Method of assessing operation of an internal combustion engine common-rail injection system

A method of assessing operation of a common-rail injection system (1) of an internal combustion engine (2); the injection system (1) having a number of injectors (5), a high-pressure circuit (6) supplying high-pressure fuel to the injectors (5), and a low-pressure circuit (7) supplying fuel to the high-pressure circuit (6); and the method including the steps of hydraulically isolating the high-pressure circuit (6) from the low-pressure circuit (7) and the engine (2); and assessing operation of the injection system (1) as a function of the fuel pressure drop in the high-pressure circuit (6).