Two astronauts aboard the International Space Station conducted a hastily arranged spacewalk on Saturday to look for the source of a worrisome leak that had forced the shutdown of one of the eight power systems that provide electricity to the orbiting laboratory.

The two astronauts, Thomas H. Marshburn and Christopher Cassidy, both American, spent 5 ½ hours outside the space station. They were unable to locate the source of the leak, but did install a new pump, which appeared to solve the problem.

The new pump started up normally, and with the two astronauts looking for escaping crystals of ammonia coolant, there were no signs of the leak.

“We’re not seeing anything,” Dr. Marshburn reported.

Pressure in the coolant loop was also holding, another indicator that the problem had been resolved.