Overheating

Hello,

Did you have this overheating issue before replacing the heater core?

It is possible that there is restriction in the line and preventing the coolant from circulating. Which is causing the engine to overheat and blow out coolant.

Besides that, the engine can overheat due to a stuck closed thermostat, bad water pump, faulty radiator, inoperative cooling fan.

https://www.2carpros.com/articles/engine-overheating-or-running-hot

If the thermostat fails at closed position, it will also prevent coolant from circulating and will overheat the engine.
A stuck closed thermostat will have significantly different temperature for the upper and lower radiator hose. One will be noticeably cooler than the other.

https://www.2carpros.com/articles/how-an-engine-thermostat-works

Please let me know if you have any questions.
Thank you.