Revisiting this issue again... I started getting secondary air pump codes but only for bank 2, which pointed to a failed O2 sensor there. I replaced both banks' pre-cat sensors, which cleared the secondary air codes but not the idle stumble.
I'm now trying to get my car through inspection and I'm not achieving readiness for fuel tank ventilation on the stock tune. I followed the procedure detailed
in this thread to a T. It says to idle "in gear" for 5 minutes. I idled around a parking lot in first gear for 6 minutes, then tried holding the clutch to the floor for 6 minutes with the transmission in 1st, then tried idling in neutral for 6 minutes. No dice. I've never gotten a code for anything related to the fuel system. Any thoughts on what might cause the system to not be ready but also not throw any codes?
I did stumble upon
this thread and the embedded video is
exactly what I'm experiencing with my idle stumble. So I'm hoping that whatever is causing my fuel vent readiness problem is also causing my idle problem. I'd rather look into this a bit more before dropping yet another $100 in parts to replace the valve without first confirming it's the problem.