Carbon buildup due to free-flowing exhaust and/or K&N filter?

Topeka, Kansas, USA
1998 Chevy Tahoe
I was wrestling some major carbon buildup until recently, on my 1998 Tahoe, upon which a past owner bestowed free-flowing exhaust duals. I ended up replacing all plugs and wires, which helped for a while, but then carbon got plug #4. I replaced the K&N air filter with a standard one, and that helped some, and then I had #4 replaced again, and she's been smooth since. I have heard that issues like these are common with free-flowing exhaust, that many owners run 92 octane to fight it. I can't get 92 locally, I do use 91 on the advice of the previous owner. Any tips on doing better than this? I'd like to find a way to replace the OEM air intake with something freer which still takes in wheelwell-air, but am concerned about handling this overall situation first.
You seem to have an unusual problem and I can't see why a free flowing exhaust system can lead to one cylinder getting more carbon build up than any other cylinder.

Your filter choice is not the culprit IMO.

Provided that you are using the correct heat range plugs then I would think that the #4 coil may be a bit weak.
We are using strictly Chevrolet-recommended plugs for this vehicle. OEM was platinum, replacement recommend now is iridium which was put in. Could have been one marginal plug I suppose. A different mechanic (known for spending my money) thought via engine sounds that there might be a crack in #4's area on the exhaust manifold, I have wondered if that could take operating temperature down and thus produce buildup. We're pretty sure that overall combustion heat generation is dropping due to the air ionization I'm doing, and I have thus also wondered if #4 is just the low-hanging fruit of buildup, whether I should therefore boost overall coil strength. But thus far, since changing that last plug and the filter, all has remained well. So I'm far from certain what to keep sharpest eye on.
If it is oily carbon on the plug there may be a ring sealing or valve stem sealing problem so have you done a compression test ?

You could go up 1 heat range IF it is oily black carbon.
My mechanic did a compression test on all cylinders, he gave a number I don't remember on #4, he said it was a little low but nothing particularly unexpected in a '98 with 237k (miles) or so on it.
Carbon build up is typically caused when the engine is running rich, and the burn is not as clean as it could be.

Does the engine temp sensors and thermostat work ok? If they think the engine is colder than it is, it will dump more fuel to warm it up.

Are the injectors in good order? A fine mist spray gives a nice clean burn, a jet doesn't as you only ever get a partial burn.

Have you checked the ECU for error codes and fault codes, that might throw something up.

The Air flow meter and exhaust o2 sensors can also affect the burn efficiency if they are not up to spec.

Are you still seeing around 12-13volts when the engine is running? It's a long shot but a lower voltage could affect so many things in and around the engine.

As regards the air filter, I recommend a higher flowing panel air filter, made from a cotton gauze to give good filtration but better intake airflow. It will fit in the OEM airbox, so no need to site a fresh air feed or the other issues you get with an induction kit.
Thanks, that helps. I don't think I'm burning rich: nothing at all is visible coming out the tailpipe shortly after startup in current weather, and never anything except a bit of gray in any weather.

I have been thinking about having sensors gone through. Two out of the three oxygen sensors are probably old; one was replaced a few months ago, and reportedly it was awful. ECU reports the other two sometimes erroring, naught else, though I'll check it more often and make a list, I want to keep this vehicle. I replaced the mass air flow sensor myself before the #4 plug was re-replaced, and that definitely helped for a while. I have been wondering if there is a list somewhere of a good order of sensors to have checked visually. My paying profession is IT, and if I mostly relied on the machines' own readouts I'd get very little done. I'll probably get the other two O2 sensors replaced next time I have her in the shop. I have been wondering about the MAP sensor, I wonder if those are prone to wear..

I read about about design of the injector system on this engine. If I understand it right it's a kind of weird hybrid between EFI and TBI, heavy on the TBI. I'm still not sure whether there's one injector nozzle per cylinder, but the one or eight are located where the carburetor would have been, with a distribution gadget. I will think that the system is probably in fairly good order, just by overall behavior, except possibly on the higher end of output when running hard for an hour or so. Apparently the next year's model replaced part of what I have with something a bit better called a "spider", which is a bit more proactive and consistent in delivery, and apparently one can swap the '99 as a direct replacement for mine, and I may have that done sometime. But anyhow, the model I have isn't known for rich, it is known for a tad lean, so I don't think this is the carbon cause.

I do send various fuel-system cleaners through every once in a while. The reason why I'm still looking at carbon, is the carbon problem first showed up immediately after I used a very commonly recommended fuel system in this area -- Chevron Techron -- for the first time. I think it's a total of three worthwhile local mechanics and four or five auto-parts store people who have recommended that stuff. So my theory (given by two of the mechanics) is that the Techron loosened some gunk which then pounced on my plugs. Thus I am looking to do preventatives :-)

Electrically we seem to be quite good. There is a custom digital voltage meter which a previous benefactor put on the dash, and it registers a consistent 13.6 or higher, usually more like 14.0 or even 14.2. One thing I did early on was to replace every single fuse in both fuse boxes, and it was amazing to see her come more alive afterwards.

The air filter and related is a very interesting thought. I did a bit of research a few years ago and found some likely sources, I'll start a new thread about that!
I can categorically state that a K&N filter with a full flow exhaust is not your problem here.

If it is only one cylinder that is suffering, then you have a piston, piston ring or valve stem and seal problem. Which at the milage you state would not be unreasonable to expect.

If you were getting a dry sooty coating across all plugs, more or less evenly then I would suspect an injection issue and a trip to the dyno tuners to remap it.

It is essential that when a K&N and and a free flowing exhaust are fitted, the vehicle must have its fuelling map adjusted to match. These items are not "fit n go".

If this has not been done then you will have all kinds of fuelling errors.

Further to the compression test. Find a reputable garage who will carry out a cylinder leakage test. This is a much more accurate test and will determine exactly where any pressure leakage is in each cylinder. Providing a very accurate map of the the state of each cylinder. Highlighting cracked or broken rings, wear, cracks, etc.

But at 200k, I suspect its getting tired, and time for a rebuild.

Good luck.

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