Weird Extrusion Problem

[Sir_Fruit_Loops 0]

tl;dr:  brim, shell and infill fine - top/bottom layers screwed up. Also curling and blotchyness. Pls help me :<

 

Greetings, fellow printing enthusiasts!

 

I've been using my UMO+ for a couple of years now and it has always worked quite well. With some fine tuning I managed to get absolutely flawless prints  comparable to SLA prints. But the Design of the Extrusion system and Hotend had some design flaws causing an unnessecary amount of filament jams, stripping and failed prints so I exchanged them for E3D's V6 and Titan. This worked quite well and produced good quality prints. Haven't used it in a couple of months, now I took it apart and reassembled it to clean it up and improve some minor things - and now I've encountered a weird problem I just can't put my finger on:

 

The Brim prints absolutely flawless every time but as soon as it gets to filling the bottom layer the lines are way too far apart and all the bottom layers as well as the top layers are this way, appearing underextruded. However the nozzle is not jammed. It has no problem extruding more filament. The outer walls and infill print just fine, it's just the top/bottom. weirdly enough, I have printed successfully before using basically the exact same setup - the only difference being a couple of months of non-usage and me taking the printer apart. But only to give the casing a new coat of painting and implementing the banana tensioner bushing blocks by alan89.

 

Normally I would assume the nozzle was blocked but then the brim shouldn't be as clean. Also I assume a too distant build plate probably isn't the problem because the top layers are underextruded as well and the shell/infill in between looks just fine. I noticed increasing the temperature to 260°C slightly improved the top/bottom quality but this is quite hot and even 5°C hotter than the manufacturers specifications (Innofil ABS). So this can't be the solution, it just reduces the symptoms.

The filament is quite old now - it's been out of the box for a year and a half or so. I've had some extremely strange behaviour in the past from filament that was too wet. But I baked it for 3-4 hours at 70°C just a couple of days ago so I'm thinking it should be fine. And I had the same problem with PLA too.

 

I used this test cube as reference model

https://www.thingiverse.com/thing:1278865

 

Also the corners came out really blotchy and curled upwards. In one of the standard troubleshooting guides I read this might be due to overheating. So at first I reduced speed and temperature down to 220°C. same problem. Strangely the print came out better when I went up to 255°C, for helping the contour of the Z at the top adhering to the top layer fill lines but still not very good.

 

Sliced with Cura 3.5, using standard ABS settings at first, then changing around initial layer line width/height, flow, temperature and skin overlap percentage etc. but to no avail. Also I feel like the standard settings should be at least somewhat okay since they have worked in the past.

 

So my question is where do you think the problem might lie? Should I buy new filament or might the problem be Cura or firmware related? Or even mechanical?

 

sorry for the long text

thanks for reading!