This is a common Canon printer problem which manifests itself amongst many models and is not a head cleaning issue, nor a timing strip contamination problem, nor a head re-seating issue, and is a problem that Canon Support has not yet released a solution for.
The problem normally surfaces after about two years of average use and begins by affecting the sharpness of black text but does not normally affect pictures.
As a test, print a body of text in Black ink which will show up as blurry, then change the text color and repeat. If the color text is sharp, then you have the same problem.
Some users have reported that changing the advanced print settings to photo and matt photo paper fixes the problem, however I have found that what this does is make the text consistent, but thicker than it should be. It is not a fix on the two units I've tested. Examples of affected printer outputs are:
https://drive.google.com/file/d/1Vm8PJhXEzdBERJ57OvQDq6WeBnLOR-Nf/view
https://drive.google.com/file/d/1cA7aOmr5gJv-yz0dknssQuVOL64loqjM/view
https://community.usa.canon.com/t5/Printer-Multifunction-Support/MX922-printing-double-lines-double-...
https://superuser.com/questions/1130333/text-is-not-printed-neatly-by-my-printer-how-to-repair-it/14...
http://multifunction235.rssing.com/chan-58362528/all_p279.html#item5574
https://www.youtube.com/watch?v=kK3u_XNvzGY
This problem is not limited to a particular Canon Model, but manifests itself across a range of models. It is possible that affected printers use the same model head, but I have not been able to validate this yet.
It appears that the problem may be related to firmware or an inner head mechanical / electronics problem, because since the printer can print some text perfectly (for example colored text) whilst displaying the problem on black lines in exactly the same region (for example red text surrounded by a black text box made up of a single thin black line). This infers that it is a logic / black print head section issue.
The other thing that is apparent is when performing a manual head alignment with this problem, many of the adjustment parameters are at +5 but the head is still out of alignment, suggesting that the required setting is out of range. Since the vertical alignment lines of the PGBK printer head alignment printout initially begin as a single line, then form two lines, and then converge back to a single line at the bottom of the 16 row printout, I tested a theory that this may be due to a bent rail, possibly caused by pressing down on the head assembly when loading and locking new printer ink cartridges.
On one faulty printer, I forced the printer head assembly upwards to try and bend the rail in the opposite direction and then conducted another print test. The result was identical to the previous output, indicating that the rail was not bent in the middle. Also if it was bent, I would expect that the color printout would be affected as well, which it isn't.
I also eliminated the possibility of faulty in cartridges or ink causing the issue by replacing them with new ones and retrying the tests. I even tried adding solvent to the black ink head input in order to clean it, but there was no change to the results. As a final test I added solvent to the black in and this resulted in a splattering of dots around the printed characters, but the inherent problem was still there, so no change. The splatter was expected due to the reduced viscosity of the ink.
In summary, none of the recommended solutions offered to date by Canon will fix this problem and I am currently engaging with Canon Support in order to try and identify the root cause of this issue.
