What was found
ENGINE RAN FINE ONE DAY, BUT NEXT DAY ALTHOUGH IT WOULD START, IT WOULD DIE AFTER JUST A FEW SECONDS. FOUND IT WOULD CONTINUE TO RUN ONLY IF BOOST PUMP WAS LEFT ON. ASSUMED ENGINE DRIVEN PUMP HAD FAILED. REPLACED ENGINE PUMP BUT PROBLEM PERSISTED. EVENTUALLY SECOND MECHANIC, SPOTTER, NOTICED FUEL LEAKING UNDER MID-FUSELAGE DURING TEST RUN, BUT AFTER SHUTDOWN COULD NOT FIND ANY EVIDENCE OF A FUEL LEAK. FIGURING IT MUST BE IN VICINITY OF FUEL SELECTOR AND/OR BOOST PUMP, THE COCKPIT CARPET AND SUB-FLOOR PLYWOOD PANELS WERE REMOVED AND BOOST PUMP WAS TURNED ON. FOUND FUEL SPRAYING INTO CABIN SIDEWALL AND INSULATION AND RUNNING DOWN INTO FLOOR STRUCTURE FROM LEAKING IN MAIN FUEL LINE FROM BOOST PUMP TO BACKSIDE OF FIREWALL. FUEL LINE APPEARS TO HAVE BEEN CHAFED AND/OR CORRODED DUE TO CONTACT WITH STEEL-WIRE SCAT VENTILATION DUCT BEHIND SIDEWALL UPHOLSTERY PANEL TO LEFT OF AND ABOVE PILOT'S LT RUDDER PEDAL. NOTE: AREA OF PROBLEM IS OUTSIDE OF NORMAL SCOPE OF ANNUAL OR 100-HOUR INSPECTIONS.