Hi I have a 2007 chevy Express van 4.3. The ac blows through the vents sometimes but after I start driving for a bit 10-15 mins it starts blowing through the defrost. If I stop and restart the engine it would blow through the vents and then 10-15 minutes later it repeats the same thing. Not sure if it's the HVAC controls or the vacuum line etc... Thanks in advance for your help

 

Vacuum is strong when you're idling or coasting because the throttle is shut and the engine is sucking against it. When you accelerate, the throttle opens, and the vacuum drops momentarily. To buffer the loss of vacuum, there is a reservoir with checkvalve. All these things have to work in concert for the HVAC to work properly. It sounds to me like the system is leaky and doesn't hold a vacuum.

So, you need to dive under the hood. Find the main vacuum line which comes from the engine intake.




The vacuum tube sits right over the hot engine, and is notorious for falling apart. It's a $10 part at the dealer. Then check that the one-way-valve works and that the sphere shaped reservoir doesn't leak. I just just my mouth.