NASA engineers, beset by minor snags, ordered a second 24-hour delay in a practice countdown for the shuttle Discovery, pushing a critical test firing of the ship's main engines from Sunday to Monday. "There was just too much to do and not enough time to do it," a NASA spokesman said. Under the new schedule, Discovery will be loaded with half a million gallons of fuel tonight for a launching simulation early Friday. Engineers then will begin preparations for the unmanned 22-second engine firing at 7:30 a.m. Monday. The practice countdown, already running 24 hours behind schedule, lost another eight hours overnight Tuesday when engineers were forced to replace a fuel valve.