Your mode doors are operated by vacuum motors that run on vacuum from the engine. Most of those systems are spring-loaded to go to defrost mode when there's a failure. That's for safety. Your feet might freeze, but your windshield will be clear.
When you accelerate or when the engine is under load, the vacuum it generates goes down, often to the point that it's too low to overcome the springs on those mode door actuators. When it never did that since the car was new, it's usually due to a tired engine with high mileage. Even when it happens on newer cars, there's a fix for it. Since it is normal for engine vacuum to go down at times, most manufacturers put a small vacuum storage canister inline with that vacuum hose to prevent the mode doors from changing position for five to ten seconds. Some manufacturers just use a check valve inline with that hose.
First look for the check valve. You should be able to blow through it one way but not the other way. Replace it if you can blow both ways through it. If the valve is okay, you can add the storage canister if there isn't one in there already. If the Ford dealer doesn't have one, visit a Chrysler dealership. When they just use a check valve, it's about the size of a stack of three nickels. You can remove and discard that and install the storage canister in its place. It is about two inches long and about an inch and a quarter in diameter.
The hose is usually pretty small diameter and made of hard plastic. Most of the time it will be shown on the vacuum hose routing diagram on the sticker under the hood.
Wednesday, June 18th, 2014 AT 12:20 AM