Originally Posted by MrChrisLS3
On the Direct Injection engine, using anything in the fuel tank isn't going to do much to anything other than the injector itself. Google Direct injection+valves or carbon. You'll get a pretty clear picture of what's going on here.
I think you posted that you have around 44K miles. If that's the case, you might be beyond the chemical top end clean. The trouble with DI that hasn't been solved yet is that there is no gas sprayed on the back of the valve to "clean" them. So, the oil residue from the PCV builds up and bakes on the valves and valve stems. This build up becomes rock hard, and chemicals, like Seafoam, or GM Top End Cleaner aren't going to do much. In fact, at this point, they more harm than good.
Once this carbon gets baked on, and builds up, it begins to flake off. There is only one place for these hard pieces of carbon to go, and that's in the cylinder. There, they can stick in the valve seat, not allowing the valve to close completely, thus losing compression. Furthermore, these pieces of carbon can work in between the cylinder walls and the rings, damaging both, again, loss of compression, blow by, and all that goes with that. The carbon pieces that do escape through the exhaust then get stuck to the catalytic converter, creating obvious issues there as well. Then there are smaller things, like the carbon clogging the injector port, getting stuck in the plug gap, things like that.
Unless a chemical top end clean is done as part of regular maintenance, say, every other, or every third oil change, depending on mileage and frequency, it really isn't effective. At this point, the only real solution is to take it in for a walnut blasting of the valves.
Is this your current problem? I have no way of knowing of course. However, think about this, if the dealer told you it was carbon build up on that cylinder, wouldn't it be a reasonable thought that the other five have the same issue? And if they did a chemical clean on it, chances are it didn't really so much actual "cleaning" at this point.
DI engines are great for producing power more efficiently, unfortunately, the PCV system on them has yet to be fully thought through.
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