Q: air vents on 1998 Dodge Ram Van 1500

Rookie cbe0621eac06868b3efe0d8d1d3611e23c60d3114864ea2ec19a68cfbd3eebab
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
(3) Answers
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.

I am having the same problem with my '97 Ram 1500.
Were you able to solve your problem and if so, how did you do it? Let me know. Thanks
Mine did the same thing. There are two maybe three small plastic vacuum valves in the vacuum lines that need to be replaced. They beging to leak by these valves and cause the flap in the vent to default to the dash or defrost. To check the valve pull them off of the vacuum hose, suck on the valve one way,(be sure to wipe them off first), and then try it the other way. You should be able to suck air one way but it should not let you suck air the other way. If it does it is bad. In my opinion they are so cheap that if you are having that problem with one of them then replace them all because you will have to do it again if the other one goes bad.