Air flow in cabin

When the engine is under load, like accelerating or going up an incline, the airflow on my 2004 Mercury Grand Marquis will switch from dash vents to the defrost or floor vents. How do I fix this problem?

You don’t fix it, because it’s not a problem. It is the WOT (wide open throttle) circuit engaging, which cuts off the A/C so you’ll have more power to go up the hill, or around the guy you’re passing. Read up on WOT and it’ll explain it all.

The only fix would be to add a dedicated vacuum pump. It is normal as the available vacuum that controls those vents goes don’t to near zero under WOT and the climate control system goes into default mode.

Unless you are infering what benny said about A/C, which I didn’t read, I’d say you have a faulty vacuum check valve, which (unless near impossible to get to) should be cheap to replace.

When you are not under load, your engine runs under high vacuum. As you accelerate, more air enters the engine making for less vacuum. So if the valve is not sealing the “more vacuum” side of your HVAC controls from the “less vacuum” side of the engine manifold, they can move. Replace it, and they will stay put.

WOT only turns off the compressor, it does not switch the airflow.

This is not true! There is a check valve and a vacuum reservoir that keeps enough vacuum on the climate control for it to work correctly. I have had several Grand Marquis, and they do not function this way. This one has a fault.

You either have a faulty vacuum check valve, or you have a vacuum leak in the AC system. Either one is pretty cheap to fix.

This does sound like a probable problem. The AC does not cut off; only the air flow changes and it changes even if the air control is on “vent”. I will check into replacing the vacuum check valve.