Mazdaspeed3 rough idle

I own a 2010 mazdaspeed3(2.3L turbo). This is my second post on this issue, as it wouldnt let me edit my previous one. I am getting a very unconsistent stumble at idle, that is worse some times than others and sometimes not there at all. It will idle high(upwards of 1000rpm) at times and others as low as 650(about normal for this car), and seems to idle the worst when idling low. The muffler tips and bumper surrounding them are very black(sooty). I have recently pulled the upstream o2 sensor and it was also covered in soot, and was not reading any value on the live data. After replacing it, the issue is still there, although the sensor actually reads values now.
There apears to be no exhuast leaks(no soot visable anywhere between headers and cat).
The live data reads -4ish on stft pretty consistently when the issue is occuring.
I have heard fuel injectors as being a potential suspect, but am not sure what exaclty to look for. My guess is a leaking injector seal?
Also is not seeing soot anyweher on the exhuast prior to the o2 sensor good enough to rule out an exhuast leak?

Have you’ve tried cleaning the electronic throttle body?


Check engine light on? Codes? Which ones?

I replaced it about a month ago because yhe tps went bad. The idle issue was not solved though.

No codes at all.

That’s not much out of spec. I don’t think that is providing much diagnostic info. Are the spark plug tips covered in black soot as well? Suggest they all be inspected and compared vs each other. You may have a stuck injector.