A method for adjusting an on-time calculation model or lookup table used for determining on-time values (t) for an injector of a cylinder in a combustion engine of a motor vehicle where said injector is connected to a fuel accumulator of a common-rail fuel injection system, the on-time values (t) for the injector being controlled by engine control means with the aid of said on-time calculation model or lookup table based on a fuel pressure value and a fuel injection quantity value (δ), said fuel pressure value representing the fuel pressure in the fuel accumulator, said fuel injection quantity value (δ) representing a desired quantity of fuel to be injected into the cylinder and the on-time value (t) representing the associated on-time for the injector, comprising the step of: A) interrupting the supply of fuel to the fuel accumulator, the method being characterized by the steps of: B) executing a test injection of fuel into the cylinder with the injector by making the engine control means apply an on-time value (t test) for the injector corresponding, in accordance with said on-time calculation model or lookup table, to a fuel injection quantity (δ test), here denominated test fuel quantity, exceeding the maximum allowed fuel injection quantity of the cylinder, while maintaining the supply of fuel to the fuel accumulator interrupted, this test injection being executed under such conditions that no ignition of the injected fuel takes place in the cylinder; C) measuring the pressure drop (ΔP) of the fuel in the fuel accumulator during the test injection; D) establishing the actual fuel quantity (δ actual) injected into the cylinder during said test injection with the aid of a fuel quantity calculation model or lookup table based on the measured value (ΔP) of said pressure drop and the fuel pressure value (Ptest) at the start of said pressure drop; E) comparing said actual fuel quantity (δ actual) with said test fuel quantity (δ test), or comparing an on-time value (t a) established with the aid of said on-time calculation model or lookup table based on said actual fuel quantity (δ actual) and said fuel pressure value (Ptest) with the on-time value (t test) applied for the test injection; and F) adjusting said on-time calculation model or lookup table in dependence on the result of said comparison. A) interrupting the supply of fuel to the fuel accumulator, the method being characterized by the steps of: B) executing a test injection of fuel into the cylinder with the injector by making the engine control means apply an on-time value (t test) for the injector corresponding, in accordance with said on-time calculation model or lookup table, to a fuel injection quantity (δ test), here denominated test fuel quantity, exceeding the maximum allowed fuel injection quantity of the cylinder, while maintaining the supply of fuel to the fuel accumulator interrupted, this test injection being executed under such conditions that no ignition of the injected fuel takes place in the cylinder; C) measuring the pressure drop (ΔP) of the fuel in the fuel accumulator during the test injection; D) establishing the actual fuel quantity (δ actual) injected into the cylinder during said test injection with the aid of a fuel quantity calculation model or lookup table based on the measured value (ΔP) of said pressure drop and the fuel pressure value (Ptest) at the start of said pressure drop; E) comparing said actual fuel quantity (δ actual) with said test fuel quantity (δ test), or comparing an on-time value (t a) established with the aid of said on-time calculation model or lookup table based on said actual fuel quantity (δ actual) and said fuel pressure value (Ptest) with the on-time value (t test) applied for the test injection; and F) adjusting said on-time calculation model or lookup table in dependence on the result of said comparison.