Rule the head gasket out first. That test just takes a few minutes.
Normally the thermostat keeps the coolant in the engine long enough to get it up to the right temperature, about 195 degrees. That's where parts expand to fit perfectly and the oil can carry away contaminants to the filter. While it's not common, a missing thermostat can cause overheating, especially with really big engines.
Feel across the radiator to see if all areas are warm. If there's a cold spot, some tubes could be plugged. More commonly, if the coolant got diluted with too much water over the warm summer months, the radiator could be frozen.
I really hate winter / cold / snow!
Wednesday, February 10th, 2016 AT 1:12 PM