My heater vents have been acting up. I have read that there may be a vacuum leak that causes the blend doors to default to defrost...

Does anyone have experience as to where the vacuum line fails? I have ESOF 4x4 and that is still working. I think there may be a correlation between engine vacuum and my vents: at cruising or idle the vents work ok (not perfect) and under acceleration (less vacuum) the dash vents stop blowing. I thought that the blend doors had their own vacuum pump, so I do not know why things change at different amount of throttle?

Thank for any help in advance-

 

Diesels have their own vacuum pump, gassers use intake vacuum which, of course, drops when the throttle is opened.

Follow the vacuum lines and cap them off one at a time. The PVH solenoid is a common leaker as is the tubing in the vicinity of the battery. A leaky PVH can still operate the hubs but cause airflow outlet problems. My 01 did that to me and it thew me off the path for a bit since I assumed that if the solenoid was leaking, ESOF wouldn't work. Mine was "slightly" leaking for awhile and ESOF worked but I had the typical problems with loss of vacuum to the climate controls.