So the exhaust cam shaft is most likely what is causing this to be this way. Basically when you increase the duration of the exhaust cam alone, you are opening the valve early and longer to expel the combustion. This causes more of the unburned fuel to go through the exhaust. Hence the long term negative fuel trim.
Just because you are watching the short term and it seems to be switching this is only a segment of data points that goes into the long term calculation. What this is saying is that the fuel trim is rich (less oxygen) and it is commanding the injectors to shorten their pulse widths.
Remember that the o2 sensor can only measure the amount of oxygen in the exhaust. So when there is less oxygen (negative number) it interprets it as having more fuel so there isn't enough "room" for oxygen so it commands it lean.
So the short term switching around 0 is a good thing, as it is showing the PCM is able to adjust but in the long term, the exhaust is more rich for more time then lean. That is due to the exhaust cam.
If the engine is running well, I would not worry about it as the changes you made would make sense.
Hope that helps.
Wednesday, September 30th, 2020 AT 2:49 PM