I have a 93 grand cherokee 6 cylinder laredo. The vacuum on the intake manifold is a steady 17 psi at a warm idle. Thing is when I turn on the air conditioner a/c the vacuum drops by 3 psi to 14. This seems to cause the motor to idle rough though the idle does NOT decrease in RPM's. I can not figure out what the connection between turning on the a/c and a drop in vacuum pressure is. Checked all the vacuum lines, just about every single sensor has been replaced by me over the last few years.

Thanks

 

I would put some oil in compressor. It my be locking-up and taking too much power from engine. You my still have some vacuum leak too.

 

When the A/C compressor kick on, the idle air control motor opens a little to raise the RPMs up for the load of the compressor. There is your slight decrease in vacuum. The compressor may be getting a little old and drawing more power from the engine causing the rough idle. Do not add oil to the compressor, as stated above.

 

X2. Sounds normal to me. Is the AC still blowing cold/working normally? If so I wouldn't worry about it. JMO
Dan