heater wierdness

[dustin g]

The heater in my 98 GC Laredo will only blow thru the dash when the engine is working hard. When normally driving on flat ground the heater works like normal but if i am going uphill or passing someone the heater shifts to dash only. This is not reliant on rpms, speed or gear. Only when the engine works hard. What would cause such a thing? And what kind of headache am i gonna have to go thru to fix it? Help please i am currently on a snowboarding trip in Colorado and this is making the drives up to the mountains a little chilly.

 

[Jester32]

Sounds like a vacumn prob. Most gms are vacumn actuated blend and mode doors. When engine is working hard it will produce less vacumn. If there is a leak the doors will move when vacumn drops off.