Fuel injectors that are clean and functioning properly are THE key element to getting the best performance and fuel mileage from your diesel engine (and gas engine, too). They're essential for injecting the fuel in an optimally atomized mist that burns cleanly and enables the engine to get the most out of the fuel.
The biggest problem with injectors is that, over time, deposits build up in the tips of fuel injectors. This typically happens when there's fuel left in the injector tips after the engine is shut down. The leftover fuel cooks and turns into deposits that alter the nice, clean spray of the injector.
If the fuel injectors have deposits, it's important to clean them to restore engine performance to the way it was. At a professional garage, they may use an ultrasonic machine to clean the injectors out. Those who have some mechanical skill can pull the injectors themselves, disassemble them, and clean the parts in an acetone bath and wire brushes. You have to be careful using this approach, as it's easy to damage the parts of tips. And you have to know how to put them back together.
If you're not up to taking the injectors apart by hand, you can try periodic fuel injector cleaner applications, which clean the injectors through the addition of concentrated detergent packages delivered through the fuel. One thing to keep in mind, though, is that sometimes the injectors are not dirty, they are worn. And no injector cleaner or fuel additive is going to be able to fix a worn injector. Short of taking the injectors to a mechanic shop for bench testing, how do you know if the injectors are worn? You can try using a chemical cleaner and see if the performance reverts back to the way it's supposed to. If it does not, it may be well worth taking the engine in to a professional who can address what is most likely to now be a mechanical issue.
As an aside, there are some references on the Internet to the practice of adding gasoline to diesel fuel in order to clean the injectors. This is a bad idea because gasoline has the potential to change the combustion qualities of diesel fuel and also to strip essentially lubricating films from diesel parts in areas like the common rail injection system. So we would never recommend doing that.
Here are some other general guidelines for keeping your diesel injectors clean and working properly.
Use High-Quality Fuel: Ensure you are using clean, high-quality diesel fuel with the appropriate cetane rating for your engine. Contaminated or low-quality fuel can lead to injector deposits.
Don't Neglect Your Fuel Filter Replacement: Change your fuel filters at the recommended intervals in order to prevent contaminants from reaching the injectors. A clean fuel filter is crucial for injector protection, especially in today's common rail diesel engines.
Maintain Proper Fuel Pressure: This is one we don't normally think about. You want to ensure that the fuel system maintains the correct pressure levels as per your engine's specifications. If you're getting performance problems that you suspect are due to an issue with fuel pressure, you'll want to have a mechanic check that out. Fuel pressure issues can come from sources like the fuel pump, fuel filter, fuel lines, or even a malfunctioning pressure regulator.
Engine Oil Quality: Use the right engine oil with the correct viscosity rating. Dirty or contaminated engine oil can indirectly affect injector performance. And you should be sticking to the recommendations for changing your engine oil, anyway.