heater not working at idle
 

When I idle, I notice my heater stops blowing hot air
I have te small grey tube plugged In too the box in the valve cover. What do I look for?

Thermostate might be stuck open as the radiator will cool down the system very rapidly at an idle if thermostate is wide open.

As you have gauges, it should be quite apparent that your coolent temp is low if in fact your 'stat is open all the time. Does it take a long time to come up to operating temp?

No. It reaches operating temp in a normal fashion. Hmmm

At idle with the truck fully warmed up, are the heater hoses as warm as the upper radiator hose?

Also, does your truck have the vacuum controlled flow valve on the heater hose line, and if so, is it closing when you turn the heat to OFF, and opening when you turn it to any other setting?

Ill check on the hoses after work. What does this vacuum controlled Flo valve loom like? Ill check that also after work. I did my injectors in may. Last winter... my truck would blow hot air at idle. So I'm assuming its something I did. The only thing I mess with is I unplugged the AC compressor. And I noticed that little grey tube wasn't hooked up, so I hooked it up to the little black box above the valve cover.

I think all trucks after 2004 have the vacuum controlled valve. I don't have a picture or diagram of it. If your truck has it, the grey hose goes there. It will be pretty obvious, though. It'll be a valve on the heater hose with a nipple for a vacuum line.

I don't think the valve is your problem. The symptoms don't fit. But it's worth checking. Vacuum should close the valve when the heater control is in OFF and MAX A/C. In all other positions, there should be no vacuum on the grey hose and the valve's spring should open it.

sometimes when there is little to no heat you may have a bad rad cap or a cracked degass bottle the system needs to be under a little pressure 15 lbs does it get warmer at high speed?

Maybe the heater core is clogged enough to prevent heat at idle? I have read several posts about this problem. No heat at idle but heat returns when rev'd up some....

Post about heater core flush....

https://www.ford-trucks.com/forums/8...-too-well.html

heater not working at idle
 

Working on my wife's 2003 Excursion with a 6.0L Power Stroke Diesel
No heat, replaced the heater core,water pump, thermostat we have heat driving down the road but not at idle,Checked the blend door and it opens and closes ,the door is not cracked and the motor appears to be operating normally
The heater core is hot above an idle is so hot you can't touch it, but at a idle its cold. any ideas out there ,Thanks

Having same problem with an '04. Customer states it worked until HE installed new radiator.

Is the belt slipping?
Check the pressure in the degas bottle

Another heater not working at idle problem..........These are almost as common as EGR delete and tuner threads now--LOL.

Is the coolant level sufficiently high in the degas bottle and is the truck up to normal operating temperature?

Yup and Yup. Coolant level fine, water pump working, housing fine, heater core clear, new thermostat. What gives?

My typical response would be that the waterpump is not providing enough flow at idle which may be an impeller problem or a housing problem. I just have a problem believing that there are that many pump/housing problems. How's the ECT/EOT delta?