CAPE CANAVERAL, Fla. — Two astronauts took a hastily planned spacewalk Saturday to find and, possibly, fix a serious leak at the International Space Station, but they discovered no problems as they worked to replace a suspect pump.

Flakes of frozen ammonia coolant were spotted Thursday drifting from the long frame that holds the solar panels on the left side. Barely 48 hours later, Thomas Marshburn and Christopher Cassidy hunted in vain for the leak as they inspected and removed an old pump.

No new, major flaking occurred, to everyone’s disappointment. NASA had ordered up the fast, impromptu spacewalk in hopes that ammonia flakes might lead the astronauts to the bedeviling hole or crack, which is too small to notice without a trail of icy evidence.

“All the pipes look shiny clean, no crud,” Cassidy reported as he used a long, dentist-like mirror to peer into tight, deep openings.

“I can’t give you any good data other than nominal, unfortunately. No smoking guns.”

Despite the lack of visible damage, Mission Control ordered the spacewalkers to install a spare pump. The spacewalk was going to last no longer than the allotted six hours, the astronauts were advised.