Q: Changed my headgasket and radiator on my 98 subaru legacy outback and my car starts overheating on long trips until heat is on 4

The most common causes of overheating include low coolant level, a faulty thermostat, a plugged radiator, a faulty radiator pressure cap, collapsed hoses, non functioning cooling fans, and a faulty water pump or drive belt. Certain engine faults, such as a leaking head gasket or poor engine running condition can also cause overheating. Unfortunately, even though you replaced the head gasket, a fault there cannot be ruled out, unless of course the overheating is found to be due to the other causes I listed. If the thermostat was not replaced when you changed the radiator, that is certainly a possibility especially if the thermostat was previously run in an overheating engine (overheating is known to damage thermostats). If you desire that this problem be diagnosed by a certified Mechanic, dispatched by YourMechanic right to your location, please request an engine overheating diagnostic and the responding certified mechanic will get this resolved for you. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.