Why does my AC start blowing out of my defrost vents when driving long distances?

Question

when i am driving with the pedal being pushed constantly for a period of time, the AC stops blowing from its normal vents, and starts blowing out of my defrost vents. when i let off the pedal, the air begins to blow out of the normal vents again. ive replaced both check valves, and still no good! whats the deal, b/c its driving me insane!!! thanks- jacob

Answer 1

Sounds like a vacuum issue to me as well. Check for leaks in the smaller black hoses under your hood. If they are old and cracked replace them all. You can usually hear a vacuum leak. Sounds like a hissing.

Answer 2

Mustang and Mommy are right. There is a leak in one of your vacuum lines. The pressure (vacuum) changes with the changes in your acceleration. When there is a change, the little doors that control which channels the heater blows though open or close.

To detect this, try listening to your dashboard while you are sitting still and idling. If you hear a whistling or sucking sound there is a leak in your vacuum control system. This happens in just about every car after a number of years because the rubber hoses succomb to dry-rot. They get hard with age and they can't maintain the seals (generally at the connection points). Then they leak. Imagine sucking on a straw. Then imagine there being a hole in the straw. Your ability to draw the liquid up the straw will be compromised in the same way as your car's ability to move a little door.

Fixing this generally involves opening up your dashboard and that is a royal pain. Try to live with it until something else goes wrong and you can justify going into your dash.Then change all of the hoses at the same time. The hoses will cost about $10. Openning the dash takes a couple of hours if you do it yourself or a few hundred dollars if a repairman does it.

Answer 3

Your vent controls are vacuum actuated. At idle, the engine creates more "manifold vacuum" than under throttle. The system is loosing vacuum while you're driving and the vent controls are returning to preset (no vacuum) settings until you take your foot off the accelerator and draw a vacuum back into the system. You probably have a bad vacuum line that is sucking air and allowing the system to loose vacuum. Shut the engine off and listen for a sucking noise. That's where you're loosing vacuum. Added: That may also cause a funny idle too.(misfire at idle)

Answer 4

if you have the air set to circulate in the car with the AC on or it set to "AC MAX" then dont do that for more than 7 minutes at a time.

my dealship told me this after our van did the same thing....it even froze up and shot ice out the vents at times for our long trips down to NC from NJ....(loads of fun for the kids though lol)

also have the dealership check the car out to make sure no pressure valves are getting stuck.

Answer 5

theres something wrong with the vaccumm system...

my husband had a similar problemw ith his truck..it actually only blew out of the defrost vents...he replaced his vacumm lines and all was better.

Answer 6

WERE R THE VACUMM LINES