Just bought a 93 Protege with 150k on it. When i crank up the heat it stays cold and then the moment it starts to get warm, the car begins to overheat. After a little while, the gauge will drop back to normal and then it will run fine. I figured it would be the thermostat so I put in a new one for 6 bucks, but it still does it. What is causing that?

 

Try backflushing the entire cooling system or have a shop do it. Also, don't forget the radiator cap. Also make sure the fan is working and that the AC condensor and radiator fins are not bent of obstructed.
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also, you could disconnect the hoses and run a water hose into the bottom hole and turn it on. if anything comes out of the radiatior other than water and coolant, the radiator could be clogged.

 

There is two additional possible issues with the cooling problem. There are two temperature sensors on this vehicle. One is located in the housing of the thermostat and the other is ontop of the radiator. Either one that fails will create the same symptoms.