You can just follow the instructions here to swap them 

https://help.prusa3d.com/guide/4-upgrading-the-nextruder_754312

A further question if I may, you said you replaced it with a new 0.4mm one, of the same type or ?

Yes, it was exactly the same nozzle, 0.4n Prusa brass. https://www.prusa3d.com/product/prusa-nozzle-brass-0-4-mm/. I also encountered the same problem with the newly purchased 0.4n Prusa Nozzle ObXidian which started to make me suspicious it wasn't actually the nozzle. 

Regardless, I've gone with the idler lever upgrade fix attempt. I've set a reminder in two months to return to this thread with my findings for you and any future readers if this fixed it. Hoping me and Brian are right on this one! 

Posted by: @brian-12

Here are 2 pictures of 1 with lost preload and another from the new design. Notice the gap between the door and frame.

 

I only printed the idler lever, not the nut and the swivel. Mainly because I wanted to minimize the disassembly needed. Do you think I it would be wise to replace the nut and the swivel as well? 

After the change the cold pull looked good again.

Posted by: @neophyl

Brian, I know why it grinds away at the filament, my point is, to make it clear, is that if there's sufficient back pressure from the nozzle to cause the idler to grind it away then its highly unlikely to be a problem with the idler.  Increasing the pressure in the hopes of 'forcing' it through is the wrong approach.  

Unless of course the idler tension was changed when the new nozzle was fitted.... Still in that case it would just be a matter of adjusting it rather than replacing it.  I  will bow to Foxrun's knowledge on how easy it to swap and if that's an easy point to check, but from the current information I would be looking for something else.

A further question if I may, you said you replaced it with a new 0.4mm one, of the same type or ?

Adjusting the tension doesn't help because the door bottoms against the Nextruder case.  Essentially what happened is the rollers in the idler door moved from getting too hot essentially removing all preload from the idler door. 

The first layer and perimeters usually print fine, but when it starts printing infill the printer speeds up and the extruder gear starts slipping creating the ground up filament. 

I've experienced this first hand on my XL.  My problems started after printing a full bed of ASA.  The temperature inside the enclosure got too hot and caused the door to deform.  I troubleshot for about 2 weeks and finally discovered the issue I described above. I reprinted the idler door and latch and the printer was solved.  I've put about 400 hours on that print head since without a single issue. 

One thing to note is that the original idler door design had very little preload.  If you compare it to the new design Prusa increased it probably 2 or 3x.

1. Here are 2 pictures of 1 with lost preload and another from the new design. Notice the gap between the door and frame.

Unless of course the idler tension was changed when the new nozzle was fitted.... Still in that case it would just be a matter of adjusting it rather than replacing it.  I  will bow to Foxrun's knowledge on how easy it to swap and if that's an easy point to check, but from the current information I would be looking for something else.

A further question if I may, you said you replaced it with a new 0.4mm one, of the same type or ?

I ran into this on my XL.  Do you have an enclosure?  What I found was that over time the heat from the enclosure caused the PETG idler to deform just as you suspected. I reprinted the new Prusa versions of the parts with PCCF and the problem went away. 

@Neophyl. It grinds the filament because there's not enough pressure on it and the back pressure from the nozzle exceeds the amount of grip on the filament. 

Yes, I always print with the doors closed. Temperature around 30 C on average when printing (which is arguably too high for PLA, I know!). I don't have any PCCF filament but planning to reprint it in ASA. I'll update you all in this forum thread in a few months for an update. 😄

Posted by: @neophyl

If you are currently grinding filament with the standard idler then fitting a tighter one isn't going to make any difference, except grind it away faster.  

You think changing the nozzle coinciding with an increase in clogging is a coincidence ? Think again.  You changed Something  when you swapped, its going to be a case of figuring out what.  

Thank you for the thoughts. Changing nozzle again will be my backup in case changing the idler lever won't make any difference. 

Posted by: @foxrun3d

That planetary gear system and idler are one of the few things I actually like about my MK4s. I'm with Neophyl and doubt that a new idler will fix the issue. But I mean, printing a new idler and installing it is a matter of minutes, so an easy experiment to do. Or maybe just adjusting the idler tension may help.

Yeah adjusting the tension was the first thing I did, I think tightening them more than the assembly instructions won't do anything though due to the shape of the idler lever (it collides with the nextruder frame if I understand correctly). 

Posted by: @ujindesign

Hi all. 

I purchased a MK4 kit almost as soon as it was released and I've been printing very happily on it since. Clogs and jams were very uncommon, maybe once every 4 months. However, since about one month ago, I've been getting a clog every week. (It happened in conjunction with me replacing my 0.4n with a new one, but I honestly think this is just a coincidence.)

What happens is that the filament clogs up in the nozzle, the filament jam detection does not go off (but it did go off yesterday when I did something weird so I know the system works), and the printer keeps on printing but without extruding any filament. Sometimes, there is a bunch of filament dust on the build plate from the extruder gears grinding on the filament. I solve it by doing a cold pull and re-load the filament, but this clog problem reoccurs quite frequently. 

Do you have any idea on why this sudden change in clogs might have happened and what I might do to fix it? 

I have a theory. I've seen a YouTuber "Terpuh Labs" make a video about a modified idler lever in Printables that is slightly more tight than the stock MK4 idler lever, and that helps the nextruder get a better grip on the filament. Maybe my idler lever has experienced some creep with just like 0.5mm and is not pressing the filament so tightly against the nextruder gears anymore. Could printing a new idler lever that's a bit more tight be the answer? 

I ran into this on my XL.  Do you have an enclosure?  What I found was that over time the heat from the enclosure caused the PETG idler to deform just as you suspected. I reprinted the new Prusa versions of the parts with PCCF and the problem went away. 

@Neophyl. It grinds the filament because there's not enough pressure on it and the back pressure from the nozzle exceeds the amount of grip on the filament.