Think easier before you go to harder. Thermostat before something like a control valve. Cougar is probably on the right track. Had my thermostat go out. When I got it out it wasn't just stuck but had actually come apart.
Anyway, if your thermostat stays open all the time, then the coolant runs through your engine and back to the radiator so fast that it doesn't have time to heat up. Especially in cold weather, your thermostat needs to close periodically to stop the flow of coolant and to allow the coolant that is then stuck in the engine channels to warm up. When that coolant gets hot enough, the thermostat opens allowing cooler liquid into the engine and allowing the warmer liquid to move out. The hot liquid then moves out to your heater core under the dash where your a/c system blows air across it to heat the cabin before it moves back to the radiator to be cooled back off.
With a thermostat stuck open, the cooling system is basically free flowing, and in cold weather the coolant will simply stay chilled all the time. Cold liquid in your heater core means no heat in the cabin. Sometimes you'll notice a difference if the car is moving vs. not moving. While it's moving, there is air blowing across the radiator meaning the radiator works better and cools everything completely. But, if you've the engine is warm and the car is stationary, then no wind across the radiator and less cooling. In other words, if you start to get a little heat when you're stopped at a long red light, but it goes away when you start moving again, then that's another sign of a bad thermostat.
Pull your thermostat and take a look at it. Shouldn't take you 5 minutes even if you don't really know what you're doing. If that's your problem it's a cheap fix.
Good luck and merry Christmas.
Last edited by jeremywatson; 12-24-2011 at 03:54 PM.