1. A method of operating an internal combustion engine comprising at least one cylinder equipped with a pressure sensor, wherein, said engine is operated in an injector learning mode, in which an injection parameter associated with a nominal fuel quantity of a pilot injection is learned based on the following measurement routine: performing a learning pilot injection based on a candidate value for said injection parameter to influence injection of said nominal fuel quantity and determining a parameter indicative of the actually injected fuel quantity based on the in-cylinder pressure during combustion of said learning pilot injection; characterized in that said learning pilot injection is operated at an early timing such that combustion thereof is complete before the next combustion starts; and said determination of the parameter indicative of the actually injected fuel quantity based on the in-cylinder pressure takes into account essentially the entire combustion event of said learning pilot injection.