Here is a stumper: 1985 300tdt w/275K. Engine has not beeen rebuilt but is tight with good compression. New water pump, radiator, thermostat & coolant. Engine runs hot at 100C during normal driving. Ran same temp before upgrades but wanted to to them anyway. Never overheats or gets hotter than 105 yet top of radiator is hot yet bottom hose is cool. Runs cool at 50-70 when thermostat is removed. New themostat from dealer. Old one has same effect. Two defective thermostats? Oh yeah, heater inlet to interior will bake a potato if you crank it on high with either thermostat in.