Components that can cause engine to run rich

First thing, go buy yourself a $30 eec4 code scanner, Actron or some other brand, and learn how to use it, it's simple, not a fix-all, but every car owner should have one, either obdI (yours) or obdII, depending on your model year. Secondly, take a fuel pressure reading, any black smoke from your car? If it's rich as you say, there must be some sign of the extra fuel. Any leaks? That K&N filter, is it the oiled one? If it is, get rid of it, will only foul out the maf sensor. The problem with computer codes for obdI, check engine light may not necessarily come on, even if the sensor is not at optimum performance. Only when certain parameters are not met will the light come on, your system is not at all comparable to the newer, 96 on up, obdII. Much more detailed is the newer. My own recent example of no cel, my trans was shifting weird in my 94 Taurus, no light, however there were 3 codes in the hard memory related to shift issues. Back to your issue, any misfiring noted? That would be a reason to have someone do a scope of the ignition system if plugs, wires didn't fix it, of course it could be fuel-related misfire. Another good thing about the little code reader is you can run a cylinder balance test where the injectors are shut down in sequence, one by one, while the pcm looks for a specfic rpm drop, you'll be alerted by codes if any cylinder looks suspicious. As far as o2 sensors, only ever replaced one, on 87 Taurus, because I got a code indicating a lean issue, turns out the sensor wasn't the problem, the dedicated ground for the sensor was, I couldn't figure that problem out for nothing, lucked into a Ford mechanic who told me to check the dedicated ground, even told me where it was, I had noticed that orange wire millions of times, had no clue what it was for, loosened it up, cleaned it, just like new. Only reason, for me, to even think of changing o2's would be if they weren't switching voltages quickly enough, in which case, especially with obdII, you would get a light, older cars such as yours might not necessarily light up, only if a rich or lean condition doesn't meet the less stringent parameters set in the pcm. Go get yourself a code reader, fuel pressure gauge, maybe a compression tester, 3 tools that can take you a long way with any car. Probably talking $125 for all, reader will work up to 95 model year with obdI, I believe some 95's may have been obdII, not sure of that, however a obdII reader can be had right here on this site for what? $70 or so, the other 2 tools can be used, with adapters, on most any domestic car, probably most foreign. Good luck