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1989 Dodge Shadow 2.5 non turbo 3 speed automatic
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We went on vacation for a week so the car wasn’t driven at all. Went out to Michigan and if you know anything about the roads there, potholes and 75mph speed limits where everyone drives 90mph. A 2.5 NA car with the automatic is not very happy going 65mph let alone 90mph.

So when we came back I noticed that the A/C was being weird. If you got over half throttle you could hear the face/foot baffle move from face to foot while still being on face at the controls.

I did some pulls on the highway and at about 1/4 throttle I can feel the air start to shift. At WOT it is only on foot. If you let the throttle snap closed it will move back to face.

What would you say is broken
 

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Failsafe for an HVAC system is always Defrost.
If mode doors are moving while driving/accelerating you are probably losing engine vacuum to the HVAC or from the control unit to the vacuum actuators, either through a leak/collapsed hose, faulty vacuum actuator or a faulty check valve underhood.
The main vacuum line comes off of the brake booster and runs through the firewall, that is where my line broke/cracked, where the rubber underhood hose joins the plastic lines at the firewall/underdash.

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After a physical check of all lines you can use a handheld vacuum pump and vacuum gauge to check all of the above.
 
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