About 3,500 Boeing jets will be required to fly with extra fuel by Thursday, making sure there is enough to cover fuel pumps in case they overheat, the Federal Aviation Administration said.

The step follows reports that pumps in three planes overheated to a point where they might have caused an explosion, the FAA said.

The reason for the extra fuel to cover the pumps is to keep them away from fuel vapors which can ignite, said Paul Takemoto, FAA spokesman.

The National Transportation Safety Board ruled that an explosion in the fuel tank of TWA Flight 800 caused it to crash off the coast of Long Island in 1996. It said vapors in the partly empty tank probably were ignited by a spark in the wiring.

"There's no danger of (the pumps) igniting fuel vapors if they're covered," by fuel, Takemoto said.

The safety warning, the third in four months, is intended as a precaution while the FAA tries to figure out what's causing the problem. The agency, in August, ordered airlines to submerge certain pumps with fuel or, if not, to X-ray them to make sure they were properly wired. If the pumps weren't properly wired, they had to be replaced.

Major U.S. carriers said they had few planes in which the suspect pumps, made by Crane Co.'s Hydro-Aire unit, were installed.

In September, the FAA found the problem potentially included every Hydro-Aire pump. The agency ordered all fuel pumps submerged or X-rayed and replaced if faulty.

Then Boeing Co. received two reports of overheating within fuel pumps that had been replaced on a Singapore Airlines 747 on Wednesday and on a Lufthansa 747 on Friday. Since both pumps had been replaced, the incidents indicated the problem wasn't miswiring as the FAA had thought, Takemoto said. A third had been removed and was found to have overheated during an inspection.

Boeing immediately notified airlines that operate the jets, said Cindy Wall, company spokeswoman.

"It's baffling," she said. "They've been working nonstop 24 hours a day trying to fix it."

Greg Ward, Hydro-Aire president, said the company has been making the pumps for 30 years without a problem. An extensive investigation includes chemical analysis of pump parts that overheated, checking into airline service records and interviewing retired employees who worked on the pumps, he said.

"We're still piecing together clues," Ward said.

The FAA issued the order to cover all fuel pumps with fuel on Sunday. Airlines have until Thursday to comply. The order will remain in effect until the agency determines what causes the fuel pumps to overheat, Takemoto said.

The order affects about 1,400 737s, 747s and 757s flown by U.S. carriers. The FAA is sending advisories about the pumps to its counterpart agencies in other countries, which cover about 2,100 jets.