Why car runs hot even when thermostat missing?

Could be one of several things. From Least expensive to most expensive 1. Low coolant level 2.Bad Radiator cap. 3. bad water pump 4. Clogged heater core.5. Clogged radiator 6. Blown head gasket. If you check your oil when the engine is cold and see a milky substance in the oil you have a blown head gasket or cracked block. Otherwise start with the least expensive and troubleshoot your way up. Good luck.