Published: Jul 13, 2010 7:43 PM EDT
Updated: Jul 13, 2010 4:35 PM EDT

NEW ORLEANS (AP) - With a tight new cap freshly installed on its leaking well in the Gulf of Mexico, BP planned gradual tests starting Tuesday to see if the device can stop oil from pouring into the sea for the first time in nearly three months.

The cap would be just a temporary solution, but it offers the best hope yet for cutting off the crude that has fouled the Gulf since the Deepwater Horizon rig leased by BP exploded April 20, killing 11 workers.

Engineers will slowly shut down three valves that let oil flow through the 75-ton capping device to see if it can withstand the pressure of the erupting crude and to watch if leaks spring up elsewhere in the well. National Incident Commander Thad Allen said the process of closing the valves, one by one, would start later Tuesday.

If pressure inside the cap stays in a target range for roughly six hours after the valves are closed, there will be more confidence the cap can contain the oil, Allen told a news briefing at BP's U.S. headquarters in Houston. That target range is 8,000 to 9,000 pounds per square inch, he said. Anything lower could indicate another leak in the well.

Allen and BP officials repeatedly cautioned there are no guarantees about the delicate work a mile below the sea. Allen urged Gulf Coast residents watching the possible fix evolve to be patient.  "They ought to be interested and concerned but if they hold their breath, they'll run out of oxygen. I won't be," Allen told The Associated Press after the briefing.

The tests could last anywhere from six to 48 hours, Allen and BP said.