Filament not extruding after filament swap
I have been having a number of issues trying to get the MMU3 to reliably switch filaments.
My biggest problem recently is that the filament was not extruding far down enough for the nextruder to catch. It would get far enough for the sensor to trigger, but not quite far enough for the gear to catch. This would not happen on the very first filament load, right when the test print started. This would only occur, and continue to occur, on subsequent filament swaps. Essentially, the first color would go down and nothing else.
To try and remedy this, I adjusted the extra loading distance option from -52 mm to 0 mm to see what happens. Now the filament seems to be more than far enough down for the gear to catch, but yet it does not! The same happens with -25 mm as well. I know that the nextruder is more than capable of printing normally since it can do the first color no problem, but now it cannot do the same. Funnily enough, after setting it back to the original -52 mm, the filament is now always seemingly far enough, but now I am getting frequent filament jams and being unable to load back to the FINDA.
Has anyone else had similar troubles?
For reference, my idler tension is adjusted and I can preload/load fine. The idler tension "should" be fine, as the first layer still prints as expected. I have recalibrated the filament sensor as well. I am on MMU3 firmware 3.0.3 and Mk4 6.1.2 +7894.
Here is where the filament sits now when I abort a print to see where it is.
RE: Filament not extruding after filament swap
I am having a similar problem. I have a Mk4s, from an upgrade kit, that worked well before adding a factory assembled MMU3. All calibrations went well for set up and the FINDA and PINDA sensors are working correctly. When I load each slot, the loading sequence seems to go correctly with both sensors working and the extruder rotating. About 10%-20% of the time the filament will extrude but only after hitting continue to extrude. The rest of the time everything works but no extrusion with no unusual noises at any time. Extra loading distance doesn't appear to have any impact. It appears that filament is not feeding enough for the extruder to grab it. I baffled and frustrated for a fix.
RE: Filament not extruding after filament swap
Make sure your PTFE tubes do not have sharp bends. The total friction may be causing your problems. Possibly, try tightening the MMU idler screws just a little. The tops of the idler adjustment screws should barely be higher than the surrounding frame. (Terrible standard, I know)
MK4S/MMU3
RE: Filament not extruding after filament swap
Thanks for the feedback. I am running from the spool to the MMU3 in one easy section. I will continue work on a solution. 
RE: Filament not extruding after filament swap
Nice rack stand! Clean & compact.
MK4S/MMU3
RE: Filament not extruding after filament swap
Thanks, I did since I am fitting everything into my existing enclosure and I didn't find anything small enough. I ditched the buffer because they caused too much friction in the location that it fit. The retracted filament is being managed with caps over the spools.
RE:
I promise, this idler will solve your Issues:
https://www.printables.com/model/1269218-nextruder-mmu-multi-material-gamechanger
I was developing that solution since months...
