I have a 1994 Mazda 626 Cronos.
Last week the car started to overheat. I took it the mechanic and he found a pin hole in my rad and the resevior for the coolant was bone dry. The rad was repaired and all was well, until today. It overheated, the coolant was full, no leaks, no problems that I can see. What are the chances this is my thermostat?

A good chance, but check for proper operation of the engine cooling fan before you condemn it.

Before doing anything, especially if the rad has been worked on, make sure you don't have air bubbles trapped in the system. An easy way to check is to start the engine, when it get up to temperature SLOWLY remove the rad cap and have a look at the coolant.

If you see bubbles passing as the coolant circulates you've probably got an air lock in the system.

You can remedy this by running the engine at varying RPM's(up and down) until you don't see any more bubbles.

Probable reasons for the bubbles:
a) you have air entering the system, by a hole (hoses or rad cap) or somewhere else

b) if coolant was added at any time (rad repairs, flushing etc. or just topping up) the system wasn't properly bled of air.

You may want to also check that your cooling fan is coming on as it should or it might just be the thermostat.

Thanks for the input everybody. I got the thermostat replaced and all seems well. Great site for those of us who no nothing about cars. Thanks again.

Thanks for the input everybody. I got the thermostat replaced and all seems well. Great site for those of us who no nothing about cars. Thanks again.