I have a bizarre issue that I can't seem to figure out on my 97 Blazer

When the truck accelerates or the engine is under load (going up hill, etc) - no matter what the control **** is set on - the blend door moves all ventilation to the defrosters.

I checked the vacuum line routing and even replaced the round canister that mounts to the hood (old one broke the inlet nipple). I even checked the vacuum lines in the interior.

What am I missing?

Mitch

 

The way I understand this is that as engine load increases manifold vacuum decreases. The vacuum accumulator in the fender helps to maintain vacuum to components such as blend doors during acceleration (load). There is also a check valve in line with the accumulator. A leak in a vacuum line and/or a missing check valve may cause the blend door to move to the default position when vacuum is lost.

 

def a vacuum issue