Pontiac Repair: overheating, coolant level, exhaust gases

Question
1995 Grand Am SE 2.3liter 4 cylinder quad engine
We have replaced the heater core twice (only owned car for 7 months).  Now, the faster you go the hotter it gets unless you have the heater on full blast.  Coolant level stays full.  Thinking thermostat, but can't find it to replace.  The fan is working.  Have also replaced the oxygen sensor and catylatic converter.

Answer
Hi Jerinda:
The thermostat is on the back side of the engine near the firs wall ther is a metal pipe that runs under the oil pan where that pipe meats the engine is where the thermostat is. I don't necessarily think that's the cause. it's a bit fishey that you have had to replace the heater core twice. You may have a far worse problem like a head gasket strting to go bad and the excessive pressure in the cooling system is making the weakest link go bad "the heater core" I would go to a shop that has the equipment to see if there is exhaust gases in the coolant. That would be the first place I would look. I assume you had to replace the cores doe to leakage issue??? Hope that helps you out. Good luck :)