2001 Blazer 2WD. While driving and heater/ac are off, air comes through the vents. What causes this, how can I fix it, and does it affect anything other than me ?

 

I think that's normal. Mine does it if the system is set to outside air. If I push the recycle inside air button it stops. Perhaps switching to the floor vents would bug you less.

 

I don't have a system that lets me choose outside air or recycle air. I have had this Blazer for 12 years and it just started doing this.

 

you might have to replace the controls they may be leaking pull them out and check all the hoses that are connected there or your vent flap (the one that shuts the outside airflow off) could be jammed or broken i believe that one online autoparts site sells new ones

 

Should be coming out the defrost not the vents. Factory description is below. Automatic air is different.

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Off Mode
The blower motor and A/C compressor are turned off. When the vehicle is moving, air flowing over the vehicle increases the air pressure just ahead of the windshield. This forces air into the outside air inlet, into the HVAC assembly and out through the defrost outlets. Since the A/C compressor is not running, the incoming air may be warmed but not cooled.

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Defrost Mode When Defrost is selected, air is delivered to the windshield outlets. Vacuum is applied to the mode actuator through the Blue vacuum line, and to the defrost actuator through the Yellow vacuum line. The mode actuator will retract, closing the Vent Door. The defroster actuator will retract, opening the Defroster Door and close the Heater Door through mechanical linkage.

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Sounds like a vacuum supply or vacuum leak problem causing the mode and defrost actuators to not retract. Could this be the cause of your PO300 as well? Maybe the vacuum line didn't get hooked back up to the new engine?

 

I bet you are right!!! Makes sense that I have a vacuum leak, and those do cause a P0300! Thank you!

 

You are welcome. Let us know!