A method for adjusting an on-time calculation model or a lookup table used
for determining on-time values for an injector of a cylinder in a
combustion engine useful in a motor vehicle, where the injector is
connected to a fuel accumulator. The method comprises the steps of
injecting a test fuel quantity exceeding the maximum allowed fuel
injection quantity under such conditions that no ignition of the injected
fuel takes place; measuring the pressure drop in the fuel accumulator
during the test injection; establishing the actual fuel quantity injected
during the test injection based on the pressure drop; performing a
comparison based on the actual fuel quantity or the corresponding on-time
value established with the aid of the on-time calculation model or the
lookup table; and adjusting the on-time calculation model or lookup table
in dependence on the result of the comparison. An apparatus which
performs the method. A computer program to perform the method.