Loss of a/c air through front vents

This is caused by a low engine vacuum, the engine vacuum is low when you accelerate, the HVAC system loses vacuum to the mode control vacuum switch and defaults to the defrost ducts for safety. You likely have a leaking mode control vacuum switch in the control panel of the heater AC system (HVAC).

the motor that select the air to blow is the mode door actuator, some models are supply by air solenoid valve, if you want to see the location of mtor actuator, this is on the left side of evaporator case, a little bit upper of accelerator pedal

what position is the control switch in? does this occur when accelerating?

the door that directs air flow is controled by a vacuum motor. the default position for the door directs air flow to the defrost vents in the event of vacuum loss to ensure the windsheild will be clear of mist. if there is a small leak, vacuum will temporarily be removed when accellerating and the motor will revert to the default position. there are vacuum lines from the control knob to the motor you must check these lines for leaks. the diaphram in the motor may be bad. also check the main vacuum supply line to the control knob. usually red in color. if there is a vacuum storage cannister. it may be leaking. this cannister will maintain vacuum for the circuit when accellerating. engine vacuum decreases when accellerating.

Does it only change when going uphill, or under heavy acceleration? Or is it just when ever it feels like it? Generally, it's vacuum controlled and on a Ford, loss of vacuum when accelerating or pulling a grade under heavy loads can be normal, causing the air to come out of the defrost. To answer your question, the blower door is located in the dash, generally behind the radio area.

Good luck!

first check to see if you have power going to the actuators when you switch the switch to it location if no power replace controller if you have power and no movement from actuators replace actuator. if actuators move remove hvac box and check for broken blend doors.

check or loss of vacuum to mode control switch in dash ac control. these are not blend doors, they are air flow control, a blend door controls the air temperature not where it comes out.

u have a loss of vacuum to the ac mode control switch, this switch applies vacuum to diff vacuum motors to move air to diff outlets, dash, floor, defrost etc. either the vacuum line or vacuum resivior at the engine is leaking or the mode control is leaking vacuum.

If it changes to the defrost, you have a vacuum loss somewhere. It may have a mode door going bad also where the acceleration of the vehicle causes it to swing and switch positions.

It sounds like you have a vacuum problem. When the engine is running it generates a vacuum in the exhaust manifold. That vacuum is used to move the actuators inside your dash to the position you chose with your controls. If you have a vacuum leak or something interrupts the vacuum, the actuator will move to its default position which is the defrost vents. It sounds like the act of accelerating is somehow interrupting the vacuum. I'd look for little cracks in your vacuum lines and check your ERG valve.

low vacuum...bad check valve o supply line likely cause