Can you help me improve my Renkforce RF1000 print quality? I've got a Renkforce RF1000 which should be a good 3D printer. I got it second-hand for my birthday one year ago. I've got no way of contacting the old owner.
I spend a good amount of hours fine-tuning the slicer settings last year but at best got mediocre prints. Between September and a week ago I lived somewhere else and didn't touch my printer.
Now here's what I don't know:

I don't know what parts are replaced
I don't know what my nozzle size is
I don't know if the limit switches are calibrated correctly

Though I think they are. This doesn't seem to be a problem. I did re-calibrate the Z-axis



Here are some important details:

I use 3 mm Renkforce PLA filament which I print at 190 °C on a bed heated at 60 °C. The PLA is over one year old now.
There's a fan on the motor on top that isn't connected to anything.

Here are some of the problems I've got:

I've had multiple prints failing due to the extruder not working properly. The motor keeps on spinning but the &quot;feed knurl&quot; remains stationary
I can't seem to get the right extraction settings
I can't seem to get my prints to consistently stick. It tends to work when I heat the bed to 60 °C and use glue and get lucky.

Feel free to give any thoughts you've got. These are the most important questions I've got:

Should I replace the nozzle with this one so that I know what nozzle I've got and so I'm sure this isn't a problem?
Should I replace the filament with new 1.75 mm PLA? If so, why?
How do I fix the extruder?


I tried getting the &quot;feed knurl&quot; off but can't seem to do this easily. I've got some super glue I could try to put in there but something's telling me this might be a very bad idea...


Is the unconnected fan important and if so: what do I do with it? There's no remaining wire to connect it to.
How tight should the 4 screws that hold the filament between the extruder and the rolling disk be?

For now, these are all hardware problems. I can post my Slic3r settings too but I believe the hardware should be fixed before going into slicer settings.
Here are some pictures showing the problems:

This is the extruder. The feeding mechanism can be seen in front. It shows the &quot;feed knurl&quot; of which the inside spins while the outside remains stationary (question 3). Next to it are 4 screws which determine how tight the filament is held against the extruder (question 5). On the back it shows a black fan, this got placed by the previous owner but isn't connected (question 4).

This image shows the unconnected fan (question 4) to the right. Behind it is the motor that's connected to the extruder. The motor works but the extruder doesn't spin with it. The extruder has a little black hole on top.

This shows the extruder from the front. The inner layer spins, the outer layer doesn't (question 3)

These are some of the prints when the extruder was still working.

Nozzle and print bed (question 1)
Should I replace the nozzle with this one so that I know what nozzle I've got and so I'm sure this isn't a problem?
  


That is not really necessary. The exact nozzle size does not really matter.


  Should I replace the filament with new 1.75 mm PLA? If so, why?


No, there's no reason to do that. Going from 3&nbsp;mm to 1.75&nbsp;mm filament usually requires replacing the entire extruder.


  How do I fix the extruder?


Tighten the grub screw (the "black hole") with a hex key.


  Is the unconnected fan important and if so: what do I do with it? There's no remaining wire to connect it to.


The fan appears to be cooling the stepper motor. This shouldn't really be necessary, the stepper should be fine without any additional cooling. People tend to "upgrade" their printers by adding unnecessary bells and whistles so it is possible the original owner added this fan just because they wanted to, and not because it is necessary.


  How tight should the 4 screws that hold the filament between the extruder and the rolling disk be?


Tight enough that the extruder grips the filament and is able to extrude it consistently. If it is too tight it might deform the filament too much which could cause printing problems but it should be obvious if this is the case. There is a wide margin here so don't worry about this too much.