E3D V6 underextruding always? 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.

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.

Here you can find some pictures that show what I'm trying to describe:


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.

Is it possible that the VMax E parameter is preventing the extruder motor to extrude properly? I mean, could it be that it's trying to extrude at a higher speed than the maximum allowed? Seems weird to me, because comments in Marlin's Configuration.h file say Default Max Feed Rate (mm/s) for that parameter, so if the limit is set to 80mm/s it should extrude just fine. If this is the issue, how should I solve it?
Major factors affecting extrusion:


Nozzle diameter
Extrusion ratio
Extrusion stepper calibration
Filament actual diameter
Extrusion width setting
Nozzle temperature


According to the photos, temperature is acceptable (may be a bit high, but it is not a problem in this case). Extrusion ratio is 1 and should not be changed in normal operation. Extrusion width is often set to auto but sometimes calibrating it gives better results in slic3r. If stepper is calibrated properly, then I would check for nozzle and filament diameters. For nozzle it should be enough to verify that 0.4 bit fits in and 0.5 doesn't. Filament can be measured by digital caliper. Difference between 1.75 and 1.73mm may have significant difference resulting in under-extrusion.