The invention relates to a method of operating a diesel engine having an engine controller which controls the operation of the diesel engine as a function of characteristic maps and permits rich/lean control of the diesel engine. The engine controller includes a computer which effects a changeover to rich or lean operation of the diesel engine as a function of predetermined changeover criteria, a sensor system which communicates with the computer and monitors parameters needed for changeover criteria, and a memory which communicates with the computer and in which the characteristic maps for operating the diesel engine are stored. The computer effects a changeover from lean to rich operation when all the changeover criteria in this respect are satisfied, and effects a change back from rich to lean operation when at least one of the changeover criteria in this respect is satisfied.