok not loosing any, no steam/water vapour from the exhaust, then hopefully something simpler and cheaper to repair.