You win if you know the answer..random lean condition

06 chevy trailblazer highly modified
Eaton M112 supercharger ~5-6 psi
custom built intake
custom header/exhaust
yada yada

Ever since I installed the exhaust I've been having some lean spot issues on tip in and cruise. The exhaust yielded about an 17% change in peak air flow through the engine. The tip in can be fixed with VE tuning I'm sure but the cruise lean condition makes no sense. Somebody suggested to me it looks like a lean cruise programmed condition and I agree, low engine speed, generally light throttle, open loop and lean commanded lambda. I didn't notice it ever doing this before so I don't know why it started now.

Go to just after 6 minutes in the log, you see commanded lambda goes from stoich to 15.8. Fuel trims stop responding indicating open loop. I slowly increase throttle and it just gets leaner and leaner if you watch the wideband. Goes into the 17's and starts bucking. It doesn't jump out of this lean condition until the converter unlocks at 610 and the rpms jump.

How is this possible? There is no lean cruise options in the tune.

Any ideas or solutions to make it not do this?

I wouldn't rule out a cat test, many GM pcm's do this.

On the Gen 3 V8 computers in the trucks if you have the rear o2 sensors deleted and removed, it will enter a cat test that you cannot shut off. If the sensors are left plugged in and just the codes turned off it will not do the test.

I guess entirely possible. I do have no cat and the rear o2 is unplugged. You my friend will be my hero if I plug in the rear O2 and it stops doing that. Is there any other way for me to find out if it is doing a cat test?

That I don't know. I learned about the test through my own truck and the situation that was going on, read up on it and figured out the cause was having them unplugged.