NASA engineers resumed a twice-delayed practice countdown for the shuttle Discovery Thursday, and managers decided to shorten a preliminary launch simulation to clear the decks for an engine test firing Monday.

A 24-hour delay in the countdown, announced Wednesday, pushed a critical test firing of Discovery's three main engines from Sunday to Monday at 7:30 a.m., two days later than originally planned when the countdown began last Sunday.NASA officials said that delay, and a 24-hour slip for a preliminary practice fuel loading session, was ordered because they did not want to rush engineers who have not conducted such a detailed launch exercise since Challenger's destruction in January 1986.

But engineers made up lost time overnight, and the schedule was compressed to allow NASA to complete the fuel loading exercise and launch simulation earlier Friday than had been planned. The countdown resumed at 11 a.m.

In the meantime, NASA managers planned to discuss recommendations on how to fix a small fuel leak in Discovery's left-side steering rocket pod that engineers hope to repair at the launch pad after the engine firing.

Two major repair options are under study, but one that involves cutting through Discovery's payload bay and into the rocket pod appears to have the inside track.

If Discovery was launched "as is," blastoff could not occur before Sept. 13. With an on-the-pad fix, the first post-Challenger mission could occur before the end of September. A roll back to the hangar for repairs would push launch into October.

The practice countdown, already running 24 hours behind schedule, was delayed an additional day Wednesday when workers fell behind preparing the ship for fuel loading as part of a rehearsal to clear the way for the engine firing.

As a result, the practice launch simulation slipped 24 hours, from Thursday to Friday and the 22-second firing of Discovery's three main engines was pushed from Sunday to Monday.