The paper describes simulation tests of a gas injector with the use of numerical fluid mechanics. The tests were carried out on a CNG flap-valve injector which was connected with an inlet manifold by an additional injection pipe. Such a model was identified and calibrated on the basis of the test stand research. The paper presents the tests results in transient conditions (during valve opening and closing) and in a steady-state flow. The analysis concerns pressure and gas velocity flow in selected points and sections of the injector and the injection pipe. The processes occurring in selected areas of the injector have been presented. Moreover, areas essential for the useful characteristics of the injector have been illustrated.