Over heating
2 Answers
If the system was sludged up from oil and coolant mixing, mix up some Cascade dish soap and water and run it through the system. Sounds like a flow or fan issue.
beatupchevy answered 6 years ago
on some cars the coolant fan will come on when you use the A/C, with yours it seems it'll come on when you use the heat , you should have checked to see if the fan came on when it got hot , then I would have suggested the sensor ,