Running five days late, NASA engineers conducted a full-scale launch simulation for the shuttle Discovery early Monday, but another launch pad leak derailed a trouble-plagued fueling test and shortened the practice countdown.

With Discovery lit up by high-power spotlights, NASA managers scrambled throughout the morning to figure out ways to work around a steady stream of glitches that disrupted a second attempt to load a half-million gallons of supercold rocket fuel into Discovery's external tank.The fueling exercise was designed to serve as a rehearsal before a crucial unmanned main engine test firing, which had been scheduled for Thursday at 7:30 a.m. Engineers were debating whether the engine firing would have to be delayed again - the fourth time so far in the countdown - or whether it can be conducted Thursday as planned.

"We're looking at a three- to four-day turnaround," said a NASA spokeswoman. "So (the engine firing) will either be on Thursday or Friday."

Even though engineers had problems with the fuel loading test, the countdown was allowed to continue to give launch crews a chance to gather data on the external tank and to exercise NASA's new mission management team, which has final responsibility for clearing shuttles for blastoff.

The test ended at 6:56 a.m. at the T-minus 11-second mark, when a launch abort was simulated. Even though it was riddled with problems that prompted managers to shorten the countdown, the exercise was the most extensive such simulation since the Challenger disaster 2 1/2 years ago.