I did a smoke test on the Blazer today to find a small vacuum leak. There is smoke coming from the hole in the top of the 4wd actuator switch. Is this normal, or the source of my vacuum leak? This is the only place I saw any potential vacuum leak.
Thanks!

Edit: I guess I should specify I have the auto-4wd system (with neutral feature). I've read the standard 3-button 4wd systems do not have the firewall mounted actuator valve/switch.

 

What you found could be the vent hole for the front axle actuator side of the vacuum switch. (We are talking about the switch mounted at the top of and near the middle of the engine firewall right?) Without a vent hole, when the axle is supposed to be disengaged, the vacuum would still be present and keep the axle locked if it did not have a vent to atmosphere.

Since you are at it, I'd suggest replacing all of engine-compartment vacuum hoses. I did it it on my 99 with the Auto-4wd and it is not too hard to do. I documented the hoses I used in the link in my signature below.

 

This!

 

Thanks all
It was indeed the vent hole for the 4wd actuator valve. The valve switches the axle side between vacuum and atmosphere, the vacuum side should not go to atmosphere, so no smoke should have been coming from the hole during the test.
I replaced the valve, which seems to have fixed the vacuum leak.
The only time I noticed the vacuum leak is under acceleration. The HVAC would switch from defrost or floor to the panel when accelerating; it would work fine while maintaining speed. After the valve replacement the HVAC works as it should cruising and during heavy acceleration.

 

Glad you had an easy fix and thanks for reporting back with the answer.