Dodge Repair: Next problem on 94 caravan 3.0, engine vacuum, vacuum lines

Question
Doug, you were right on with the previous explanations you gave me. I now have all working fine except when I accelerate with the a/c on, the vent swithes from the front to the defrost and when i take off my foot from the accelerator, it goes back to the front vent. I removed the bypass valve to the heater core to get the a/c to work and want to know where those two tiny nylon lines go to that enter through the firewall next to the txv valve. I think if I can get the vaccume to these back, the valve will close the heater core. Thanks.

Answer
The lines lead to the dash, specifically the HVAC box.  The lines are used to move the blend door and the mode door.  When the vacuum lines have vacuum in them the doors are able to move properly as the vacuum is directed to different places.  The vacuum source is your engine.  

Engine is present in the manifold when at idle and light acceleration.  When you accelerate heavily engine vacuum goes away.  There are check valves present in vacuum lines leading to components that utilize engine vacuum.  These check valves present engine vacuum from bleading off and the end components failing to do their jobs as a result.

It sounds like you've got a bad check valve because at idle you've got working mode/blend selections.  Additionally, the problems apppear only under acceleration.

Follow your vacuum lines back to the engine (intake manifold) and make sure that the one you've got a cehck valve in place and two the vacuum lines are properly secured.  Next, remove the check valves one at a time.  Try and blow or suck through the  valve in both directions.  A properly working valve will only allow airflow to move in one direction.  A valve that allows movement in both directions is bad.  Replace it.  These check valves are also used on 96-01 Rams for certain as well.  If you cannot get a check valve because of the year model of your vehicle ask the parts person to find one for a truck instead.
Doug