Narrative:

While heading north at 17000 feet the left engine began shaking/vibrating. Manifold pressure was erratic as was fuel flow on the digital meter. A check showed normal cruise egt and cht (cylinder head temperature). I reduced manifold pressure to about 26 inches and the shaking continued. The instruments continued to read normal except the manifold and fuel flow. I considered fuel starvation but there was plenty of fuel. The transfer pumps were on from the auxiliary tank to the main. I shut the transfer pumps off and reduced power to about 15 inches to stop the shaking/rough running. I called center to let them know that I could not maintain altitude and [a suitable alternate] was at my 10 o'clock and nearby. I informed them that I wanted to land [there]. Center approved my descent and eventually handed me off to tower. I set up for a 4 mile final to runway xx. During this 4 mile final I powered up the engines and they appeared to come up to 25 inches manifold and 2400 RPM with no vibrations or roughness. I proceeded to a normal landing on runway xx. I had the a&P mechanic there check out the left engine for any problems. They spent about an hour looking for issues and none were found. A run-up and mag check was performed and everything was normal. I made a test flight and the takeoff; climb out and remainder of the flight was normal. I returned to [the airport]. Upon my return I was told to contact the FAA and provide [them] via email a copy of the maintenance logbook entry provided by the a&P mechanic and also provide a written statement via email before I could return the aircraft to service. This information and emails were provided via email and I was told that I could return the airplane to service.the airplane is maintained properly and no corners are cut in maintenance. As the pilot I train once a year in the airplane for the insurance certificate and I just received my ATP one month ago. One consideration is that there was water in the fuel; although all 12 sumps were checked prior to the flight. The auxiliary tanks have bladders and it is possible water was trapped in a fold. I am not sure what else could have been done to avoid this.

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Original NASA ASRS Text

Title: C414 pilot reported diverting to an alternate airport after the left engine began to run rough. Water in the fuel was the suspected cause.

Narrative: While heading north at 17000 feet the left engine began shaking/vibrating. Manifold pressure was erratic as was fuel flow on the digital meter. A check showed normal cruise EGT and CHT (Cylinder Head Temperature). I reduced manifold pressure to about 26 inches and the shaking continued. The instruments continued to read normal except the manifold and fuel flow. I considered fuel starvation but there was plenty of fuel. The transfer pumps were on from the AUX tank to the main. I shut the transfer pumps off and reduced power to about 15 inches to stop the shaking/rough running. I called Center to let them know that I could not maintain altitude and [a suitable alternate] was at my 10 o'clock and nearby. I informed them that I wanted to land [there]. Center approved my descent and eventually handed me off to Tower. I set up for a 4 mile final to Runway XX. During this 4 mile final I powered up the engines and they appeared to come up to 25 inches manifold and 2400 RPM with no vibrations or roughness. I proceeded to a normal landing on Runway XX. I had the A&P mechanic there check out the left engine for any problems. They spent about an hour looking for issues and none were found. A run-up and mag check was performed and everything was normal. I made a test flight and the takeoff; climb out and remainder of the flight was normal. I returned to [the airport]. Upon my return I was told to contact the FAA and provide [them] via email a copy of the maintenance logbook entry provided by the A&P mechanic and also provide a written statement via email before I could return the aircraft to service. This information and emails were provided via email and I was told that I could return the airplane to service.The airplane is maintained properly and no corners are cut in maintenance. As the pilot I train once a year in the airplane for the insurance certificate and I just received my ATP one month ago. One consideration is that there was water in the fuel; although all 12 sumps were checked prior to the flight. The AUX tanks have bladders and it is possible water was trapped in a fold. I am not sure what else could have been done to avoid this.

Data retrieved from NASA's ASRS site 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.