Diomande and Steven Beitashour answered with goals in the 15th and 19th minutes, respectively, to give LA a 2-1 lead.
Two goals by Chris Wondolowski, in the 52nd and 63rd, put San Jose (2-9-3) back in front before Diomande scored from point-blank range in the 90th minute. Beitashour, from the left side, sent an entry toward the far post where Walker Zimmerman played a sliding cross to Diomande for the empty-net finish .
LAFC snapped a four-game winless streak.
The Earthquakes have lost four in a row – their longest such streak since 2015 – and are winless in their last five games.