I am also struggling with the same issue with my filastruder. After extrusion for about 3 hours (filament was very light grey here), my filastruder got jammed, so, i took everything apart, cleared the stuck plastic in the barrel and removed the filter. Later, i tried running the ABS and extruded good filament (light grey) for about 5 hours. but all of a sudden the filament started turning grey to dark grey, and I also found some greyish metallic scraps in the barrel. I cleaned them for the first time. But, when I started extruding the filament I found more of it in the barrel. I tried flushing them out by running the filament for about 5+ hours, but it is of no use. Then i fixed a new filter expecting an improvement in the color, but still it did not work (extruded for about 3+ hours).
I am extruding at a temperature of 170c-180c and with 1.75mm nozzle.

Could you please let me know how did you tackle with this issue?

We shall see...

All of the controller settings match the default except the following:
M50:  150
P:  192
t:  55

Also, there is 3/4" between the end of the collar and the auger.  Not sure if I should decrease it or if that's correct.

I did check temp with another device and it's within range. Yes, this is ABS and I purchased it from your recommended supplier.

Thanks for all your help!

When you assembled, did you check that 1/4" of the auger was sticking out of the barrel with the thrust bearing fully compressed?

Can you post some photos of your setup?

Can you independently verify temperature? Can you tell me what the settings are in the controller? I've had one other user discover that the wrong settings had been set at the factory.

http://www.sestos-hk.com/english/download/d1s-en.pdf

Let me know all of the values, from HiAL to Ep9

I've never seen behavior you describe in over 1200 hours of Filastruder operation so it is difficult to diagnose, but I will try!

I have disassembled and reassembled the extruder several times, as it kept jamming up after a couple of hours of printing.  The filament would start normal, then turn dark grey and extrude so slow you almost couldn't see it move. 
In my last reassembly, I have insulated the entire barrel.  It extruded at about 9" minute at 187 degrees for about 8 hours before turning dark grey/black and coming to a halt.  I know that when I pull the end off I'm going to find a bunch of plastic stuck in the end of the barrel that will go about 2 inches up.
I thought that the heater was too far up the barrel, so I even moved that closer to the nozzel, to no avail.

You can always drill the nozzle bigger. I ship them slightly undersize because it is a whole lot easier to make a hole bigger than it is to make a hole smaller. I insulated the entire barrel, drilled the melt filter nozzle to 1.65mm, and I see 1.6mm filament at 190C, 22"/min.

Hmm, that's low. Try bumping up to 185 or 190C.

Ok, I'll try that.

Btw, I just realized how dirty my prior post sounds. Lol.

At what rate?

I've left ABS (MG94) sitting for over an hour at 185C with no detectable discoloration. I wonder what the difference is.

I get around 6 inches a minute.

I've left ABS (MG94) sitting for over an hour at 185C with no detectable discoloration. I wonder what the difference is.

To chime in, natural ABS is very sensitive to discoloration. For true white, you should be using master batch. What extrusion rate? 1.75mm or 3.0?

Yes, I'm aware that it won't be "true white", but it goes from white to dark grey.  I really don't care what color my filament is, but I'm more interested in what is going on that is causing the discoloration.

I'm extruding 1.75mm.