More oil is not expected to leak into the sea, but crews are on standby with collection vessels just in case.
The cap was removed Thursday as a prelude to raising the massive piece of equipment underneath that failed to prevent the worst offshore oil spill in U.S. history.
The government wants to replace the failed blowout preventer first to deal with any pressure that is caused when a relief well BP has been drilling intersects the blown-out well.
Once that intersection occurs sometime after Labor Day, BP is expected to use mud and cement to plug the blown-out well for good from the bottom.
The April 20 rig explosion killed 11 workers and led to 206 million gallons of oil spewing from BP's well.