We have looked back into all Fermi-LAT data to search for possible missed transient sources. We expected to detect new extragalactic gamma-ray emitters which showed faint and fast time variability typical of high z FSRQs. We have chosen 1-month time bin of integration because it is a reasonable time interval for gamma-ray transients to be detected with good confidence. We have integrated photons between 0.1 and 300 GeV using the Pass-8 event-level analysis with the boundary of galactic latitude |b| >10°, to avoid confusion with low latitude diffuse emission. Our analysis spans the first 8 years of data, to be consistent with the forthcoming 4FGL, divided in 96 months. We have also performed a 15-day shift of each month in order to not lose any flare at the time limit of each bin. Most of the sources we find are already included in past LAT general catalogs or in the preliminary 8-year catalog (FL8Y). Here we report on those sources, however, which are not included and we present their spectral and time characteristics.