Diesel fuel injector issues
From reading the posts regarding fuel injector operation, it would seem that there are varied ideas about how they actually work.
The job of the fuel injector is to turn the liquid fuel delivered to it whether by a digitally controlled unit injector (PD) or mechanical Injection pump (ALH) and turn it into a fine mist that ignites and burns in the presence of highly compressed, hot air. That is all the injector does.
It does this job by basic principles of fluid under pressure. The pump portion of the injector or the injection injection pump produces high or extremely high fuel pressure and tries to force the fuel past a needle and seat, the needle being held on its seat by a strong spring. The pressure builds up because fuel under pressure is acting on a very small area to try to unseat the needle. When it finally does unseat the needle, this is called the nozzle's opening pressure or VOP.
Once fuel passes the needle and seat, it enters the sac hole area from which the tiny orifices radiate. The orifices are drilled at specific angles to suit the engine they are intended for, this angle and the size of the orifices is critical for good combustion and engine performance.
Once the needle has been unseated, the pressure drops rapidly, the needle reseats, pressure builds again, needle unseats, again and again till delivery from the pumping device is cut off. The rapid seating, unseating is called nozzle chatter and is audible in some bench testing. The needle finally seats at the end of injection and from then until the next injection cycle seals the fuel supply from entering the combustion chamber.
Fuel injectors do wear, needle to seat wear does happen which affects VOP and chatter. Parts that affect needle lift do wear but we are talking "micro" wearing surfaces. Strong springs lose spring rate over time. Orifices do wear and become larger but microscopically. Orifices do become plugged but this is usually the result of micro beads of water contaminating the fuel, the water turns to steam in the heat of combustion but leaves residue behind.
When enough or all of the orifices plug, the nozzle tip breaks off because of hydraulic forces, injection pumps have no relief valves so the nozzle tip is the weakest point. Pressures are extreme when a nozzle tip fails.
If you have read this so far you will see that there is no place for injectors to get "dirty", they perform all their useful lives having been delivered ultra clean fuel. Understanding this, you will see why I don't get too lathered up about fuel additives purporting to "clean" dirty injectors. The tips protruding into combustion chambers get discoloured from soot but that is all.
That is all from me as well.