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Q: air vents on 1998 Dodge Ram Van 1500

when the truck is under a load the air will only come out of the defroster as soon as i let off the gas the air will go back to the dash vents
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If you are not towing or carrying a heavy load or going up a really long steep hill, you have a vacuum leak, or a bad engine. I know the range sounds unbelievable, but that is the quick and dirty answer.

The vent and blend doors that make the HVAC system operate in your vehicle are operated by vacuum. This vacuum is produced by the engine, in the intake manifold. If you have weak vacuum in the intake, you probably have some underlying problem with the engine (burnt valves, bad rings, manifold leak etc.)

However, most likely, the issue is simply a cracked, leaky or disconnected vacuum hose somewhere in your engine compartment, or under the dash.

Start the engine, and listen for a whistling or hissing noise. This should allow you to narrow it down to a general area. Then have a partner shut off the engine, while you listen for the leak. It will not last long after you shut off the engine, but you should be able to pin point it further without the engine sound for a second or two.
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