| "heats up ok in warmer weather" Three things come to mind: 1. The actuators for the blend doors may be sticking or too stiff when very cold. 2. What kind of hot water control do you have to your heater core: Continous flow or flow only when heat is called for? 3. Engine thermostat not working The continous flow type depends on good control of the blend doors to shunt the heater core out of the cabin air flow when no heat is wanted. For switched hot water, the heater core is in the cabin inlet air flow at all times. Both heater hoses are connected to a diverter valve. This valve can be seen under the hood. When it is "off", hot water from the engine goes to the valve and is immediately returned to the return line thus not interupting the flow of engine coolant. When it is "on", hot fluid from the engine flows on the the heater core and then comes back through the return line. This diverter valve may be operated by a vacuum acuator or a push/pull wire. Many use a vacuum actuator. If you have a diverter valve, check it for proper operation. If the engine thermostat is opening too soon or not shutting off when cool, you'll not have hot enough fluid through the heater core, and the engine will not come up to temperature in very cold weather, although I suspect that you would have noticed this. | 
