I have a 1999 F-150 V8 5.4L. The temperature control on my A/C has stopped working. If I'm idling, the engine is cold, or running at a steady state, it will blow full cold air. Once the engine has warmed up, when there is a load it will blow full hot air. When I accelerate, I can hear something in the dash click, kinda of like a door or flap is closing. I think it might be a vacuum leak, but I don't have the first idea of where to start troubleshooting it.

 

Sounds like a broken blend door.

It flops either open or closed when you start or stop the truck.

Replacement requires removal of the dash - not for the faint of heart.

Steve