I have read numerous posts on this but I feel that I have a different problem. Here goes. When I first crank the truck there is obviously cold air, I can wait forever and let the truck warm up and the air stays cold. Once the truck is warmed up and I finally take off, I hear a loud thump and there is now hot air. That air will stay hot as long as the truck runs. You cant cool it off. Now, if you turn the truck off and crank it 5 or so minutes later you once again have cold air, once the truck is moving you once again have hot air. I pulled the temp/setting controls out and metered it and checked for resistance and what not. I also turned the temp **** while listening to the blend door actuator and you can hear it moving. Does anyone think they know what this is? Any and all help is greatly appreciated.

1999 f-150

 

Vacuum leak. There is a vacuum line that runs through the firewall near the battery to the heater controls. Good luck!

 

the blend door should be electric control. it is variable from cold to hot to get the exact temp you want. the thump could be the door sticking or the motor bearings sticking, etc.--- there are 2-3 other doors that are vacuum controlled to direct air to the floor, defrost, inside air supply, outside air supply, etc. if the vacuum hose fails, the air will come out the defroster, but temp will not change.
you have a problem withe the blend door or motor. pull the plastic panel off the transmission hump under the dash. its in that area. you cant get it out, but you can see whats going on. what year?

 

read "help me" post #12.

https://www.ford-trucks.com/forums/6...irections.html

 

1999 F-150, I hear you but why does it wait til I move the vehicle. I have read that post and others but it just doesnt add up. This is a big job if done right. And I say a job worth doing is worth doing right. So I should probably just go ahead and order the blend door and actuator and replace them both with a full day planned?

 

well, first check out the basics, startup the truck and wait afew minutes or drive it around the block. now check the radiator hose, is it hot? is the dash gauge at normal? are the heater hoses hot? if everything is hot and working and the truck is putting out cold air, it is the blend door. if the temp gauge is cold or radiator hoses are cold, then you need to look at the thermostat or antifreeze level.