Chevrolet Repair: 95 blazer AC induced engine surge, vents not working, intake manifold gasket, vacuum leaks

Question
My 95 4.3L 4x4 Blazer's engine surges only when the AC is on AND not idling.  The vents also close when this happens.  So basically AC only works when parked.  At highway speeds the w/ the AC on, the periodic surge drops the the revs by about 500-1000 rpm - this is a lot, so we can't use it.  We have had carbon build-up issues w/ the EGR, but I have cleaned it and it appears to be working fine (these symptoms are much different).  The AC system alos has intermittent issues w/ not switching between zones (e.g., defrost vents to upper panel vents.  I'm guessing it is a check valve to the intake manifold or something like that.  Any ideas?

Please let me know where the parts on the engine are located if you have a solution.

You can post this, not sure why it says not to post on Board.

Answer
Hi Chris,
It does sound like it is vacuum related.
There is a canister under the hood connected to the intake manifold with vacuum hose. In that hose should be a "T", and on the engine side of that T should be a check valve.
I doubt the valve is bad. I suspect the hose is cracked, or some rodent has chewed it.
The surge could very well be the ECM trying to compensate for the vacuum leak.
The brake booster, as well as the EGR valve can be causes of large vacuum leaks, as well as the intake manifold gasket. Spraying some carb cleaner around the gasket while the engine is running can tell if it is leaking, if the engine speed changes when you spray a specific spot.

Loss of vacuum can also cause the airflow doors to not operate correctly, and another source of leaks is the door controllers and the mode control valve, behind the levers.

Van