Vacum Problem

Whenever I step down on the throttel my heater or air conditioner blows out my dash ports no matter where I have the controls set at. I have been told that this is controled by vacume. Is their any way to check this out without taking the dash apart?

not really.

There is probably a vacuum check valve that has gone bad. You could install one in the hose in the engine compartment right before the firewall and see if that fixes it.

that will just control whether or not coolant is being supplied to the heater core. it sounds like his problem has to do with the door that decides if the air goes to the floor/panel/windshield. that is usually vacuum but sometimes cable controlled. and its inside the dash.

I was refering to the vacuum line that goes to the HVAC actuators. Although it's check valve would probably be under the dash, there's no reason one could not be added to the line somewhere under the hood where it can easily be accessed.

As with any thread, year make and model would be helpful.

Depending on the car, most of the AC/heater system is controlled by vacuum pots. The valve that turns the water on and off to the core, the doors that blend air, the doors that send air to the vents... all of it can be controled by vacuum. What is happening is one of those vacuum pots has failed. Under normal high vacuum operation like at idle and cruise, there is enough vacuum to hold the pot, but when you go WOT and vacuum all but disappears, the pot can't hold itself against the spring and it relaxes. Find the pot that controlls the directional doors, take the vacuum hose off and test it. If you suck on the hose, it should actuate. Then quickly put your thumb (or tongue) over the hole and watch it. It shouldn't move. If it does, replace it.

Many of the pots can be accessed by just looking under the dash.

Whenever I step down on the throttel my heater or air conditioner blows out my dash ports no matter where I have the controls set at. I have been told that this is controled by vacume. Is their any way to check this out without taking the dash apart?


as JR stated the vacuum to the hvac door/controls there should be a check valve to trap vacuum in the hvac portion..... but it is important that you have a softball sized ball that feeds your hvac controls after this check valve and if not and you have a small leak your problem will occur...on fords they use a metal can.....if a ford this can will rust out.....