I've got a B16 with an OBD II P72 ECU. I noticed that my A/F gauge reads significantly leaner at WOT when the AC is on. It's especially noticeable while VTEC is engaged between 6500-8200RPM.

Has anyone else noticed this? This means that the O2 sensor is reading more oxygen leftover in the exhaust while the AC is on. Could the AC be affecting my ignition timing, or does putting extra load on the crank somehow affect combustion efficiency?

 

Are you using a real wideband air/fuel gauge or are you using a "bouncing lights" gauge? The air conditioning shouldn't be playing with the air/fuel mixture of the car.

 

i know even on a socketed obd1 p28, with no ac even on the car, hitting the button(AC) can make the car idle up, because the ecu is expecting the extra load of the ac...just my experience

 

It's just an autometer pro-comp A/F gauge. It lights up different LEDs for rich, lean and stoich. I'm using the OEM O2 sensor before the cat.

Last time I took it to the dyno, the A/F gauge matched the wideband tail pipe probe pretty closely.

 

Don't bet on that AF gauge to be correct. And as mentioned before, when the AC is activated the ELD tells the ECU to increase idle speed. However, I can only assume you are doing this while driving and in this case, it would be addtional load on the engine that is changing the A/F ratios.