Let’s talk about adjusting the air-fuel ratio with an adjustable fuel pressure regulator, and whether it is even possible.

In closed-loop (cruising), the ECU injects fuel based upon it’s programmed tables, but feedback from the primary O2 sensors tell the ECU whether it is rich or lean, and the ECU adjusts the fuel calculation based on the O2 sensor feedback via the short-term and long-term fuel trims.

As I understand, the ECU adjusts the fuel based on the fuel trims, even in open-loop operation. If so, you could make a fuel pressure adjustment, and initially see a corresponding change in open-loop (WOT) air-fuel ratio, but over time, driving in closed loop operation, the ECU would see that the base fuel calculation is running richer or leaner than expected and adjust the short, and eventually long, term fuel trims to compensate, effectively cancelling out your adjustment of the air-fuel ratio via fuel pressure change.

Thoughts?