Dual extrusion invalid feedrate

I'm setting up a dual extrusion printer, one head is printing well while the other one have not enough feedrate, putting much thinner lines of plastic (about half the size of the expected amount). I am using the latest version of marlin, Ramps 1.4 and pololulu stepper drivers; the slicer is slic3r 1.2.9.

I tried to figure out the source of the issue by doing this:
- switching extruders + hotend wires : the insuffisent feedrate moves to the other extruder while the previously broken one starts to work well.
- checking drivers microstepping : both of them are set to 1/16th
- replacing pololulu driver on the faulty extruder... problem still occurs

I saw in some forum topics that marlin is considering E step/mm to be identical for all extruders; so I am running out of ideas.... really don't know where it could come from...

If you have an idea about where this behavior could come from, your help would be very much appreciated.

Tried to tighten the grub screw on the stepper gear?
Are you sure the nozzle is free and of the same size?
-Olaf

Well, both steppers and nozzles are fine, because they both work well when plugged as E0, and they are both faulty when plugged as E1 : I figured that out when I reversed board connections.

I had little time to do further testing (I started a print with Cura sliced object and it seemed all right), but my assomption is that there is something wrong with the slicer. Do anyone knows about a feedrate bug in the latest version of slic3r (1.2.9) ? It's like slic3r is assuming that I am using 3mm filament on E1 while I have 1.75mm on both hotends.

I will update my answer after I finish my tests..

Thanks

> It's like slic3r is assuming that I am using 3mm filament on E1 while I have 1.75mm on both hotends.

But you can select filament settings for each extruder separately in Slic3r and you can choose them to be the same (see attached screenshot).
Have you done this?

Indeed !!! I was looking on filament setting tab, while the selection of filament per extruder was on the plater tab... And, as expected, the second extruder had the default 3mm filament selected.

Thanks a lot fixerT Problem is solved.