A method, carried out in an ECU, for determining the needle opening delay ODinjx of a solenoid actuated fuel injector thereof, said fuel injector including a solenoid actuated valve adapted to actuate a needle valve, said needle valve comprising a needle adapted to move from an closed state to an open state and to a closed state during an operational cycle of said injector, said needle opening delay being the time between when an activation pulse is sent and when the needle starts to move, comprising the following steps: a) providing, for a series of test injection cycles, respective solenoid actuator drive pulse duration information Ton; for each test injection cycle, performing the following b) to g): b) determining, the closing time of the solenoid actuated valve VCT; c) determining, the needle closing time NCT; d) determining, the needle falling start time NFST; e) determining a Control Chamber filling Time CCFT value from the values determined from step d) and step b), where CCFT = NFST - VCT; f) determining, a needle closing delay NCD from the values at steps b) and c), where NCD = NCT -VCT; g) determining a Valve Closing Delay VCD; said Valve Closing Delay being the time between the end of the drive pulse and the valve closing time; h) providing a test plot by plotting the value of needle closing delay NCD for each cycle against the respective sum of the values of (i) drive pulse duration information Ton, (ii) Valve Closing Delay VCD, and (iii) Control Chamber filling Time CCFT; i) determining a threshold value of CCFT, being the maximum value of CCFT obtainablethrough the use of the series of test injection cycles with different respective drive pulse duration information Ton; j) from the determined plot, determining the value of said sum at the intersect of said plot of the CCFT threshold on the NCD axis; k) determining the opening delay ODinjx as the determined sum value at the intersect. a) providing, for a series of test injection cycles, respective solenoid actuator drive pulse duration information Ton; for each test injection cycle, performing the following b) to g): b) determining, the closing time of the solenoid actuated valve VCT; c) determining, the needle closing time NCT; d) determining, the needle falling start time NFST; e) determining a Control Chamber filling Time CCFT value from the values determined from step d) and step b), where CCFT = NFST - VCT; f) determining, a needle closing delay NCD from the values at steps b) and c), where NCD = NCT -VCT; g) determining a Valve Closing Delay VCD; said Valve Closing Delay being the time between the end of the drive pulse and the valve closing time; b) determining, the closing time of the solenoid actuated valve VCT; c) determining, the needle closing time NCT; d) determining, the needle falling start time NFST; e) determining a Control Chamber filling Time CCFT value from the values determined from step d) and step b), where CCFT = NFST - VCT; f) determining, a needle closing delay NCD from the values at steps b) and c), where NCD = NCT -VCT; g) determining a Valve Closing Delay VCD; said Valve Closing Delay being the time between the end of the drive pulse and the valve closing time; h) providing a test plot by plotting the value of needle closing delay NCD for each cycle against the respective sum of the values of (i) drive pulse duration information Ton, (ii) Valve Closing Delay VCD, and (iii) Control Chamber filling Time CCFT; i) determining a threshold value of CCFT, being the maximum value of CCFT obtainablethrough the use of the series of test injection cycles with different respective drive pulse duration information Ton; j) from the determined plot, determining the value of said sum at the intersect of said plot of the CCFT threshold on the NCD axis; k) determining the opening delay ODinjx as the determined sum value at the intersect.