The U.S. government gave BP the green light Wednesday to try choking off the Gulf of Mexico oil gusher with the untested metal cap after a daylong delay. (BP PLC/Associated Press)

BP has started testing the tighter-fitting cap designed to stop the flow of oil into the Gulf of Mexico from a broken well for the first time in almost three months.

The test began Wednesday with BP shutting off pipes that were funnelling some of the oil to ships on the surface, so the full force of the gusher went up into the cap.

Then deep-sea robots began slowly closing — one at a time — three openings in the cap that let oil pass through.

Ultimately, the flow of crude will be blocked entirely. All along, engineers will be watching pressure readings to learn whether the well is intact.

Former coast guard admiral Thad Allen, the Obama administration's point man on the disaster, said the government gave the testing go-ahead after carefully reviewing the risks.

"What we didn't want to do is compound that problem by making an irreversible mistake," he said.

Second thoughts

The cap — a 68-tonne metal stack of pipes and valves — was lowered onto the well Monday in hopes of either bottling up the oil inside the well machinery or capturing it and funnelling it to the surface.

But before BP could test the equipment, the government intervened because of second thoughts about whether the buildup of pressure from the gushing oil could rupture the walls of the well and make the leak worse.

"We sat long and hard about delaying the tests," Allen said.

He said the pause was necessary in the interest of the public, the environment and safety before officials were convinced the test could go forward.

Allen said BP will monitor the results every six hours and end the test after 48 hours to evaluate the findings.

The oil giant had been scheduled to start slowly shutting off valves Tuesday on the cap, aiming to stop the flow of oil for the first time in three months.

BP was initially ahead of schedule on its latest effort to plug the leak. The cap was designed to be a temporary fix until the well is plugged underground.

Initial progress stalled

A series of methodical, preliminary steps were completed before progress stalled.

Engineers spent hours on a seismic survey, creating a map of the rock under the sea floor to spot potential dangers, like gas pockets. It also provides a baseline to compare to later surveys during and after the test to see if the pressure on the well is causing underground problems.

'We decided that the process may benefit from additional analysis.' —Thad Allen, national incident commander

An unstable area around the wellbore could create bigger problems if the leak continued elsewhere in the well after the cap valves were shut, experts said.

"It's an incredibly big concern," said Don Van Nieuwenhuise, director of Professional Geoscience Programs at the University of Houston. "They need to get a scan of where things are, that way when they do pressure testing, they know to look out for ruptures or changes."

The leak began after the Deepwater Horizon offshore drilling platform exploded on April 20, killing 11 workers. As of Tuesday, the 84th day of the disaster, between 342 million and 676 million litres of oil had spewed into the Gulf.