Heater stops blowing heat when idle

[motc777]

I have a 2004 stock Mountaineer. I noticed the other day that it blows heat fine after the engine is warmed up. When we come to a stop light, or are idling, then the temperature both in the front and rear steadily drops to where it's blowing cold.

Is this a heater core problem?

 

[Tekreck]

I am having the same issue. I have been flushing my core with water, which typically restores great quick heat for about 2 weeks or so, then I have to do it again. However, this last time it was not as effective. I am going to take the vacuum-operated heater value (it's what the other end of the heater core hoses attach to) off tonight or tomorrow and examine. I'm beginning to suspect it is sticking and slow to open. MY blend door seems to be working fine (when I have heat, blend door switches from hot to cold with not problem).

 

[motc777]

I am having the same issue. I have been flushing my core with water, which typically restores great quick heat for about 2 weeks or so, then I have to do it again. However, this last time it was not as effective. I am going to take the vacuum-operated heater value (it's what the other end of the heater core hoses attach to) off tonight or tomorrow and examine. I'm beginning to suspect it is sticking and slow to open. MY blend door seems to be working fine (when I have heat, blend door switches from hot to cold with not problem).

same thing here. Changes from hot to cold fine on the thermostat, but when the car comes to a stop and idles, the temp drops to just blowing air, and in the winter, no hot air is going to be cold air.

 

[Tekreck]

Thermostat fixed mine

While working through the heat issue, my thermostat housing cracked and spoiled my day. Had to get replaced at dealership along with new thermostat. Everything then was fine. Suspect old thermostat was not opening far enough to allow for proper flow pressure of water at idle speeds.

Summary - no heat at idle - 1) fill coolant, 2) look for leaks (even small) in system that could serve to lower water pressure at idle, 3) replace thermostat (could not be opening far enough for proper water pressure), 4) check heater control valve visually for proper (and full stroke) operation of mechanical arm (replace if suspect - only $20 - partial opening will lower water pressure to heater core).

Note - I have seen some ask about how to determine which heater core hose is inlet, then (aside from knowing the outlet goes to the water pump) look for directional arrow on heater control valve.

Keep on it.

 

[fifthranger]

Had that same problem with the wife's Ford Edge. We replaced the thermostat, and the heat was only temporarily working correctly. The problem was with the heater core.