A computer simulation is used to investigate the motion of a marked particle of mass M in a free gas of particles with mass m = 1, for large times. Previous results seem to indicate a non-Wiener behavior for the rescaled trajectory when M != m. The results reported here, with better statistics, are compatible with the Wiener hypothesis. The Einstein relation between mobility and diffusion coefficient is also investigated. The results indicate that it holds both for M = m and for M != m.