This overcooling is a problem I usually see on fords. My F150 would overcool so bad in the winter I had to block 75% of the radiator with cardboard.
The cure is relatively simple. Drill a bleed hole in the thermostat. Usually 3/32 is big enough.
That will stop the thermostat surge. I bet you can watch the thermostat open on the gauge. With a small flow across the thermostat, there is no surge of cold water from the radiator into the pump when the thermostat opens. It acts to damp the swings.
Now, what year is this Dart, and what exactly is this radiator & it's source?
B.
