Hi guys,

(2000 xj, 4.0, auto)
Yes I know, this has been covered 9000 times already. I've seen all the diagrams, both engine bay and air box/HVAC. I followed the vac line from the intake all the way to the vacuum bulb in the bumper and didn't find any leaks. So I took some of the dash apart and watched the blend door (or defrost door?) behind the radio move when changing the mode door selector.

So say I turn on the AC and set it to the front vents, it will only come out the defroster. BUT after 30 seconds or so, it will start to come out the front vents and progress from defroster only, to some defroster some front, to full front vents. This all happens over a minute or two, sometimes longer. I can literally watch the door actuator start to subtly move, then move a little quicker, until it finally rotates fully. Is this looking like a vacuum leak still? Or just the door hinge binding maybe?

Any input would be great!

 

You might look real closely at the selector unit behind the dial. Mine was cracked/leaking and had to be replaced.

 

Yeah I might have to pull the selector off again and inspect it more closely.

On a recent 1,100 mile trip I did, it was giving me more symptoms. If I was stopped, or going slow, it would switch to the front vents pretty easily. However, if I was going 65+ mph it wouldn't switch off of defroster. Even when the front vents were working, sometimes if I went over 65-70 it would actually revert back to defroster only.

 

Defrost is the default setting for a no vacuum situation. You're loosing vacuum control when engine vacuum is reduced by higher throttle settings. Surely a leak in the control console.

 

Replacing the two vacuum check valves on the vacuum lines fixed the air switching to defrost under load for me.