I have a 97 cherokee, and I am having problems with the heater. When I am in park or not pushing on the gas everything is fine. No matter what setting I put the heat on ( AC side is ok) and give it gas or when I am going up a hill my heat goes strait from the front vents to the defrost vent near the windshield. Is their a one way valve in the heater (vacuum) lines? Like I said when I am just idling it is fine, and when i am going up a hill it goes to the defrost vents no matter what setting it is on. Any suggestions would be greatly appreciated!! It's starting to snow in Montana now, so I would like to get this fixed asap!! Thanks all

 

I don't know your 97 well. I don't even thoroughly know my 90! Guess you know the flaps are controlled by vacuum. What you are describing is the vents working at higher vacuum and not at lower. (bet it works great going down hill) Could be a vacuum leak in the line or vacuum ball in front of the right front wheel/in the corner there up behind the bumper. (no not kidding!). On my 90 the line runs clear back from the manifold, across the firewall, then forward all the way down to the vacuum ball.

Or you could have "chronic" low vacuum period. An intake gasket/loose bolts. You would expect it to run poorly if that where the case though.

Any help here? Is it a single line down to the ball that can be pressure checked?

 

either theres a vacuum leak or the check valve is shot in the supply system to the vent,page 24-47 show a sample http://www.jeep4x4center.com/jeep-se....swf#toolbar=0

 

Nice file freedgr. So, come to think of it the whole system there has no metered leaks so could be checked.
Then I suppose the check valve on the manifold lets the vacuum "accumulated" (saved in the ball/canister), do work while the engine has low vacuum, ie, accelerating.

 

you got it under acceleration a engine does not creat vacuum that why the check valves in the line to hold vacuum to the ac/cruise .