A ground-shaking test firing of the shuttle Discovery's three main engines has been delayed at least two days to July 30 because of slow progress finishing work in the ship's cramped engine room, officials said Friday.

Work to ready the rebuilt shuttle for its pre-launch trial by fire proceeded in parallel Friday with work to devise a way to fix a fuel leak in a steering rocket system aboard Discovery that threatens a major delay for the first post-Challenger flight.The leading candidate for a launch pad repair job involves cutting through a payload bay bulkhead to reach the leak. Discovery's launch, which had been planned for the first week in September, could be delayed until mid-October or later if the shuttle has to be hauled back to its hangar for repairs.

The fuel leak, however, does not involve the shuttle's main engine system and it was not involved in the decision to delay the engine test firing.

The 22-second unmanned "flight readiness firing," or FRF, had been scheduled for 7:30 a.m. July 28 but engineers were running at least two days behind schedule Friday because of slow progress completing preparations in the ship's engine compartment.

"It's just a lot of little things, it's nothing major," one engineer said.

NASA officials said late Friday the test would be delayed until at least July 30 and it could slip an additional day if preparations take longer than expected.

The flight readiness firing represents a major hurdle for the overall shuttle system in that it will subject the ship's many systems - and the NASA launch team - to the stress of the launch environment for the first time since Challenger's destruction more than 21/2 years ago.

To practice for the flight readiness firing, NASA engineers will conduct a full-scale dress-rehearsal countdown that calls for filling Discovery's giant external fuel tank.

The so-called "wet countdown test" will end July 27 at the T-minus 10-second mark, and if all goes well, a new countdown will start leading to the engine test firing three days later at 7:30 a.m.

As for the fuel leak problem, engineers Friday continued to assess three launch pad repair options in a bid to avoid hauling Discovery back to its hangar.

The leak, discovered July 15, is situated in a half-inch fitting in a propellant line inside Discovery's left-side orbital maneuvering system pod. The shuttle is equipped with two such pods, one on either side of the ship's tail fin, that each house 15 rocket engines used for steering.