hello I have a 2006 Ford f250 with the 6.0 liter powerstroke sometimes I will have heat at idle and no heat going down the highway sometimes I will.
Today for example, started it let it warm up and had a little heat then backed out of my driveway and drove 2 blocks got a soda and then continued to work. 4 miles in i had no heat and pulled over, revved engine
to 2000 rpm's and got heat, continued to work and lost heat within a couple miles. Got to work and am sitting here with it high idled and it's burning me out its so hot.
Other times it will do the opposite of above, there are no air volume concerns besides the cold air coming out when there should be heat.
The truck has 302388 miles on it, stock head gaskets, no studs, egr has been deleted at 175,000 miles. Replaced water pump
, and radiator at 260,000 miles.
When cruising at 70 mph, with heater on, you have cold air coming out vents, you can shut the heater to off and in a few minutes turn it back on and you will have heat for a few minutes. I have checked the heater control valve and it has been replaced with a on/off valve 2 years ago, it is positioned to allow coolant