Diesel injectors can suffer from the same kinds of aliments as petrol injectors, including varnish deposits, clogging, wear and leakage. Because diesel injectors operate at a much higher pressure than petrol injectors, their opening pressure can drop over time. Up to 300 psi is considered acceptable, but more than 300 psi means the injectors should be replaced or reset back to their (fully rebuilt) original operating specs.
You may find that one or more injectors need nozzle tips. As the spray holes wear, spray pattern and fuel atomization are affected as well. This in turn, can have an adverse effect on fuel economy, starting, noise, smoke and emissions. Used nozzles tips also lose their temper from long-term exposures to operating temperatures, which means they wont hold up over the miles like a new or fully rebuilt injector.