I have an Anet A8 printer (no modifications) and first printed a calibration cube. It came out okayish, within expectations. Afterwards I flashed the firmware to Marlin 1.1.9 and since then have very high over extrusion. In this picture you see on the left the original calibration cube with the stock firmware and on the right a cube with Marlin (unfortunately not a good picture): In fact on the right cube you can see two narrow vertical lines in which extrusion appeared to have come out fine.
The cube on the left was without calibration. On the right hand side all axes, including the extruder, where correctly calibrated. The bed is level. The extruder has been calibrated in case of the cube on the right hand side (95.6 steps/mm). Material is PLA.
After flashing to Marlin the over-extrusion became such a problem that the nozzle is constantly dragging over the printed plastic. You might notice, that these are in fact different cubes, that is correct. I tried the same G-code from the left cube again, but aborted the print, because the nozzle was hitting the print too much.
I calculated a modified flow rate according to this tutorial and found that I would have to reduce my flow rate to 44%.
This appears quite wrong to me. What could be in the firmware change that causes such a massive over extrusion?