Lean Under Decel with Over-run Fuel Cut Turned Off
 

I am just starting the tuning on my new motor yet when on deceleration with my foot completely off throttle I get over-run fuel cut. I had the over run fuel cut turned off but after seeing this I tried to turn it on and set the perimeters so that it shouldn't cut fuel or spark when below 5000. This however still did not fix my problem and I am still getting max lean readings until eventually fuel turns back on around 1100 or 1200 rpm. I am running real rx8 yellows and have them at 420cc and I have the injector dead time at 0.8ms.

I have both the tunerstudio file and a data log that I took to show the fuel cut

My MS3PnP on a 2001 did the same thing. No matter how much fuel I put in, it still goes full lean when the vacuum drops to 15 kPa or so. As soon as you tip in the throttle at all, so that you go to around 30 kPa, it goes to whatever AFR you are targeting, so long as the VE table is OK.
It did it when the car was stock, and after I added bigger injectors, and after I added the turbo.

Given you are coasting and in over run the high lean is no problem.
I resolved it by adding a 15 kPa line in the VE table, set AFR target to 18, and as a result, VE analyze live, or VE analyze in the Megasquirt Viewer, stopped trying to do weird fuel and VE things to the actual, functional load values at 30 kPa and above.

As to the reason, I have concluded it is because it is an O2 sensor and not an AFR sensor, and at 15 kPa there is simply not enough O2 getting through the intake, engine and to the sensor. But I don't know that with certainty.

Reliable and repeatable fuel control during decel is challenging to start with, and will be impossible with your setup. My recommendation would be to use fuel cut and move on.

Thank you, I adjusted to allow for a lower kpa and interpolated the rows. Going on another drive later to see if that will fix the issue.

I want to just try since I tuned my other motor set up naturally aspirated and want to learn more about how to use tuner studio. I also love the challenge of tuning since it's still new to me.

The engine does not need fuel in that situation and that is why fuel cut exists. It improves economy. Also when there is no fuel there is no heat being produced which has a cooling effect.

I had the same thing going on with my car for a while. Full lean immediately upon decel even before fuel cut would enable. Funny enough, it went away completely after installing my turbo kit with no changes to the tune. AFRs now stay around 12-15 off throttle depending on the situation before fuel cut enables.

I went back and smoke tested my NA exhaust manifold afterwards and found my EGR blockoff cap was leaking pretty good. The O2 sensor is located pretty close to the EGR port on the stock manifold so I’m assuming the exhaust leak was contributing to the problem.

however, like stated above, it’s not really a big deal since it’s on decel anyways.