Disclaimer: I am a novice, so I possibly have made many errors during the process.
I have a Mercedes-Benz A Class w168 (2001).
It has a problem that sometimes, when running low RPMs it shuts down (often while stationary - but it did happen low speed corners).
I have brought it to two different mechanics but they said it's all fine and they can't find the issue. So I have decided to learn about modern engines myself, and bought a OBDII device to read ECU values.
I have read engine errors and it turns out I have one P0100 (Mass Air Flow Sensor).
So I took it apart and cleaned it, cleared the error, and put it back.
Unfortunately I did not measure LTFT and STFT before that.
The problem still persists, so I am trying to understand better what's going on. (although the error did not come up again yet).
All the data below has been collected with engine in closed loop.
I have noticed that the car has LTFT and STFT at ~0% when idle, but when I open the throttle (even moderately), LTFT goes straight to -20%, however STFT goes up, proportionally with RPMs (the more the throttle is open, the more STFT goes up), let's say between 10% and 25%.
Both idle condition and open throttle condition, O2 sensor reports normal conditions.
When returning to idle from an open throttle condition, LTFT stays -20% and STFT get temporarely "stuck" at 10%, and in those condition 02 sensor reports lean condition, which it kind of makes sense.
The thing that I don't understand is, how can LTFT be negative and STFT be positive so consistently? What am I missing?
Idle: LTFT 0%, STFT 0% Throttle: LTFT -20%, STFT 10-25%