What was found
ODOR OF OIL IN CKPIT & CABIN DURING APU OPS & DURING LOW THRUST ENG OPS. RAN ENGS W/ APU BLD VALVE CLOSED & ODOR IMPROVED, RAN APU & ODOR RETURNED. R & R HEPA FILTER IAW AMM 2 1-70-10. DURING CLOSE UP, FOUND OIL IN MUFFLER DRAIN LINES. REMOVED LOW LIMIT DUCT TEMP SENSOR FROM MUFFLER & BORO MUFFLER. OIL FOUND IN MUFFLER & MIXER. BORO INSP THRU MIXER TO A/C PACK & FOUND OIL SITTING ON FLANGE BETWEEN MIXER & PACK. REMOVED, CLEANED & INSTALLED MIXER IAW AMM 21-20-05. BORO DUCTING FWD OF MUFFLER, ALL DUCTING GOOD EXCEPT AFT BLKHD CHK VALVE FLANGE. CLEANED OIL ON CHK VALVE & REINSTALLED IAW AMM 2 1-20-20. REPAIRED APU INSTALLED. AFTER APU POST CHECKS. NO ODOR. OPS CHECKED GOOD IAW AMM 21-50-00.