Welcome to the forum GeeTee, one question, is this a Nox you have owned since new, or just new to you? I'm a big believer in Techron, mainly because it keeps the injectors clean and for some reason prevents problems with the fuel level sensor on the fuel pump in the tank, maybe because it removes varnish on the float shaft. Anyhow at this point in your case I doubt it will help much with combustion chamber carbon, and certainly not with carbon buildup on the back of the valves. I always stock up on Techron when one of the big box auto parts stores run a 2 for 1 sale on it as it's spendy. As far as the valve cover, someone posted on here not all that long ago about having that hot tanked at a machine shop to clean up the crud, mainly to prevent timing chain and guide problems and he said it's not all that hard to remove, although my wife has a 17 Nox with the 2.4 I haven't gotten that far into it other than routine maintenance as it only has 20K on it and I'm an old SBC V8 guy. Only other thing I can add is do frequent oil changes with full synthetic Dexos oil.