Divers searching the depths of the Potomac River yesterday found the second engine from the crashed Air Florida jetliner, but failed to locate the last of the bodies of the 78 persons killed in last week's accident.

For the most part, the Navy divers recovered only small bits and pieces of the shredded wreckage left on the riverbed. The bulk of the wreckage is now at National Airport's Hangar 12 where it is being examined by National Transportation Safety Board investigators.

They have been studying such technical items as the wing flap settings, rudder positions and settings on instruments, safety board spokesman Robert Buckhorn said, in an effort to determine why Flight 90 hit the 14th Street bridge and plunged into the river moments after taking off from National in a blinding snowstorm a week ago last Wednesday.

Investigators said they have not yet found out whether the plane's landing gear was extended at the time of the crash, although they have determined that there were no wheel marks left on vehicles that were hit on the bridge.

They said one means of learning whether the gear was down and other important details about the plane may be through laboratory examination of several "peanut size" light bulbs to determine whether they were lit at the time of the crash. These instrument panel bulbs, which have been recovered, show such things as status of the landing gear and temperature of the engines and wings.

Navy divers are still trying to locate an outer portion of the right wing and the main landing gear in the river, in addition to the body of 2-month-old Jason Tirado, the only victim not recovered. The infant's father, Jose, 23, of Spain, was identified yesterday as one of those recovered from the river Thursday. His mother Priscilla, 22, was one of five survivors of the crash, but suffered a broken leg and various cuts and bruises.

Yesterday the two-man helicopter crew that rescued the five survivors from the frigid Potomac received the Interior Department's highest medal, the Valor Award, from Interior Secretary James G. Watt, as well as a letter of commendation from President Reagan.

But the crew members, pilot Donald W. Usher and paramedic M.E. (Gene) Windsor, said after the ceremony they still are unable to pinpoint which of the victims was the bald man, about 50 years old, who they said selflessly passed a lifeline to other survivors in the water before perishing.

"That's being left to the safety board," Windsor said. "I'm not ruling anyone out."

He reiterated his earlier recollection that the man did not have a beard. According to the D.C. Medical Examiner's Office, the only man who drowned in the accident was Arland D. Williams, a 46-year-old directing bank examiner for the Federal Reserve Bank in Atlanta, but he had a beard. All of the other victims who died were killed from the impact of the crash.

Two victims identified yesterday were Jane R. Burka, 37, of Bethesda, president of the sisterhood of the Washington Hebrew Congregation, and David Krzanowski, 4, whose mother, father and infant sister were also killed in the crash.

Highway officials said yesterday that the last impediments to bridge traffic are being removed this weekend and both north- and southbound express lanes of the 14th Street bridge's center span will be open by the Monday rush hour.