Vents change to floor when going uphill

[Skandocious]

So when I'm running air through the vents in my truck, the truck changes the air output to the floor when I'm going up hill (or whenever the engine gets loaded). I've been told that this is caused by a vacuum leak but I have yet to find one. Anybody know exactly what causes this and where to look?

 

[blacksnapon]

Look at the "tin can" vaccum reservoir. Then you can plug one of the vacuum lines at the hvac controller, using a vacuum pump, you can check integrity of lines.

 

[Brian_B]

I would say it is a vacuum leak. On the explorers it is typically a "blend door" problem somewhere behind or near the glove box. I have never repaired one myself, so I don't even know what to look for.

I have also seen vacuum lines come off under the dash. My ranger did behind the AC controls and my brothers old T-bird got kicked off under the passenger side dash. Those were different issues, but they were both vacuum leaks.

 

[Skandocious]

Forgot to mention... When it changes between A/C and floor on its own, you can hear the vacuum motor THUNK into the other position. A couple times my brother has been able to reach under the dash near the passenger side and push the vacuum motor arm manually to get it back into the correct position. So it sounds like the vacuum line is ATTACHED because it IS actuating the motor, but when the engine loads up I guess it loses vacuum.

 

[bowtiehatr]

anytime the engine is under a load or the throttle plate opens the engine loses vacuum. the more closed the throttle plate is, the more vacuum you have.

 

[Skandocious]

Yeah that's kind of what I figured. The vacuum system in these trucks is one of the things I still have not learned.

So it seems pretty definite that this is a vacuum problem. Anybody have an idea of where to start looking?

 

[bowtiehatr]

post #2 or 3 has some ideas

 

[econo61]

little plastic thing about as big around as a quarter, and about 3/8 th inch thick, in the line between the manifold and vac. can is a one way valve. it is made to stop a backfire from blowing out the whole vac. system under the dash. if it goes bad you will have the problem of loosing vac. on hills and under load.

pull it out and blow thru it both ways. if you can blow both ways you have found the problem! you should only be able to blow thru it one way.