vacumn leak ?

[Marty064]

I'm sure someone must have the answer....Thinking I must have a vac leak somewhere....with heater blower running why does the damper system change from lets say "floor and face" to maybe "defrost", whenever you accelerate or put more load on engine? Do I have a bad air solenoid somewhere. as soon as you let up on gas it switches back. 99 B3500. Scratching my head.......Thanks Marty

[alloro]

You have a leaking check valve (one way valve) in your vacuum line that runs the environmental controls. Pick one up at the dealer (about $5-$10 or so), then pull the engine cover to replace it. It's on the side of the intake manifold (passenger side) about half way back on the engine. You'll find a single vacuum hose, possibly with a 90 degree elbow in the line and the check valve will be right there. Pay attention to the direction it's connected because if you put it in backwards none of the controls will work.

[Marty064]

Thanks Alloro, I'll give that a try..... Marty