About a month ago I purchased a '93 Sentra XE. The car runs great, though its idle kind of fluctuates sometimes. Today I got around to checking the plugs, and though the plugs for cylinders 2-4 are very clean (more white than light brown), I assume that it's because the plugs are brand new. However, the number 1 plug had a lot of black soot, more chalky than wet or soft, making me think rich mixture rather than oil. Then I noticed that the injector for the number 1 cylinder had red plastic, and the other three had tan plastic. I don't know what color the OEM injectors are supposed to be for the GA16DE, but I did find a photo online showing the SR20 injectors to be red. My diagnosis based on that little information is that the previous owner replaced one injector but accidentally used an injector meant for an SE-R, and despite running on the same injection cycle, has a mechanical difference that is allowing an excess amount of fuel into the cylinder, and with the rail pressure of the GA16 isn't being injected efficiently either. I'm hoping a more knowledgeable person on here can concede to, modify, or dispute my diagnosis.