GMC Repair: 83 S10 blazer misfire, engine vacuum, vacuum test

Question
QUESTION: '83 S10 blazer 4x4 2.8 engine, auto, air. Engine stumbles as if ignition problem but only when the a/c control is in the a/c, vent, or defrost setting. When in the off position runs great, new plugs, wire, cap, rotor, o2 censor. Thank You: Jim

ANSWER: Hi Jim
So let me just clarify, is it ONLY when the AC is on, as in the compressor being on , or can the compressor be off and the engine still misfire just have the control in these settings?
Please follow post back
Pawl

---------- FOLLOW-UP ----------

QUESTION: Hi
The compressor does not have to be on, I unpluged the compressor and it still misfires. Just having the controls in these settings makes it misfire. One other thing I forgot to mention that might shed some light is it is not constant only intermitant. Had it to two shops so far one said he did computer diognastic but could not show the printout, recomended the tune up with o2 censor. It ran a little beter but still the misfire. The other shop said maybe the control itself. Before I went further just replacing parts, thought maybe you had heard of this befor. Thanks for you time: Jim

Answer
Hi
After rersearching the only things I could think of are
1)Low engine vacuum. The controller uses engine vacuum to change modes and mode doors and keep them held. If the HVAC controller is using vacuum and the engine is not able to produce enough for the demand, it could make a rich condition.
2) As you stated the controller itself.
Have a shop preform an engine vacuum test.  See what vacuum is at idle and running down road. They should be able to see if it is low.
If not, controller.
Hope this helps.
Just advice from experience.
Good Luck
Pawl