With a new, tighter fitting cap in place on the gushing Gulf wellhead , BP is preparing Tuesday to test whether the flow of oil could be stopped completely.

Kent Wells, a BP senior vice president, said a pressure test, which would allow for the valves on the cap to be closed thus completely shut down the leak, would probably start later Tuesday and last for six to 48 hours.

If the tests show the pressure rising and holding, which is an indication that the well is intact with no significant damage to the casing pipe that runs the length of the well below the sea floor, BP could decide to leave the valves closed, effectively cutting off the flow of oil.

However, if the tests show pressures that are lower than expected, said Wells, the well is damaged, meaning that oil and gas are leaking into the surrounding rock. If this is the case, then keeping the cap closed could damage the well further. The cap valves would have to be reopened, allowing oil to start escaping from the well again, although much of it would be funneled through pipes to surface ships.

Though the new cap could significantly help in efforts to contain the spill, the well will not be completely killed until relief wells are completed at the end of July or later.