i have a 1997 plymouth neon, SOHC engine, auto. for some reason ym car doesn't want to warm up. i have checked the fluid level and it doesn't go down like the thermostat is openig and it never gets all the way to operating temp. what could be wrong?

Optional Information:
1997 4 door 1.6

Already Tried:
letting it run for ever , checked fluid level cold and as ht as it gets