wife has a 06 explorer 4.0 it has front and rear a/c and heat. Problem is in real cold weather heater will not put out much heat at idle. It can warm up for 30 to 45 mins and still barly warm air. Engine is getting up to temp. all blend doors are working right valve in heater hose is working. Will not put out much heat till you start driving it or sit with the rpms up. when driving it has great heat stop and let it idle for a few mins and it cools back down until you start moving again. Also have replaced water pump flushed heater core and new radiator. I have ran out of ideas to fix it and don't want to get rid of it. If anyone has had this problem or has any idea whats wrong please let me know. Thanks

 

since it works on the road and problem is at idle / low speed, you would normally thing the water pump is wearing out, or the heater core is partly plugged. Since you have replace the pump, you can assume that is not the problem. Check the temp gauge and make sure it does not drop when setting idle. I might even be tempted to change out the thermostat as the only thing left....... If the gauge is stable, then I would say your heater core is partly plugged even tho you have flushed it out....... It is about 10 years old..... why was the radiator changed out ?

 

Ive changed the thermostat twice once the whole housing. Temp does not change from idle to running. Radiator was changed before I got it not sure if it was leaking or another problem. I also thinking heater core.

 

well, we have missed something. If you have 190 degree water going thru the heater core, and core is not plugged, and the fan is on, and blend door works, then you would have heat.