A single spark plug out of 4 on our 1.4MPI engine keeps going dirty from carbon buildup. This causes engine issues (losing power). After being cleaned it's fine for a while, but the issue returns after a month. The engine is also a bit more dirty on the outside where that spark plug is located (last one on the right). I found out bad spark plugs could be caused by bad injectors, incorrect air/gasoline ratio or oil in engine. Since only 1 keeps going bad I assume that rules out air/fuel ratio. Does it also eliminate bad injector or could it be either of those other 2 issues?
In my experience, new car, low mileage, carbon build up in all cylinders = bad quality fuel, or wrong additives for the application (been there, tested that..)
High mileage vehicles... carbon build up in only one cylinder? That has to be either a leaky fuel injector (putting too much fuel into that cylinder) or a leaky valve stem seal.
You can test the fuel injector (or have it done by a shop)... they accurately measure the output from an injector and compare that volume to specification.
A leaky valve stem seal? As far as I know the only way to verify that is to remove the head, pull the valves out and inspect the stem seals. And as cheap as those seals are, if you are going that far, you might as well replace them all.
I know they make Official Sleazy Sam's Snake Oil Goo** advertised to help seal valve stem seals, where you dump a tube of the stuff to the top of the engine, but I don't trust those type of products.
**Obviously not the real name.