NASA managers today ordered engineers to replace a suspect positive pressure relief valve in the shuttle Endeavour's crew module to fix an apparent leak that showed up over the weekend during routine launch pad testing.

While the schedule is tight, engineers expect to complete the work in time for Endeavour's launch Tuesday on a space station assembly mission.

"It's extremely tight," a NASA spokesman said late today. "On paper, yes, we can make it. But everything has to go right from this point forward."

The valve in question is one of two in the crew module that ensure the cabin air pressure stays within required limits. Engineers plan to replace the valve, located behind the shuttle's toilet on the lower deck of the split-level crew cabin, with one taken from the shuttle Atlantis.

The work should be completed Thursday. Engineers then will have to carry out additional pressure tests to confirm the system is operating normally before Endeavour can be cleared for flight.

While the unplanned work poses a challenge to the shuttle launch team - there is no contingency time left in the schedule - engineers say Endeavour should be ready for blastoff on time at 7:02 p.m. Tuesday.

Earlier concern about thermostats in one of the orbiter's hydraulic power units has been resolved and NASA plans to start Endeavour's countdown on time at 9 p.m. Saturday.