Only one spark plug is getting carbon fouled?

Hey guys,

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?

Try running a slightly hotter spark plug in that cylinder.


I have thought about that and will probably try that next. I just can’t think of WHY only that one plug is getting fouled and not all of them.

Is there any other part that I haven’t listed that could be the culprit? I just did the fuel pressure regulator today so only time will tell if that fixed the issue

Although it is rarely confined to one cylinder, I wonder if the valve stem seals for that cylinder may be defective.


Run a dry and wet compression test on that cylinder. You may be losing one; or all of them actually and those deposits could be oil instead of gasoline.

Just my 2 cents, but a compression test is mandatory with any engine suffering a performance problem, fouled plugs, dead misses, and so on.


Smell the plug to see if it smells like gas or oil.


Wouldn’t low compression in a cylinder almost always throw a code? I don’t have any codes

There is no low compression code. There is no sensor to measure that. You might see a misfire code and it could be intermittent. Leave that plug in a while longer and you are highly likely to see a misfire code set.

You can’t rely on the cars computer to tell you everything that is wrong. It isn’t that smart. Sometimes you have to go back to the basics. Vacuum tests and compression tests.

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Maybe this will help


If you do a google for fouled plugs, one of the first things that comes up is “if #4 is fouling, check valve guides”. Not that anyone on the internet knows what they are talking about.

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You mean everyone but you and me I’m sure.

Me anyway.