Different lengths of filament is extruded with different temperatures TL;DR: I'm looking to change the steps per mm but I'm not sure what to set it to. Do I decide on a temperature and change the rate based on the percentage for that temp? Or is there a more general setting I can put it to? Or is there a different underlying problem here?

I'm running into an issue with my Ender 3 where when I print PLA at different temperatures I'm getting different extrusion rates. I've done the test where marking it at 120&nbsp;mm and then extruding using PronterFace 100&nbsp;mm and measuring the distance and this is my results. I did two tests of each to confirm results


185&nbsp;°C 80&nbsp;mm left = 60&nbsp;% under extrusion
200&nbsp;°C 31&nbsp;mm Left = 11&nbsp;% Under Extrusion
210&nbsp;°C 32&nbsp;mm Left = 12&nbsp;% Under Extrusion
220&nbsp;°C 28&nbsp;mm Left = 8&nbsp;% Under Extrusion
230&nbsp;°C 25&nbsp;mm Left = 5&nbsp;% Under Extrusion


When I feed 100&nbsp;mm through the Bowden tube without the filament going through the hot end I get exactly 20&nbsp;mm left meaning:


0% under Extrusion

echo: Steps per unit:
echo: M92 X80.00 Y80.00 Z400.00 E93.00



I have been having this problem since I started. I normally print at 210&nbsp;°C at 50&nbsp;mm/s with a flow rate of 103&nbsp;% which seems to work well for most prints but on flat walls on prints it under extrudes in the same spot each time it comes around to that point.  

Steps I have taken so far:


Different PLA filaments. All perform the same
Replaced the nozzle with a new one
Checked all fittings with the bowen tube system
Printed a spring spacer to make the spring tighter on the extruder gear
Changed the Vref from 0.75 to 1 for the extruder stepper


The reason I changed the Vref on the extruder stepper is because the system would click back when printing as if the pressure was too great in the tube. Changing this has helped some of the under extrusion a bit.

So. depending on the temperature, it's harder or easier to push the filament by hand through the hot end. Maybe I need a better hot end?

Examples: 

My test print to replicate the issue. The ripple bit is brittle and under extruded.





This was a Prototype piece for an up-sized print I was planning. After going around it under extrudes at the same point each time 210&nbsp;°C



Most of the print is fine just some parts are different. This is an inner wall.



Heat towers
Obviously, your extrusion process is troubled by a lot of pressure. This can be seen from the extensive experiment you conducted with PLA extrusion at different temperatures. Please do note that 230 &deg;C is considered pretty high for PLA! Usually it should be in the range of 185 - 205 &deg;C:


  In general, PLA filament settings have an optimal printing PLA
  temperature range from about 185C to about 205C. If you’re using
  1.75mm as opposed to thicker 2.85mm (or 3.00mm) your optimal print will be closer to the lower end of this PLA filament temperature
  range.


The temperature dependency of filament diameter is explained that small diameter filament warms up way faster in the heating zone of the hotend than large diameter filament as the heat travels less far to the filament core. Basically, with 1.75 mm filament you should be able to print at 195 &deg;C. The pressure that the filament exerts on the hotend and extruder is clearly too much and leads to skipped steps.

It is strongly discouraged to create a function of steps per millimeter (or an over-extrusion by specifying a more than 100% flow modifier). This is a mechanical issue that needs to be fixed by addressing the hardware problem. Usually this is done by:


fixing the extruder; 


is it skipping steps? 
does the filament tension get too high that it skips back pass the extruder gear?
does increase the stepper current work?

fixing the Bowden tube;


is it clean?
is there too much friction?
are there kinks?

fixing the hotend;


is the temperature that is reported the correct one? (thermistor problem?)
is the heat conducting properly to the nozzle?
is there a tolerance issue in the hotend/heatbreak?
is the coldend properly cooled?



A few of these you already explored, others you have not. Unfortunately, you have to do a little more troubleshooting the get to the bottom of the actual problem that is causing this pressure preventing the extruder to extrude the proper commanded length.