Noticed when I have the AC on the idle will smooth out compared to when it’s off. Any reason?

 

When the AC is on, the compressor will load the engine. The ECU is probably adjusting the Idle Air Control solenoid duty, to give the engine a little more air to keep the same RPM when the AC is on. This will also change the manifold pressure by a small amount, you might see a little less vacuum if you have a boost gauge installed. Because of this change in vacuum, the ECU is probably referencing in a slightly different part of the fuel lookup table. It might be smooth because the different part of the fuel lookup table works a little better for your engine. I've also heard some people claim that there is less EGR (exhaust gas sucked back into the intake ports due to overlap) when the engine pulls less vacuum, but that's pretty hard to measure.

 

The idle speed is increased a bit to compensate for the added electrical load when the AC/radiator fans kick in. Same thing happens to the idle speed when you turn on the parking or headlights, or just set the cabin air fan to 3.

 

Very low engine load at idle means a very sparse intake mixture, i.e., the molecules of oxygen and fuel are spaced pretty far apart. That makes it harder for the spark to ignite anything, and you get a "lean miss."
The slightly increased load with AC, etc., means that there is a more ignitable mixture, less lean miss, and idle smooths out.

 

Another possiblilty is a leak past the throttle. If the ecu is trying to achieve a low idle but cant due to a leak it will bounce, then you turn on the ac and the computer raises the setpoint due to the load and it smooths because it can tolerate the leak at that level. So i would check vaccum leaks into the intake or intake gaskets maybe. Probably looking for a small leak.

 

If your oxygen sensor is old, the idle will get too rich and you will feel the individual combustion pulses. When I replaced mine the idle smoothed out.
Link to related thread:
https://www.rx7club.com/3rd-generation-specific-1993-2002-16/driveability-improvement-secondary-throttle-plate-adj-o2-sensor-repl-687859/?highlight=hesitation
https://www.rx7club.com/3rd-generation-specific-1993-2002-16/driveability-improvement-secondary-throttle-plate-adj-o2-sensor-repl-687859/#post7329814