  The majority of galactic chemical evolution models assumes the instantaneous
mixing approximation (IMA). This assumption is probably not realistic as
indicated by the existence of chemical inhomogeneities, although current
chemical evolution models of the Milky Way can reproduce the majority of the
observational constraints under the IMA. The aim of this paper is to test
whether relaxing this approximation in a detailed chemical evolution model can
improve or worsen the agreement with observations. To do that, we investigated
two possible causes for relaxing of the instantaneous mixing: i) the ``galactic
fountain time delay effect'' and ii) the ``metal cooling time delay effect''.
We found that the effect of galactic fountains is negligible if an average time
delay of 0.1 Gyr, as suggested in a previous paper, is assumed. Longer time
delays produce differences in the results but they are not realistic. We also
found that the O abundance gradient in the disk is not affected by galactic
fountains. The metal cooling time delays produce strong effects on the
evolution of the chemical abundances only if we adopt stellar yields depending
on metallicity. If instead, the yields computed for to the solar chemical
composition are adopted, negligible effects are produced, as in the case of the
galactic fountain delay. The relaxation of the IMA by means of the galactic
fountain model, where the delay is considered only for massive stars and only
in the disk, does not affect the chemical evolution results. The combination of
metal dependent yields and time delay in the chemical enrichment from all stars
starting from the halo phase, instead, produces results at variance with
observations.
