I have a 2011 Ford F-150 and the heater will not blow warm air until I am on the highway even though the engine is at operating temperature. as soon as I come off the highway and operate the vehicle at lower speeds or idle it blows cold air. I do not see much movement of the engine temperature gauge during this process. What could be causing this problem.

 

Possible vacuum leak? I have seen it on other cars and trucks but not sure if the F150 uses vacuum motors or electric for that part of the heater system.

 

Are you low on coolant? Have you had any work done on your coolant system? Are your fans running more than usual?

I would check your coolant level first.

Something similar happened to my truck after I changed the coolant. Turns out I didn't completely purge the system and there was an air bubble stuck in the heater core and I was low on coolant. A strange symptom I noticed was a lack of heat when doing down a hill, but a burst of heat when going up a hill. I imagine this was the air bubble moving into and out of the heater core as the truck tilted.