First post here, so hello!
I have a 2008 Honda Fit Sport, manual transmission. It has ran great since I got it at 80k miles. It currently has 128,000 miles and has started having misfire issues. The first codes were misfire on cylinder 1, 2, and 4, as well as an upstream o2 sensor fault. I replaced the upstream o2 sensor, reset the codes and ran it again. Now it's only misfiring on cylinder 2 (code P0302). I inspected all the spark plugs, cylinder 2 spark plug was black, indicating that the car is running rich on cylinder 2, the other spark plugs looked fine. I replaced all the spark plugs (NGK) and replaced all the coil packs (Hitachi). The car gained a little power, but it is still misfiring on cylinder 2. My next thought was to do a valve adjustment.
I'm not a professional mechanic, but with enough research I got the gist on how to do a valve adjustment. When doing the valve adjustment, a handful of the valve gaps were too tight, but none of them were too loose. I adjusted all the valves and put it back together.
Now, the cars idle sounds a little smoother, but it is still misfiring on cylinder 2. Somethings I have noticed: When the car is cold, it seems to run better, doesn't misfire as bad, and the engine light doesn't come on. Once the car has warmed up, the check engine light comes on and the misfires become very apparent. I have also tried disconnecting the battery for an extended period of time after resetting the codes, and trying it again, no luck.
Banging my head against the wall with this thing, my next thought is to check fuel pressure for an issue on the injector perhaps? Or maybe an issue with the MAP/MAF sensor? I am open to any advice. Thank you in advance!

 

Checking for a leaky injector sounds a good move. If you can measure the fuel pressure you could check if it holds after the engine is turned off (if not, could be a leaky injector). Also try listening to each injector with a stethoscope while it's running, and see if #2 sounds different.
You could also try disconnecting each injector one at a time and see if one of them makes a lesser (or possibly greater) difference in running/idle speed. Also, check the resistance of the injector coils in case there's an odd one out (I think they should be around 12 ohms). Or, more of a rigmarole: you could swap the suspect injector with another one and see if the misfire and rich condition moves with it.
I'd also suggest doing a cylinder compression test, in case cylinder 2 is leaky.
Good luck!

 

Thank you for the response, I'll start out with checking the resistance of the injectors and fuel pressure, I think that's the best move. Appreciate your input!

 

i had a similar issue with my 2007. Before you do a lot of work switch the coil pack from #2 to another cylinder and see if the problem follows it. I had two brand new coil packs fail on me. If it doesn’t move you could also switch out the spark plug after you test the coil. One thing at a time. If the problem doesn’t move then you have eliminated those items as being suspect.

 

+1 on switching coil packs. I've had them fail a few times too, even the Hitachi.

Another thing you could try is some top end cleaner, in case there's gunk in the cylinder causing misfire from the previous rich condition.

Finally, and this is unlikely, but you could also check the EGR and PCV and see if either one is clogged or not moving correctly.