I have a 1994 Camry with a 4 cylinder the car runs fine and stays cool but the heater didn't work very well so i replaces the thermostat and the car started to overheat so I installed a second thermostat and had the same problem so I just removed the thermostat and noe everything is back like it was. anyone have any ideas on what may be the problem?

Welcome to the AF!

Make sure you are installing the thermostats in the correct orientation. Align the jiggle valve of the thermostat with the upper stud.

That's good advice about the "jiggle valve" being aligned with the mark on the water inlet - also, I believe it is possilbe to install the thermostat backwards - with the wrong side facing out. Not sure I remember anymore, I just put mine in the same way it came out, but I think the imprint on the thermostat that gives the opening temperature faces out.