Ford vent problem

I have a little bit of an odd problem in my 93 f-150. When I turn the vent/ac on it will blow out of the vents for a little while but if i get on the gas going uphill, or taking off, the air starts to blow out of the defrost vents. It will stay there unless I back off the gas, then it will start blowing out of the vent again. Anyone know what might cause something like this?

Check your vacuum lines to the a/c control head/door actuators. They are vacuum controlled. This is a common problem on Ford trucks around your year. Look around the firewall for a broken line or one with a hole in it, most likely it will be hard plastic. If you don't find it there, look under the passenger side of the dash... Hope this helps... Mike

yep, its a vacuum leak or a bad actuator. Had the same thing happen on late 80s GM stuff. The plastic vacuum lines get brittle and start leaking and the heat/ac goes haywire.

The reason it changes when you get on the gas is the engine produces less vacuum under load, so thats when the leaks are most noticable.

ahhhh I see! thanks guys, I guess the fun begins now...

What I suspect is happening is:

Under the hood there will be a plastic canister with a vacuum line or 2 going to it. It stores vacuum for when you are under load and engine does not produce vacuum. It stores just enough to allow the vacuum actuators to control vents etc.. Between the canister and the firewal where vac lines enter, you will find a small plastic round item right in the line. Maybe 1" in diameter. I think there are 2. What they are . are check valves. They prevent the vacuum stored from backing up. Under a load without vacuum,.most vehicles will go the defrost mode as default then back to floor and vent once vacuum returns. One of those check valves are bad. You should be able to blow through it one way and not the other. The vacuum lines I think are colored and hard plastic rather than the old rubber style.

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