Wiggis

Any clues as to what would be causing the long term fuel trims on both banks to be running at between -6 and -9 most of the time? the short term trims fluctuate between +1.8 and -2. This is with the car idling, and readings through torque.

No apparent performance issues. No fault codes. It's a 2012 C63.

Thoughts? Or just one of those things?

Celicasaur

I want to say perhaps give your maf a clean with carb cleaner and check over your air filters to ensure they're not badly clogged, but then your car isn't exactly old either. Could even be the sensors themselves. I'm not sure, so hopefully somebody will be along shortly. I am pretty sure though that a dirty or tired maf can cause a negative fuel trim.

bhamg

Definitely try cleaning them but never with a carb cleaner and only with a MAFS-safe cleaner that leaves no residue like the one made by CRC. A carb cleaner will kill a MAFS.

Vladds

These are not very bad numbers. Drive it and see what the numbers do. They may change from cold to warmed up.
With my car, they were smallest when cold and by the time I got to work, they increased by 4-5 points.

skratch77

Not sure if this is normal but I just loaded my 104 tune and my ltft are 19.xx and 20.xx

Stock exhaust but headers are coming in soon

Vladds

I would watch these numbers. The code threshold should be not much over 20. See if they drop on their own after few days, being that the tune is changed.