A BE58P IN CRUISE AT 15000 FT DECLARED AN EMER AND DIVERTED DUE TO LOSS OF PWR IN THE R ENG. CAUSED BY A FAILED OIL DRAIN LINE FROM THE TURBOCHARGER.

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I WAS FLYING AS A CFI/PEACE OF MIND PLT WITH MR X. WE WERE FLYING HIS 1979 P BARON WITH CONTINENTAL TSIO 520 WB ENGS. BOTH HAVE APPROX 400 HRS AND WERE ABOUT 10 HRS OUT OF A COMPLETE TEARDOWN INSPECTION. HE WAS TRAVELING FOR PLEASURE. BASICALLY; I WAS THERE 'JUST IN CASE' SOMETHING WENT WRONG. PIC IN THE R SEAT. WE HAD 1 PAX. I WAS READING CHKLISTS AND ASSISTING AS REQUESTED. THE WX WAS CLR; GOOD VISIBILITY AND FORECAST FOR VFR. FILED AN IFR FLT PLAN. WE WERE CLRED V21 DIRECT AT 15000 FT. CRUISE SETTINGS WERE 33 INCHES AND 2300 RPM. WE HAD COMPLETED ALL CHKLISTS AND THE AUTOPLT WAS FLYING THE PLANE. WE HAD A SUDDEN LOSS OF PWR IN THE R ENG. PWRED UP BOTH ENGS AND STARTED TO IDENT THE PROB. AS HE PWRED UP THE PLANE; HE WAS STRUGGLING WITH ACFT CTL AND WAS HYPER-FOCUSED ON THE DECREASING AIRSPD. I TOOK CTL OF THE AIRPLANE; ESTABLISHED POSITIVE CTL; TRIMMED UP THE PLANE; AND DECLARED AN EMER WITH ATC. AT THAT POINT; L ENG WAS 35 INCHES AND 2500 RPM; TEMPS WERE WITHIN NORMAL LIMITS. R ENG WAS ABOUT 20 INCHES AND ABOUT 2200 RPM WITH VERY LOW CHT AND EGT'S AND SHAKING MODERATELY. I WAS CONCERNED ABOUT THE PROP AND IF THE ENG MIGHT COME OFF THE AIRPLANE. IT SEEMED THAT THE R ENG WAS GONE AND WAS ONLY CAUSING MORE PROBS SO I DECIDED TO SHUT IT DOWN COMPLETELY AND FEATHER IT. I BEGAN THE SHUTDOWN MEMORY ITEMS BY REDUCING PWR ON THE R ENG TO IDLE. AS I REDUCED PWR TO IDLE; THE PLANE YAWED EVEN MORE INTO THE DEAD ENG. IT SEEMED THAT THE R ENG WAS DEVELOPING PARTIAL PWR. AS I ADJUSTED THE R THROTTLE THROUGH ITS RANGE; THE ENG SEEMED TO RUN SMOOTHLY ONLY AT ABOUT 21 INCHES MANIFOLD PRESSURE. HIGHER OR LOWER MANIFOLD PRESSURE CAUSED SHAKING AND ROUGH RUNNING. AT THAT POINT; WE WERE DSNDING; BOTH ENGS WERE RUNNING SMOOTHLY AT 20 INCHES AND EVERYTHING SEEMED OK. I UPDATED ATC AND THE DSCNT CLRNC TO 9300 FT MSL. I DIRECTED MR X TO COMPLETE THE DSCNT CHKLIST AND TO RPT IT COMPLETE. ONCE WE HAD IN SIGHT; I INFORMED ATC THAT I WOULD ACCEPT TERRAIN CLRNC AND ASKED FOR THE TWR FREQ SO THAT I COULD SET UP WITHOUT HAVING TO PULL THE CHART. HE CLRED US FOR A VISUAL APCH TO RWY 21. HE GAVE ME THE FREQ AND HANDED ME OFF TO TWR. I FLEW A HIGH L BASE LEG TO RWY 21. AS I TURNED FINAL I CALLED FOR THE BEFORE LNDG CHKLIST; COMPLETED THE ITEMS; AND CALLED IT COMPLETE. IT WAS A HIGH FINAL AND I HAD TO DO A FEW S-TURNS TO LOSE ALT AND REDUCE PWR. THE R ENG RAN VERY ROUGH AS I PULLED THE PWR IN THE DSCNT. AS WE CAME ON SHORT FINAL; I ADDED PWR BACK. THE LNDG WAS NORMAL AND UNEVENTFUL. ON TAXI TO THE RAMP; THE R ENG RAN VERY ROUGH. MAINT DETERMINED THAT AN OIL DRAIN LINE ATTACHED TO THE HIGH PRESSURE AIR FROM THE TURBO HAD BROKEN OFF LEAVING A QUARTER SIZED HOLE. THE HOLE CAUSED PARTIAL PWR LOSS AND VERY ROUGH RUNNING ENG EXCEPT WHEN THE PWR WAS SET TO AMBIENT MANIFOLD PRESSURE.