Mazda is a Japanese car company that has traditionally struggled in US markets. The 626 was first released in 1979 with the goal of capitalizing on Honda and Toyota's market share with the Accord and the Celica, respectively. The 626 never achieved the reputation for reliability that its two main competitors did, and sales suffered as a result. The 626 could have mechanical problems that included engine overheating. If your Mazda 626 starts to overheat, there could be several different causes.
The 626's radiator plays the most vital role in keeping your engine at a safe temperature. It dissipates heat from the engine coolant. Radiator fluid can run low over time, either from to evaporation or from a leak somewhere in the system. If your coolant level is low, the engine will overheat. The radiator and overflow reservoir need to be filled and any leaks need to be repaired. In addition, if the cooling system has not been regularly flushed and serviced, or has been filled with water at some point, rust and mineral deposits can build up in the tiny channels within, gradually clogging the radiator and decreasing it's efficiency. In this case, the radiator will need to be flushed, re-cored or replaced.
The head gasket is the seal between the engine block and cylinder head. It not only maintains the pressure of each cylinder's combustion, it also maintains pressure for the engine's oil and coolant systems as they circulate through the head. If the gasket is bad, the 626 can easily overheat. Conversely, an overheating engine can also cause the head gasket to go bad. Replacing it is very expensive due to the labor involved since, the entire top of the engine has to be disassembled in order to reach it.
The thermostat is a valve that opens and closes to govern coolant flow, thereby maintaining the engine at it's optimum operating temperature, usually around 190 degrees Fahrenheit. If the thermostat isn't working, it can stay closed and prevent coolant from entering the engine and this causes the 626 to overheat. Replace the thermostat.