Method of determining fuel injector characteristics

The invention concerns a method of operating an internal combustion engine with a fuel injection system comprising at least one fuel injector for performing injector events in a respective engine cylinder, the fuel injector being associated with an intermediate fuel tank of pressurized fuel. The method comprising the steps of: a) performing at least one injector event by applying to said fuel injector a drive pulse having a predetermined pulse width; b) measuring the resulting pressure in said intermediate fuel tank; c) repeating steps a) and b) with drive pulses of increased pulse width until a predetermined pulse width is reached; d) storing a test data set comprising data indicative of the rate of pressure loss for corresponding pulse widths; e) determining an injector characteristic from said test data set by detecting expected variations of said data indicative of said rate of pressure loss.