Narrative:

On a morning in jul 08; after an uneventful preflight and run-up; I took off from ZZZ1 in my piper saratoga (PA32) en route IFR to ZZZ2. I landed at ZZZ2 and I experienced no problems with the aircraft en route. I topped off at ZZZ2. After a second uneventful preflight and run-up; I left ZZZ2 later in the day intending to fly IFR to ZZZ1. I had filed for 6000 ft. I experienced no problems and noted no discrepancies during the preflight; run-up or on takeoff or climb out. After I climbed to 6000 ft; I engaged the altitude hold. I noticed that the altitude hold on the autoplt would not hold altitude. I so advised center; and I asked permission to descend to 4000 ft while I attempted to diagnose the problem. This request was granted. I descended to 4000 ft and during the descent I began to hear a knock in the engine and the engine began to run very roughly. When I arrived at 4000 ft; I still heard the knock and still experienced the roughness and I could not maintain airspeed or altitude despite going through the emergency procedures. I so advised center to whom I had been passed. Since I could not hold altitude and had to continue to descend to maintain flyable airspeed; I could not maintain the minimum altitude for the sector. I advised center of the serious situation and I asked to be vectored to the closest airport. The controller then vectored me to ZZZ where I was safely able to land without incident. The 'knocking' and roughness continued throughout the remainder of the flight. I have asked a mechanic to investigate the problem.

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Original NASA ASRS Text

Title: PA32 PILOT REPORTS ROUGH RUNNING ENGINE AT 6000 FEET AND RECEIVES VECTORS TO SAFE LANDING AT NEARBY ARPT.

Narrative: ON A MORNING IN JUL 08; AFTER AN UNEVENTFUL PREFLT AND RUN-UP; I TOOK OFF FROM ZZZ1 IN MY PIPER SARATOGA (PA32) ENRTE IFR TO ZZZ2. I LANDED AT ZZZ2 AND I EXPERIENCED NO PROBS WITH THE ACFT ENRTE. I TOPPED OFF AT ZZZ2. AFTER A SECOND UNEVENTFUL PREFLT AND RUN-UP; I LEFT ZZZ2 LATER IN THE DAY INTENDING TO FLY IFR TO ZZZ1. I HAD FILED FOR 6000 FT. I EXPERIENCED NO PROBS AND NOTED NO DISCREPANCIES DURING THE PREFLT; RUN-UP OR ON TKOF OR CLBOUT. AFTER I CLBED TO 6000 FT; I ENGAGED THE ALT HOLD. I NOTICED THAT THE ALT HOLD ON THE AUTOPLT WOULD NOT HOLD ALT. I SO ADVISED CTR; AND I ASKED PERMISSION TO DSND TO 4000 FT WHILE I ATTEMPTED TO DIAGNOSE THE PROB. THIS REQUEST WAS GRANTED. I DSNDED TO 4000 FT AND DURING THE DSCNT I BEGAN TO HEAR A KNOCK IN THE ENG AND THE ENG BEGAN TO RUN VERY ROUGHLY. WHEN I ARRIVED AT 4000 FT; I STILL HEARD THE KNOCK AND STILL EXPERIENCED THE ROUGHNESS AND I COULD NOT MAINTAIN AIRSPD OR ALT DESPITE GOING THROUGH THE EMER PROCS. I SO ADVISED CTR TO WHOM I HAD BEEN PASSED. SINCE I COULD NOT HOLD ALT AND HAD TO CONTINUE TO DSND TO MAINTAIN FLYABLE AIRSPD; I COULD NOT MAINTAIN THE MINIMUM ALT FOR THE SECTOR. I ADVISED CTR OF THE SERIOUS SITUATION AND I ASKED TO BE VECTORED TO THE CLOSEST ARPT. THE CTLR THEN VECTORED ME TO ZZZ WHERE I WAS SAFELY ABLE TO LAND WITHOUT INCIDENT. THE 'KNOCKING' AND ROUGHNESS CONTINUED THROUGHOUT THE REMAINDER OF THE FLT. I HAVE ASKED A MECH TO INVESTIGATE THE PROB.

Data retrieved from NASA's ASRS site as of May 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.