The test involves slowly closing the valves of the cap, ultimately blocking the flow of oil entirely. High pressure is a good sign, because it means there's a single leak.
BP says the testing will take up to 48 hours before they know if the cap works as planned.
Meantime, scientists are reporting early signs that the spill is altering the marine food chain by killing or tainting some creatures and spurring the growth of others.
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 92 and 186 million gallons of oil had spewed into the Gulf.
If the cap works, it will only be temporary. Two relief wells are being drilled that should be ready by next month.
- A House committee wants to put BP in a penalty box of sorts. The Natural Resources Committee approved an amendment Wednesday that would ban companies with significant workplace safety or environmental violations over the preceding seven years from new offshore drilling permits.
- The amendment has a long way to go before becoming law. It was passed as part of a larger energy bill that the committee was considering Wednesday.
- A government investigation of the deadly explosion on the Deepwater Horizon oil rig is set to resume next week. The Coast Guard and Bureau of Ocean Energy Management, Regulation and Enforcement have scheduled five more days of hearings, from July 19 through July 23, at a hotel in the New Orleans suburb of Kenner.
The Associated Press contributed to this report.