Question about 1999 Dodge Ram
The air/duct controls on this are vacuum operated and you most likely have either a leaky vacuum line or a bad vacuum check valve. When the engine is running at a steady cruising speed there will be quite a bit of vacuum in the intake manifold and thus in the vacuum lines and control system. This vacuum is held there by a check valve so when you are accellerating and the intake vacuum drops you won't lose this vacuum. If there is a leak or the check valve is bad then as the vacuum drops the air doors move to the default position which is where the air is directed to the defrost vents. The vacuum line under the hood is usually black plastic and about 1/8" diameter. The line and check valve are on the right side of the intake manifold area towards the rear. I'd start my search there as that's where you'll probably find your problem. Hope this helps!
Posted on Jul 15, 2010
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Posted on Jan 02, 2017
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