CAPE CANAVERAL, Fla. (AP) _ The countdown for a Thursday test-firing of space shuttle Discovery's main engines proceeded Tuesday while technicians searched seals, fittings and welds for an elusive hydrogen leak that could once again delay the vital test.

Shuttle managers said they hoped to avoid a fifth postponement of a test which is necessary to certify Discovery for the first shuttle flight since the Challenger explosion 2 1/2 years ago.

They said they would meet Wednesday morning to assess the leak situation and decide whether to continue or delay the countdown.

The problem is in a launch pad service mast and not on the shuttle itself.

Officials said there were indications the leak could be temperature related and occur only when hydrogen flowed through the service mast into the shuttle's fuel tank at a temperature of 423 degrees below zero, necessary to keep it in liquid form.

If that were the case, the leak might be difficult to find at air temperature and suspect parts might have to be removed and subjected to tests in a supercold laboratory chamber.

The leak is in the same service mast as another hydrogen leak that developed last Friday in a pressure monitoring connector. That leak was fixed, but it forced a three-day delay in a practice countdown.

The new leak, believed different than the first, was detected Monday while hydrogen was flowing into Discovery's tanks during a renewed practice countdown. The launch team stopped hydrogen loading and continued the countdown to conclusion without a full load.

A new countdown then was started, aiming for the firing of Discovery's three main engines at 7:30 a.m. Thursday.

The 20-second ignition is to test modifications made to highspeed turbopumps and other engine systems and to provide the launch team with practice in new countdown procedures initiated since the Challenger accident.

Officials believe the practice countdown and firing delays, combined with an unresolved leak in the spaceship's steering thruster system, could slip the launch date from mid-September into October.

NASA has deferred action on the steering thruster leak until after the engine test. Then shuttle managers will determine whether to move Discovery from the launch pad to a hangar for repairs or if engineers should try to fix the leak on the pad by entering the shuttle's cargo bay and cutting a hole through the rear bulkhead to gain access to the thruster compartment.

A fix on the pad would delay the launch about two weeks, while moving Discovery to a hangar could put it off as much as two months.