Presumably one of two things has occurred. Either you failed to glue the new thermosdtat into place before bolting the thermostat housing into place, or it wasn't the real cause of the leak. Check the engine coolant temperature sensor. More likely is a bad lower intake manifold gasket. These are a very common failure and very easy to replace. Do not forget to change the engine oil after you fix the leak and fill the radiator.
First thing you should check is the antifreeze level. If that is fine, replace the thermostat. If it still over heats after the thermostat, then most likely you need to replace the waterpump. If your antifreeze is low, check for leaks, the radiator, hoses, heater core, and water pump. If the water pump is leaking, that is your problem, replace that and thermostat all in one.
Residue from the prior leak, cracked the neck when installed, defective core. Any leaks on engine side of firewall leaking in somehow??
If your fanbelt is old, it may be slipping or loose. If so, replace it. Aaaaand, your thermostat could be stuck. Also Check to make sure that your head gasket is not leaking, the exause could be heating, and boiling your coolant.
You've got either a leaking intake gasket or a leaking head gasket. It's a serious problem which needs to be repaired.
Blown head gasket? Air trapped in the system (hasn't bled out yet)? Wrong cap?
yes, leaking coolant is what happens before the pump totally dies, then you have overheating and possible engine damage soon after the leaking.
heater core....
Yes, however, not recommended Your engine still needs coolant, hot or freezing outside.
I have a promblem with that too, my car would overheat very fast, it was a faulty NEW thermostat. It still runs hot, I think I need a new water pump
Possible cracked block or a blown head gaskets.
check your fluids it is probably low on antifreeze and if it still does it change your thermostat that should fix the problem
The thermostat might be installed backwards. The thermostat will still work, but not as efficiently as if it were installed properly. It sounds like engine is still not receiving proper water flow. This is a common mistake.