My 97 Cherokee seems to have a vacuum leak somewhere behind the dash. When I start the engine, my dash vents work for about 2-3 seconds then they quit working(hear the doors slam shut). The fan keeps blowing but nothing comes out. When I switch the setting from defrost to anything else you can here the doors moving around but still no air. Before this, it worked fine until i was under a hard throttle, then anytime I accelerated and now I have nothing.. Anyone know where a common vacuum leak is behind the dash. I have already checked the engine bay and the vacuum line going through the fire wall..
Thanks for any help..

Is there a way to pressurize the system to hear for leaks?

 

I just went through a similar ordeal with my wife's Blazer. Vacuum lines at the reservior were rotten. Check at the reservior inside the right corner of the front bumper.

 

i have noticed i have the same issue when accelerating the fan stops blowing. i really think it has to do with the relays behind the glove box, i have 94. I bought the two relays, just need to get back there to swap them out to see if that fixes the issue. other than that, i can only guess its the res as stated above.

 

I found the res. and there is no hose going to it

I looked under the hood and found a bolt to plug where a vacuum line is supposed to be.

 

I have a buddy who was having this problem just last week. He said the piece of metal that goes from the actuator to the door itself was loose, so the door just shut back after it opened.

 

Do you know where in the dash the actuator is for the defrost?

 

Looks like I will be taking my dash apart this weekend..

 

Under the glove box. Sorry I been busy today.

 

Default setting is defrost sounds like vacuum hooked up backwards ,
.

 

Isn't that the one for recirculating the air?

 

With no vacuum being applied vent should be in defrost mode it sounds like vacuums being applied to wrong valves behind dash is the vacuum ball hooked up behind front bumper? Should be hooked up thru the fitting with the bolt in it ?

 

This may be a good time to examine that little vacuum check valve at the intake manifold. It may not be the main problem, but that little valve gets ignored a lot and can cause control problems under load. It's designed to keep vacuum stored in the reservoir so that the various doors/actuators hold their positions during open throttle operation (like climbing a hill with a trailer).

 

First reconnect the vacuum line from that tee fitting (bolt) to the vacuum reservoir then try it before nuking your dash.

For your reference, here's a pic of three of the vacuum valves on the HVAC unit. The Recirculation Door Actuator is not shown (on cabin side of the HVAC unit). The Blend-Air Door electric motor is on the bottom of the HVAC unit.

 

No the ball is not hooked up yet. I'll have to get that done.

 

Thank you all for your help, I picked up a V line tonight and will install it in the morning. it the problem still occurs then I will go from there.