vents/defrost door problem
When I turn on A/C, air comes out both vents and defrost. If I cruise on highway at a steady speed, sometimes air comes all out the vents but as soon as I step on the gas, it goes back to defrost. I know it's a vacuum problem. I know it has a check valve right by vacuum connection by rear of the 4.0 liter engine valve cover. I thought this check valve should prevent this problem. I then went under dash and see the vacuum valve that controls this door for vents/defrost. It appears that there is not enough vacuum to pull the plunger all the way in to get all vents. I removed the vacuum line and hooked up my hand held vacuum pump and plunger moves all the way in with no problem so I'm sure the vacuum plunger is working correctly. I checked as much of the vacuum lines as possible and I see no breaks. It's the green vacuum hose that controls this vacuum plunger. Why would I have low vacuum on this line? I swapped out another heater control head from another same year Jeep and it did the same thing.