**Vehicle:** 2019 Lincoln MKZ AWD

**Issue:** The engine temperature rises when the car is idling for more than 10–15 minutes with the AC on. However, the temperature stays normal while driving at higher speeds.
**Work already done:**
- Replaced the thermostat
- Replaced the water pump
- New battery installed
- Radiator fans are working
- No coolant leaks
- Tried bleeding the cooling system (air removal)

Despite all this, the problem still happens at idle. The coolant reservoir was left open temporarily during bleeding. Could this have caused air to enter again? Also, could a partially clogged radiator or bad head gasket be the reason?

Any advice or similar experiences would be appreciated!