Extruder tuning - different results every time

Every time I run the feedrate tuner, I get a different result. First it was 24mm,t hen 17, then 20. This is on an i3 plus (no mods). When I perform the test each time is it starting from the same default, or does it start off using the last feedrate change I made?

The last one (if you hAve saved it)

I would also check your extruder motor drive gear as the teeth can get clogged with plastic from the filament. This will defiantly affect your extrusion rate.

I’ve disassembled the head, put in a new PTFE tube and a new nozzle. Gears are clean. Still every time I test the feed rate, I get a different result. I’d think that once I adjusted it once or twice, it should stay reasonably the same?

Yes.

So what else could cause this?

BTW, recommendation for a tweak: before the extruder test runs, the z axis should raise up off the bed much higher, to make room for the extruded material to flow and not tangle and then back up and melt onto the nozzle.

Faulty step motor or driver?

A loose gear screw maybe?

I do this as a cold extrude with the nozzle removed.