MECHANIC REPORTS AN B757-200 MADE AN AIR TURNBACK AFTER REINSTALLING THE LEFT ENGINE INLET NOSE COWL FOLLOWING AN ATTRITION LINER REPAIR. THE P-1 SENSE LINE FOR THE EPR TRANSMITTER AND FUEL FLOW GOVERNOR WAS FOUND LOOSE.

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ON DEC/XA/08 A B757-200 ACFT WAS IN THE HANGAR FOR THE L ENG ATTRITION LINER REPAIR. IN ORDER TO ACCOMPLISH THIS REPAIR; THE INLET NOSE COWL AND ALL THE FAN BLADES NEEDED TO BE REMOVED. I WAS ASSIGNED TO OVERSEE THE INSTALLATION OF THE L ENG INLET NOSE COWL AFTER THE REPAIRS HAD BEEN ACCOMPLISHED. THE INSTALLATION CONSISTED OF ABOUT 30 BOLTS; A CLAMP FOR THE ANTI-ICE DUCT; AN ELECTRICAL CONNECTOR; AND A P1 SENSE LINE FOR THE EPR XMITTER AND THE FUEL FLOW GOVERNOR. THE INLET NOSE COWL WAS INSTALLED PER B757 AMM. DURING THE ASSEMBLY PHASE; THE COMPANY HAS DEVELOPED THE NEED FOR THE AIRPLANE. THE AIRPLANE STILL HAD TO GO TO THE RUN-UP BLOCK TO PERFORM THE ENG VIBRATION SURVEY WHICH ENTAILS A HIGH PWR RUN. IN ACCORDANCE TO THE AMM; A PRESSURE TEST OF THE P1 SYSTEM NEEDED TO BE ACCOMPLISHED. IN THE SPIRIT OF EXPEDITING THE ACFT READINESS; I WAS UNDER THE PRESUMPTION THAT; PERFORMING A HIGH PWR RUN ON THE L ENG WOULD IN FACT PRESSURIZE THIS LINE AND ANY DEFICIENCIES RELATED TO THIS P1 SYSTEM WOULD HAVE LIKELY BEEN PREVALENT DURING THIS HIGH PWR RUN PHASE; AND UNDER THOSE ASSUMPTIONS I IN FACT THOUGHT I WOULD BE ACCOMPLISHING THIS LEAKAGE CHK. AFTER THE COMPLETION OF THE ENG RUN; THE TECHNICIANS PERFORMING THE RUN DID NOT NOTICE ANY ABNORMALITIES AND THE ACFT WAS TAXIED TO THE GATE FOR ITS SCHEDULED DEP. SHORTLY AFTER THIS ACFT TOOK OFF; THE PLT RPTED A DELAYED THROTTLE RESPONSE WHICH HAD EVENTUALLY CORRECTED ITSELF WHEN AT 10000 FT. THE ACFT MADE AN AIR RETURN BACK TO ZZZ. IT LANDED SAFELY WITH NO EVENTS AND NO PAX WERE INJURED. IT WAS DETERMINED THAT ONE OF THE LINES THAT ATTACHES TO THIS P1 SENSE LINE WAS LOOSE. IT WAS RE-TORQUED AND THE ACFT DISPATCHED SAFELY TO ITS DEST. I HAVE LEARNED THAT THE P1 SENSE SYSTEM IS NOT PRESSURIZED DURING HIGH PWR RUNS AND THAT IT IS; IN FACT; PRESSURIZED BY RAM AIR WHEN THE ACFT IS FLYING. I ALSO LEARNED THAT THIS P1 SENSE SYSTEM IS SENSITIVE TO AIRPLANE ALT.