Question about 1997 Ford Expedition

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1997 expedition vent chages air flow from panel to defrost when under load

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Suspect a problem with a check valve in the vacuum lines somewhere. When you ask for "power", the manifold vacuum dives. This is affecting your ventilation. It is not supposed to happen as you know.

Many ventilation controls use engine intake manifold vacuum as power to move miniature air valve actualtors. Actuators may be balanced by springs. Springs pull one way, vacuum (controlled by the dash knobs) pulls against springs. When the throttle is opened (accelerator pushed hard) the manifold vacuum drops.

When the vacuum drops, then the spring would be strongest and pull the control valve over. When the throttle goes back to normal--- vacuum is back up, and air flow changes again.

There should be a check valve in (one-way valve) in the system so that the vacuum is held during periods of acceleration. If this valve is stuck open by wear or contamination, everytime you ask for power you would lose the vacuum in the controls.

A full diagram of the system is needed to trace this which I do not have.

This should give you a place to start or at least know what you are hearing if you have someone troubleshoot this.

jp

Posted on Sep 07, 2010

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