I have a 1991  loyale  spfi .  when I accelerate  the vents go to center . and my acceleration is sluggish .  I used a vacuum tester and found no leaks in the duct system  lines ,  I tested the egr , and even the manifold lines .  im wondering if the leak is only under load ,  perhaps its the manifold gaskets ,  but I get no change  in rpm with  carb cleaner .  I cant seem to find it

Edited October 1, 201510 yr by ivantruckman

There is supposed to be a check valve and storage tank in the feed between the manifold and the heater controls. If the check valve is missing or stuck open, that's what you get. Or if the tank is moderately leaky.