Filament Not Getting Pushed Through Extruder & Nozzle

[jumi1174]

UPDATE: I think it may be a hotend temperature issue now. See post #4.

I know this seems like a common and easy issue to address, I thought so too, but I'm all out of ideas and need help from the community.

All of a sudden, my filament stopped being pushed through the extruder-nozzle assembly. The hobbed bolt began "carving" out a chunk of the filament line:


Seemed like a simple fix, either:
a) the idler springs were too tight
b) the hobbed bolt teeth were gunked up with filament flakes
c) the hot end / nozzle was jammed

I quickly eliminated (c) since I was able to manually push filament through the nozzle without issues; still, I removed the filament from the extruder and was able to run a thin guitar string through the nozzle opening all the way up through the extruder opening to ensure no major blockage was present. I then cleaned some flakes off the hobbed bolt and put it all back together with looser idler spring compression. The filament still got chewed up. I repeated the process until the idler bolts were so loose that the springs had no compression. Filament still got chewed up.

I cleaned the hobbed bolt again and figured that something must then be wrong with the hot end. I'm using a 3mm Hexagon. So I took the hot end completely off, put the filament just through the extruder, and was able to extrude and retract filament for long stretches (~0.5m) through the extruder using both the LCD and Pronterface with "normal" idler spring compression (not overly tight or overly loose); the hobbed bolt didn't chew up the filament at all other than the regular teeth marks.

So I figured I had some kind of blockage in the hot end that wasn't allowing the filament to melt and extrude properly. I had previously experimented with bambooFill and copperFill on that hot end, maybe something from those messed up my hot end.

I ordered a completely new 3mm Hexagon hot end, installed it, and got it set up. I brought it up to 250 C, let it sit for a bit, and manually pushed out filament; it all worked fine. I put the filament back on the hobbed bolt with normal idler spring compression: got chewed filament and no extrusion again...

I'm all out of ideas. I feel like I've tried every avenue of troubleshooting this issue, and I don't know what else to try. Other ideas I've seen mentioned haven't panned out:
- the gears on the extruder aren't slipping and aren't significantly worn
- there's plenty clearance between the hobbed bolt and x-carriage, so there's no friction or rubbing
- my filament spool has plenty slack

The only possible thing that is different from before the issue is that the filament I'm using now (White ABS from ToyBuilder Labs) is measuring ~2.9mm in diameter whereas my previous filament (Blue ABS from MakerFarm) was ~3.0mm. But I don't think that is the case because this issues sprang up all of a sudden; I was previously able to get completed and nice prints with the ToyBuilder filament before these issues occurred.

Any possible ideas? I'm not sure what else to try.

Thanks in advance!