I think the physical problem in my HX is in the EGR passages, and that Marvel Mystery Oil cleans up just enough crud to allow some flow there. From photos the EGR passages look to be about 1/8 inch diameter channels machined into a casting below the fuel injector rail. (See this post:
How to Clean EGR Ports / Passages - Honda Civic Forum. And you can google for more writeups, youtubes etc.)
Somewhere I read the HX opens the EGR valve during lean burn; clogged passages would prevent the "wanted" flow of exhaust gas back to the intake system. I've occasionally had CEL stating insufficient EGR flow, CELs not helped by cleaning and later replacing the EGR valve.
I know, really I should clean the EGR passages but I'm not too much of a mechanic and don't relish the thought of taking apart all that stuff including the well-sealed fuel system components. One of these days in warmer weather I might get up the nerve and time to do it.