A critical test-firing of space shuttle Discovery's three main engines on the launch pad has been put off 24 hours, until Sunday, to give crews more time to seal the engine compartment.

A practice countdown, the first of two leading to the firing, was halted Monday when the launch team decided it could not close out the compartment work in time to conduct the test on Saturday as intended.The postponement was announced as engineers assessed two major candidates for fixing a small shuttle gas leak without having to take Discovery off the launch pad, a move that could delay the planned early September launch by as much as two months.