A process for leak testing in fuel injection pumps that have a pump housing that can be plugged into a motor housing, in which an outlet of the fuel injection pump, which feeds into an annular chamber that is divided by sealing rings and is normally discharged by way of a line, is closed with a material in order to carry out a leak test. The material is plastically deformable and dissolves with the heating of the fuel, and due to its plasticity, produces a sealed closure of the annular chamber in relation to the interior of the fuel injection pump so that the annular chamber can be loaded with a testing pressure via the line.