My 2001 V6 mustang cylinder 4 spark plug has been repeatedly getting carbon fouled. I have changed that spark plug twice now in the past month due to it fouling out. Today I noticed car was hesitating/stumbling so I removed the cylinder 4 plug and sure enough it had a ton of carbon on it. This plug is only a week old. The same thing happened to the brand new plug before that. I checked a couple other plugs and they had a small amount of carbon on them but nothing abnormal. Only cylinder 4 is having this problem.
I have already tried the following in the past month…
-Used iridium and platinum plugs the first and second time changing them respectively, and used ones specifically meant for my car.
-Changed the coil pack a month ago before replacing the first set of plugs
-Replaced both upstream O2 sensors
-Cleaned the MAF sensor
-Put a new K&N filter on my cold air intake
-Replaced the fuel injector for cylinder 4
-Ran a bottle of fuel system cleaner and also a bottle of cataclean through the gas tank
-Replaced the coolant temperature sensor
There are no check engine light codes.
Despite doing all these things, I removed cylinder 4’s plug again today and found it was covered in matte black residue (carbon) I am positive it is carbon and not oil as the plug is not wet at all.
I cleaned the spark plug and put it back in. Also today, I just replaced the fuel pressure regulator an hour ago and hoping this fixes my issue.
I know carbon fouled plug(s) indicate a rich condition but I have now done everything that could be causing the issue. I removed the other plugs and they have a very small amount of carbon on them except cylinder 4 plug which continues to get a lot of carbon buildup. Any suggestions for things I haven’t tried yet? Am I likely looking at an internal engine problem?