Air pocket removal from cooling system

Maybe it is overheating? An air pocket can form in one of two ways. Refilling the cooling system after a drain and fill or a flush can leave an air pocket under the thermostat. A leaking cylinder head gasket can allow combustion gases to get into the cooling system and pool under the thermostat. That air will cause the thermostat to not open. Thermostats have to be hit with hot liquid. Hot air will not do it.

A lot of thermostats in import engines have a small bleed hole to let that air get out and go to the reservoir. Most V-6 and V-8 engines self-bleed by themselves. On many smaller engines, that has to be done manually. Look for a bleeder screw on the thermostat housing. If there is not one, look for a temperature sensor or a threaded plug on or near the thermostat housing that can be removed while you fill the system. If you have to unplug a sensor to remove it, do that with the ignition switch off to avoid setting a diagnostic fault code that could confuse someone later.