A CRJ-900 (CL600-2D24) ACFT #2 ENGINE HYD DRIVEN PUMP WAS REPLACED BY A CONTRACT MAINT PROVIDER UNDER DIRECTION OF CARRIER'S MAINT CONTROL. SUBSEQUENT LOSS OF ENG OIL IN FLIGHT; ACFT DIVERTED.

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I WORK FOR CONTRACTED MAINT PERFORMING ON CALL MAINT ON COMMERCIAL AIRLINERS AT ZZZ. I WAS CONTACTED BY ABC MAINT CTL; TO INVESTIGATE AN OIL LEAK FROM THE #2 ENG ON ACFT X. I WAS INSTRUCTED TO OPEN THE #2 ENG NACELLE COWLING TO FACILITATE LOCATING THE LEAK. THIS ACFT HAD COME IN THE AFTERNOON WITH A #3 RECAP MAIN GEAR TIRE BLISTERED IN 4 PLACES. I TOOK THIS CALL AND AFTER NOTIFYING COMPANY OF THE TIRE'S CONDITION THE FLT WAS CANCELED AND A TIRE ASSEMBLY AND SOCKET WAS SENT TO ZZZ TO CHANGE OUT THE #3 WHEEL ASSEMBLY. THE NEXT SHIFT CREW REPOSITIONED THE ACFT TO A REMOTE PAD; RECEIVED THE WHEEL ASSEMBLY AND COMPLETED THE CHANGE. THE CREW ARRIVED AT THE REMOTE PAD ON THE NEXT MORNING FOR A NON-REVENUE REPOSITIONING FLT TO ZZZ1 AND WROTE UP THE LEAK HE NOTICED. AFTER OPENING THE #2 ENG NACELLE COWLING I DETERMINED THAT THE HYD EDP (ENG DRIVEN PUMP) WAS LEAKING HYD FLUID FROM ITS CASE HALVES AND I NOTIFIED MAINT CTL. THE CTLR INFORMED ME THAT THEY WANTED ME TO REMOVE THE HYD EDP FROM THE #2 ENG. THAT THIS WAS COMING BACK TO THEM AND THEY COULD COMPLETE THE CHANGE IN ZZZ1. HE STATED THAT ALL I HAD TO DO WAS PULL THE PUMP INSTALL A BLANK-OFF PLATE TO COVER THE HOLE AND CAP OFF THE HYD FITTINGS WHERE THE LINES WERE REMOVED THAT HOOKED UP TO THE HYD EDP. I LOCATED THESE LARGER PARTS IN THE FLY AWAY KIT. WHILE PULLING THE PUMP I DIDN'T NOTICE ANY OIL WHEN SEPARATING THE PUMP FROM THE GEAR BOX. THE HYD LINES HAD QUICK DISCONNECT FITTINGS AND I USED QUICK DISCONNECT CAPS AT THE CONNECTORS. AFTER I FINISHED THESE TASKS I INFORMED MAINT CTL OF EXACTLY WHAT I HAD DONE AND THAT I WAS GOING TO CLOSE THE #2 ENG NACELLE COWL. I WAS GIVEN THE GO AHEAD AND I WAS TOLD NEXT WE WOULD COMPLETE THE LOGBOOK WRITE-UP. THE LOGBOOK MAINT DISCREPANCY READ: #2 ENG; OIL LEAK. THE CORRECTIVE ACTION/DEFERRAL THAT I WAS INSTRUCTED TO WRITE READ: CONTINUED OPS PER MEL. I WAS TOLD THAT I HAD BEEN FOLLOWING THE (M) AMM TASK NOTED IN THE MEL PER MAINT CTL DIRECTION/INSTRUCTIONS. THE ACFT LEFT ZZZ BOUND FOR ZZZ1 BUT DIVERTED TO ZZZ2 WITH LOSS OF #2 ENG OIL. I THOUGHT THAT THE GEAR BOX PAD WAS DRY AND ONLY NEEDED A BLANK OFF PLATE. IT REQUIRED A SPLINE PLUG TO BE INSERTED IN THE SPLINE SHAFT. A PART AND STEP THAT I WASN'T INFORMED OF. THE AMM TASK ALSO CALLED OUT FOR AN ENG RUN AND LEAK CHK TO BE PERFORMED BEFORE CLOSING THE COWLING. ANOTHER STEP OMITTED. I FEEL THE REMOTE PAD/LOCATION; ALONG WITH THE CREW ON BOARD WAITING TO LEAVE. THE FACT THAT IT WAS GOING RIGHT TO COMPANY'S MAINT FACILITY; BEING A NON REVENUE FLT. ALL CONTRIBUTED TO ALLOWING MYSELF TO BE PERSUADED TO ONLY FOLLOW THE COMPANY MAINT CTL'S INSTRUCTIONS WITHOUT HAVING A COPY OF THE AMM TASK ON HAND DURING THE TASK.