I have a 1997 mercury villager van it started blowing cold air instead of warm out of the vents I replaced the thermostat and still having the same problem the heat only works on and off and now the van overheats, i also noticed that one of the radiator hoses is extremely hot and the other is cold even while overheating this is getting to be a real problem when the van is first started it runs great until it overheats then it stutters i imagine that is some type of override system , there is no signs of white smoke coming out of the exhaust nor any liquid so what now?

Not in my 99 it doesn't, the bigger end faces away from engine the tip where it opens when water is hot faces in to engine.

Okay it sounds like you need to replace the Thermostat. It isn't opening to let the coolant flow through the Radiator to cool down. Thats why the other hose is cold. Replace the thermostat and it will be fine. Make sure you replace it the same way as the old one. There is a spring on the thermostat that when hot coolant hits it it expands and it will open to cool the motor down then it will gradually close again and then the cycle keeps going. Thats the problem your having. It will over heat. i would flush the system first when you have the thermostat out stick the garden hose in thermostat hole and let it flush out. But only do this on a cold engine.

You have fitted the thremostat in the wrong way around ,the body of the stat goes in towards the engine and thats why you have a hot hose and a cold one the water isnt circulating,when you get it sorted then if still cold heater put some cardboard in front of the radiator to stop the air flow then it work right