So when I am driving around town vents work fine etc. But on the highway, particularly when the engine is under load two things happen, sometimes 3. In 5th gear with it floored and going up a hill, the air comes out the defrost and it goes from cold to hot, like the heat is on, also sometimes the up shift light comes on, as soon as I get off the gas all goes back to normal. Its a 5 speed with 179 k on it.

I checked several of the vacuum hoses, the expansion bottle etc, My question is, is it possible that only when it reaches operating temp a vacuum issue appears? What would cause this? Leak on the intake, exhaust?? Or maybe I still have a hose that needs attention? Also while in 4wd the 4wd light goes off under load.

Any thoughts??

Mike