Rat - Don't take this wrong. I say that because I have a way of sounding harsh, but it's not meant that way. I don't know what AFR gauges you've been looking at, but you're not seeing what I see. There's no way reading plugs can provide such continuous, instant, accurate info. My car's engine responds just exactly as it would be expected to react based on what the AFR gauge says. There's no issue with the AFR gauge.
If I had been driving and it went lean I might question the gauge. But it happened EXACTLY after pulling off the air filter housing then putting it back on. That exact timing. So, something happened at that event that led to this.
And yes, the engine behaved differently after this. When I open the throttle to pull away from a stop, or begin to accelerate it naturally moves lean. The AFR reflects the same. But all the AFR readings have shifted up / lean after this event. And under lean enough conditions it gets weak and can sort miss some too if it goes lean enough.
