Influences of deoxygenation on the resistivity and the magnetic properties of La0.7Ca0.3Mn0.92M0.0sO3-δ (M =Al, Cr) are compared to understand the peculiar doping effect of chromium in the ferromagnetic manganites. It is found that the double resistivity bumps exhibited by the Cr-doped compounds can be tuned by the oxygen content. We have related the double-bump feature with the well-known inefficiency of Cr in lowering Tc, and interpreted both the features consistently in terms of large-scale phase separation. It is suggested that the large scale phase separation is a general characteristic of Cr-doped manganites, and many unusual phenomena exhibited by Cr doping are directly related to the large-scale phase separation.