Japanese investigators have found problems in equipment that controls a fuel-tank valve on a Boeing Co 787 Dreamliner jet that was being probed for fuel leaks, the Nikkei newspaper reported on Thursday.

The fuel leak, found in a Japan Airlines Co <9201.T> jet as it taxied to a Boston runway on January 9, came amid battery problems that have since grounded the Boeing 787 Dreamliner fleet globally.

There were deficiencies in the way electrical-insulating paint was applied to a mechanism that opened and closed the fuel-tank valve, the Nikkei said, citing people familiar with the matter. Transport ministry investigators also found foreign matter on a switch that operated the same mechanism.

The investigators believe that this caused the switch to send a signal that the valve was closed when it was still half open -- causing the leak, the Japanese daily said. (http://link.reuters.com/tuq26t)

The ministry is in talks with the U.S. Federal Aviation Administration and Boeing about ways to resolve the problem, the newspaper said.

The same 787 involved in the January 9 fuel leak in Boston leaked fuel from its left wing nozzle at Tokyo's Narita International Airport on January 13.

(Reporting by Tej Sapru in Bangalore; Editing by Rodney Joyce)