Anyone have any ideas why my i3 might be suddenly over-extruding?

I've got a Prusa i3 (rod version) that I've been running for about a year with the Itty Bitty Belted (single) extruder. It's been running well, but it's been sitting for a couple of months while I worked on other projects. Now that I've started it back up, it's overextruding significantly. Plastic is piling up and the nozzle is plowing through and re-melting the excess plastic. It's oozing out over the sides of the parts and making a real mess. It's probably over by 10-20%, so after a couple of layers, the parts start to look like Dr. Seuss.

I've been through everything I can think of, and I haven't found the cause. Any idea what might have happened? Where else should I look?

Here's what I've tried:


  • Sliced with Slic3r 1.1.7 stable and 1.2.9 stable, with the same results.
  • Installed the printer on a flat Travertine tile and re-leveled the X axis.
  • I'm using ABL, and the loops measure with a micrometer between .21 and .23mm all the way around the bed. First layer should be .2mm.
  • The Z axis is moving correctly. 50mm vertical movement measures 50.01mm with digital calipers.
  • X and Y steps are correct and parts are the right size.
  • E-steps is still 644.2, the same as it was set up originally.
  • Extruder is moving filament 98mm for 100mm commanded extrude, so it's under-extruding slightly.
  • Filament measures between 2.93 and 2.98mm, measured at various angles.
  • I can't find any compensation settings in the slicer or in the printer that are set to anything but unity.
  • Motor is running smoothly. No step or bolt skipping.
  • Cleaned out the bolt with a dental pick.
  • Sitting next to the printer and watching it print, I'm noticing nothing unusual, except the overextrusion and the eventual clatter of the nozzle running into the cooled, overextruded plastic.


I tried to calibrate the extruder, with odd results. At 644.2 e-steps, it's extruding 98 instead of 100mm, so I calculated the new value of 657.3. At 657.3, it extrudes 103mm. Re-doing the math with 657.3 e-steps, extruding 103/100, it says I should be at 638.2, which is less than where I started. Bumping it up and down slightly seems to be resulting in non-linear response, with no change for small differences followed by a big step in the response. I can repeat the same test over and over and get exactly the same result, so it isn't intermittent.

Please check my math. I must be overlooking something obvious.

Running through Triffid Hunter's guide, with rectilinear fill at 95%, I had to pull all the way back to 550 e-steps before I started getting gaps. That's 15% down from a setting that was only 2% off by measured filament movement and working previously to print all manner of parts.

I can keep tuning by printing, and I'll zero in on something that works (probably around 575), but why is it so far off from the measurements at the extruder?

Is there some global compensation value somewhere that I bumped and I'm forgetting?

Tool chain: STL -> Slic3r 1.1.7/1.2.9 -> Octopi -> Marlin (8 months old, not re-flashed) -> RAMPS 1.4