I have been logging/tuning AFR in the cruise region for a while now. Unfortunately, I have had trouble getting some cells lean enough, but I think I have discovered why!

I noticed it when trying to hold the car in just a few cells to get alot of samples. When I would close the throtttle partially to keep the RPMs down, the AFR would drop.

Conclusion: partially closing the throttle reduces the incoming air enough to change the AFR, but not enough to move the engine to a different cell. This seems obvious enough as you wouldn't try to log AFR during deceleration. However, it happens with a very slight reduction in throttle opening. So it seems the best way to avoid this is to log only accelerations. Don't bother trying to hold a particular rpm to get samples.

Another solution is the data log analyzer software I have developed It allows you to exclude samples that don't meet your criteria such as "water temp > 83C" or "duty cycle >0". It works great for most cases I have tried (cold engine, engine braking,etc), but my TPS is not able to distinguish this partially closed throttle case.