Chevrolet Repair: 2001 Chevy Blazer Vacuum, flow control valve, chevy blazer

Question
I have a 01 Chevy Blazer with 105k mi. The vehicle is in top shape except this nagging issue. When climbing a steep grade or putting any amount of stress on the engine, the vent controls will shift from blowing on the floor to blowing out of the dash vents. The temp will stay the same, just a changed outlet. I'm guessing a vacuum issue, but how related to stressing the engine??

Answer
Hi Jarrod... thats a strange one...look at the vac line that comes into the heater box and see if it has a vac flow control valve in it. it limits the amount of vacum thru that line....thats the only thing i can think that would effect that...this is something i have never come across and would be intrested to know what it ends up being.. sorry i couldn't be of more help....