I have a 99 monte the air works fine but the problem is when I accelerate the air moves from the vents in the dash to the floor vents all by its self. When I lift off the gas the air moves back to the dash vents. I think It has something to do with engine vacuum. So where do I begin to look??? THANKS!

 

I would try spraying ether around the vacuum lines and see if it raises the rpms...If it does...you found your vacuum leak!

Just be careful.... as this is not a Factory Recommendation...

I have even used a spray bottle and water....when the engine stumbles(misfires) you found the culprit...But its a much slower process..you may have to go over them 3 or 4 times for a positive results...

 

Thanks I will give it a try

 

Checking the vacuum lines is a good place to start, but it sounds to me like the vacuum tank is malfunctioning. The vacuum tank is supposed to provide vacuum for the vent flaps (which direct the flow of air/heat) during times of low engine vacuum. When you accelerate engine vacuum drops and (because the vacuum tank isn't working) the vacuum holding the vent flap drops off. The result is all of the air being directed to the floor instead of the dash vents.