VT Calais vents change when accelerating

[LordMayhem]

have an issue with a 97 model VT Calais V6 if you have the A/C or vents on the dash and or floor on when you accelerate it changes to the dimister vents then returns to the dash/floor vents when at a steady speed or completely off the gas. any idea's what it could be and how to fix it?

 

[Tree cutter]

The check valve on the vacuum hose at the back of your inlet manifold might not be working or you could have a slight vacuum leak somewhere in the hose going to the heater controls. When there is no vacuum to the heater controls the air goes to demisters. Your engine has more vacuum at idle and when you open the throttle the vacuum drops, but there should be a vacuum tank under the dash near the heater box that should hold vacuum when you open the throttle because the check valve at the back of your inlet manifold should stop vacuum leaking back to the engine.

 

[LordMayhem]

thanks mate i'll check into it

 

[aussieakito]

I have the same issue let me know if you fix it

 

[commodorefanatic]

I did have the same problem, there is a vacuum line which controls the heater vents. There is a little valve that is plugged in between the inlet manifold and the firewall and for some unknown reason mine had no valve. Check to see if there is the valve or not, if not you can get them at the wreckers for 2 bucks. It may also be the hose as over time the hoses do crack with expanding due to heat, cut a small amount of the end of the hose at the inlet manifold end and it should seal better. Good luck!

 

[aussieakito]

I have 3 hoses off the back manifold but the one that has a doohickey on it isn't the one that meant to give vacuum to the vent controls. It goes in the opposite direction.

 

[aussieakito]

Had a look at everything it's all good there.
Tank behind the glovebox is fine. No obvious leaks anywhere. Could be the switch itself