I have a 97 dodge van conversion, it is a B3500 360cid. Problem is that when I'm heavy on throttle engine vac drops and the heater/defrost/floor flappers move location so I get cold air. My conclusion is that it is just a poor system, however, would a leak in vac also produce this condition? Am I the only one who has experienced this issue?

 

My personal experience with a '94 Ram is that you're correct on both things, bad system and a leak. I had the same issue and it would go to defrost. I just decided to live with it, but there is a good chance there is a leak, intake manifold leaks seem to come to mind as a common problem, but it's been a while since I've thought about it.

 

Look for a one way check valve in the vacuum line that runs through the firewall to the heater controls on the dash. If you find it, take it out (note the direction for reinstallation)and blow through it both ways. It should only flow one direction, if it flows in both directions that is your problem.