I've had the same issue - for a couple of years now!! I can print perfect hi-res photos with full colour range including blacks, but if I print a document, the black ink is awful - it is grainy & misses every other line!
What I have to do now is to screenshot my document, import it into Adobe Lightroom, adjust the tones etc & then export as jpeg. When I then print it, the black is rich & almost perfect...
So it seems that 'quick print' fails & photo print works... the fix I will apply tomorrow if I haven't sorted it in 24hrs time is to simply throw it out of the window & buy another brand.
(KODAK customer support is the worst, not even gonna try that avenue again!)