Hi, new to this forum, hoping someone has heard of this issue : heat only works good when the truck is moving. When i start the truck in the morning and let it idle for a while, with the temp contol on hot, only cool air blows from the vents. When i start driving the truck hot air blows from the vents like normal. Sometimes when i am at a stoplight for longer than a minute the vent temperature starts getting cool again until the truck is moving again when the hot air returns. Any ideas ? I already replaced the "heater line control valve box.

 

welcome to R-F, B!

 

i have the same issue, when i rev the engine the heat starts but it dont go out at stoplights... that part suggest to me either you have a bubble in the lines or low coolant

 

Try a radiator flush? mine used to take forever to heat up and i found out it was becasue my rad was full of crappy antifreeze (looked like Coffee) once i replaced the Rad and flushed engine it heats up a lot quicker

 

welcome