Abstract: Cleaning, maintaining, refurbishing, and/or diagnosing engine components including fuel system components, e.g., such as fuel-injectors. In one embodiment, a method of cleaning a fuel system component is provided. The method includes supplying a cleaning solution through the fuel system component, discontinuing the supplying of the cleaning solution, repeating the supplying of the cleaning solution and the discontinuing of the supplying of the cleaning solution a plurality of times, decoupling the source of cleaning solution, and connecting a source of fuel to the fuel system component, and supplying fuel through the fuel system component to re-establish fuel-based operation. In additional embodiments, a method of processing a fuel system component while it remains connected to an engine assembly, and a method of diagnosing a defective fuel system component, are provided.