I have a 99 Expe with a 5.4. The air has started acting up and I'm unable to figure out the problem. When cold and I start up the air works fine both front and back. However after the engine heats up the front air starts blowing warm air, the back unit still works fine. The blend door works fine, I have charged the system and the compressor is turning. Anyone have any ideas what the problem could be. Once it turns warm it stays warm regardless if idling, accelerating, or cruising.