Well a little while ago I posted about no heat. I took some advice and put a thermostat in. (BTW the gasket never fits right no matter which place I go to get it) I still have no heat--at first. If the car idles to warm up, the switch can be to defrost and the temp select all into "red" and cold air comes out. If I start down the hwy or to work, it starts getting hot. It doesn't really feel like it used to hot, but some heat is better than none in freezing weather. I also notice an EXTREME amount of coolant loss out of the res. I mean in less than a day the bottle would be empty. The NEVER overheats and doesn't drive or feel like a HG prob. I think theres a leak somewhere, but I'm not sure where. The bottle doesn't leak and I tightened up everything pretty well from the thermostat. Any ideas?

Shit is all apart. Typical...

there is the possibilty that the new thermostat you got isn't functioning correctly, and that is not totally out of the question. However, that doesn't explain your loss of coolant. Loss of coolant could be due to overheating or a blown HG, both of which you said shouldn't be a problem. Are you sure your HG is fine? What does your oil look like?

also replace the radiator cap, believe it or not its prone to leaking and causing similar problems.

You might want to do a rad. flush and fill

1998 dodge avenger 4 cyl AT
Mods are:
Not enough

do what mcgyvr said replace the radiator cap. i went through 3 or 4 new caps before i got one that would keep a seal. also check the coolant lines running to your heater core.

dsm/mitsubishi service manuals https://www.mediafire.com/folder/5k14qlaulb9s1/dsm_mitsubishi

service manuals(my whole collection) https://www.mediafire.com/folder/ac51jwwa3g1n1/automotive_manuals