How quickly does the temperature reach hot? Also, how low was the coolant in the radiator? Also, the idea that the heater helps cool it leads me to think the radiator may need to be flushed. If the thermostat was the issue, the heater wouldn't help. Are the radiator fins physically clean? If not, take a garden hose and clean it off. If it isn't dirty, then I would try flushing the system. Since you will have it drained, replace the thermostat at the same time.
Here is a link that explains how to flush the system:
Additionally, here is a link that explains how in general to replace a thermostat:
Let me know if this helps or if you have other questions.
Thursday, June 30th, 2022 AT 9:17 PM