Fuel Monitoring System

A method of compensating for fuel injector performance variations in a fuel injection system (1) of an engine, the engine comprising a fuel pump (3,4) arranged to supply pressurised fuel to a common accumulator volume (5) that supplies the pressurised fuel in turn to a plurality of fuel injectors (8); and the method comprising: measuring (202) a first pressure within the accumulator volume; sending (204) a drive pulse signal to initiate an injection event in a first injector; measuring (206) a second pressure within the accumulator volume; calculating (208) a pressure difference between the first and second pressure measurements; using (212) the calculated pressure difference to compare actual fuel injection system performance with expected fuel injection system performance; compensating (214) for any deviations between actual fuel injection system performance and expected fuel injection system performance.