RUnning Rich

Start by replacing your oxygen sensors. If they're old that is. Air filter next. Set your throttle position sensor and replace if necessary.

Do you have a check engine light?

Clean your MAS and replace plugs/cap/rotor/wires if they have not been changed. Check your fuel pressure if you have an adjustable regulator.

What kind of mods do you have?

It appears the car was recently purchased. Has it always done this? If not, what was your last modification? If so, this is a little harder since you don't know what mod/event caused the problem. This is strange since most of the time problems cause the car to run lean, i.e. vacuum leak. You really need to address this quickly since you'll burn out your cats. In my humble opinion, I would look at the following:

a. Too much fuel pressure; this sort of happened to me when I had too much pressure. Be sure to test it with a reliable gauge at idle and off idle, i.e. with vacuum to fpr and without it.
b. Problem with fuel return line?
c. Verify that your ECT and ACT are working, if not the computer could think your motor is colder than it really is and thereby making your F/A ratio too rich.
d. Check MAF
e. MAF to injector not matched correctly?
f. Verify O2s readings
g. Leaking injector

Since you didn't say anything, I'm assuming the car idles steady, the car doesn't search for idle, and the car doesn't surge.

Good luck,

P51-302, welcome!
i like your thinking. i agree, especially with C, A, D and F

just cuz your CEL is not on, does not mean that there are not codes stored in the KAM. check them.

as said, the ECT and lambdas can read within spec, but not correct for the situation (i.e., ECT says motor is cold when it is warm). this will not trip a code, since the sensor reads within spec, but not correct for the situation (in this example a warm motor). leads to overly rich

the other thought i have is that you said you cleaned your K & N a month ago. if you liberally oiled it, the excess oil can and will travel upstream and coat the MAF wire. that can cause your problems.

i would swap plugs, cap and rotor. they are cheap and important. people do stupid things when it comes to plug choice and gapping and such. plug wires are more expensive, so i would recommend ohming them out before buying new (unless your pocket is smoldering with green smoke.
good luck.