Hey guys, I'm sure this has probably come up somewhere but couldn't find any info on it. Now my issue in my 95 chevy blazer the heater blower works fine but as you go down the road it changes from vent to say defrost. but only does it under load, do i maybe have a vacuum leak in the dash? just wondering if anybody else has had this problem. I've checked all the lines that i could see and they all look fine. Thanks any help would be great.

 

same problem 94 thought replaced my check valve at the manifold connectors didnt help . i have 18inHG at idle all systems seem to work fine till i go underload for awhile. lmk

 

Probably unplugged from the vacuum reserve tank inside of the left inner fender close to the firewall. Difficult to see where the line goes so someone may have plugged the line off without the vacuum reserve tank teed in.

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thanks for the response guys .. the vacuum is ok at the check valve i replaced that and the reserve tank on mine is a bulb attached to the hood. do those even go bad?. I'm thinking it a line in the dash, does anybody have a vacuum diagram of the vacuum lines in the dash?

 

There is a diagram...but the "short" is that all vacuum lines begin at the HVAC controls and go to each vacuum actuator inside the dash.

And yes the reserve tanks on the hoods do go bad it someone has cracked it. With the engine running for a minute or so, pull the hose off the bulb and listen for the vacuum escaping.

But also check all of the vacuum hoses under the hood. These rot-out and leak. Replacing is easy, cheap and easy to do.

 

Ok, I'll have to check that out thanks Lanny. so basically each actuator works for each selection like vent, defrost etc?