Marlin/extruder underextruding

I have an E3D v6 1.75mm bowden.

When I print, there is a gap between infill lines, and also when printing rectilinear/hexagonal infill lines seem to be pretty slim. I'm attaching some pictures to show the issue.

My settings:

Slicer: Slic3r
Nozzle size: 0.4mm
Print speed: 30mm/s
First layer print speed: 15mm/s
Travel speed: 90mm/s
Retraction length: 0.8mm
Retraction speed: 40mm/s
Extrude ratio: 1 (tried also with 0.48mm extrusion width, I remember it's the default S3D setting, so I gave it a try)
Print temperature: 220 °C
Material: PLA


Firmware settings:

Firmware: Marlin 1.1.0-1
Motion system: CoreXY
Acceleration: 1000
VMax X: 300
VMax Y: 300
VMax Z: 5
VMax E: 80
Xsteps/mm: 80.05
Ysteps/mm: 80.05
Zsteps/mm: 400
Esteps/mm: 96.5


Disclaimer: I set Esteps/mm to 96.5 because I calibrated the printer as described here, so I'm 100% sure it's an accurate setting. Same for X,Y,Z steps. I printed a cube and calculated the real steps per millimeter, and it's accurate too. The extruder motor works, it's not the issue.

While printing I tried to release the extruder motor clamp and push the filament by hand: it extruded 1mm thick lines, so the extruder is not clogged and is perfectly capable of extruding the required amount of PLA.

I saw this thread and decided to try lowering the max extruder feedrate from 80 to 25, I still have the issue. What is goin on here? The V6 seems to be extruding fine if I push the filament by hand, so I assume the issue is with Marlin? How does that parameter affect the whole print?

Thank you

Some E-step fine tuning will fix this issue [reprap.org]

It almost looks like your bed is not flat. In the second picture you can see it filled in properly in one area but then filled in with gaps in another area. That makes me wonder if your bed is not flat and the gapped area is lower than the properly filled area.

Hi PDBeal.

I can confirm that the bed is flat. I'm using the same bed since an year, I never had this issue. The issue came after switching to Marlin firmware and E3D V6. Before the change I had Repetier with a direct drive MK9 extruder. But I really don't know what could cause the issue