I've got a 96 blazer 2wd 2 door automatic. A few months ago I replaced the AC compressor. AC works great but I've noticed the engine is idling faster when the ac is switched on. Plugged in my scan tool and it's idling at around 800-850rpm with ac on and around 650 with the ac off. The scan tool says throttle is at 3% with ac on and around 1% with ac off. Is this normal on the Blazer? I was under the impression throttle should be 0% without my foot on the gas but maybe that's wrong. In addition, it does this weird thing where if i'm driving on the highway for at least 10 miles or so, all of the vent doors close and the ac stops blowing out. I can still hear the fan blowing. If i stop the engine for a few minutes and restart it, the ac blows great again. I wonder if it's something to do with engine vacuum causing it to idle fast and causing the hvac doors not to work right. Already replaced the hose going to that vacuum ball thing under the hood. Any help would be appreciated.

 

A little idle RPM increase when A/C kicks in isn't anything unusual, I wouldn't worry about that. Neither the 1% 3% reading change.

Now, to the hwy air con issue - a/c compressor does not blow nothing.
A/C compressor delivers compressed refrigerant to an evaporator. While evaporation, the temperature drops. You may see if the a/c compressor is running by looking if the serpentine belt clutch is turning or not. If it's disengaged while it should be on, You may check the refrigerant amount in the system. You have two pressure switch on hi end and low end. If either of them opens, the a/c compressor would stop (clutch will disengage).
Vent doors are controlled by a vacuum. I've seen several topics on the flow selector fix.
The air flow is supported by a blower motor. You may turn the air flow to a maximum, definitely You'll hear whether the blower runs or not.

 

Thanks for the reply. AC compressor is engaged the whole time the ac is on so it isn't that. And it isn't the blower because I can play with the fan control and hear the blower speed up and slow down. I'll just be driving and all of a sudden all the air will stop coming out of the vents. If i switch the ac off and use fresh outside air, the air starts blowing again, but if I switch back to ac mode, no air comes out even though the fan is spinning at the same speed. I'll look into the flow selector.

 

I haven't been much into details of the dash vacuum actuators, probably that would help. But first, I'd check if Your vacuum system does not have a leak. On hwy Your engine runs at a relatively low rpms, and if You run out of a vacuum, the system might play tricks.

 

This is an old post but I replaced all of the vacuum hoses under the hood and it fixed the problem.

 

Thanks for the update, wont help me now, but itll help someone sometime. Good to hear the problem was fixed.