M-20 J (201) / Allegro
Flight Crew Commercial; Flight Crew Flight Instructor; Flight Crew Instrument
Flight Crew Last 90 Days 25; Flight Crew Total 1100; Flight Crew Type 1050
Human-Machine Interface; Situational Awareness
Aircraft Equipment Problem Critical; Inflight Event / Encounter Weather / Turbulence
Air Traffic Control Provided Assistance; Air Traffic Control Issued New Clearance; Aircraft Equipment Problem Dissipated; Flight Crew Diverted; General Maintenance Action
Weather; Human Factors; Aircraft
While flying IMC in rain at 15;000 FT my engine suddenly began running very rough and the aircraft began shaking mildly. The engine monitor showed that cylinder #4 (of the 4-cylinder Lycoming) wasn't firing. At this time; I also noted my level-cruise airspeed had gone from 150 KIAS (normal) to around 80 KIAS. I noted; too; that the 'ALT AIR' (alternate air intake) indicator light was illuminated. I alerted Center that I had engine trouble and requested an immediate descent. My onboard database showed that ZZZ was the nearest IFR airport. The Controller gave me a heading there but also offered another airport as an option because it was ahead of me and would not require turning back. I stayed with my decision to land at ZZZ because I had already programmed it as 'direct' into my GPS navigation system and was bringing up that airport's approach information on my computer. I didn't want to waste valuable cockpit management time reprogramming a marginally more useful airport into my navigation system.Within a minute or so; cylinder #2 also stopped firing as indicated on the engine monitor.The Center Controller was an excellent resource during this engine malfunction event. He provided me with headings to suitable airports for immediate landings. He also provided me with winds from nearby airports as ZZZ's AWOS was out of service; the preferred runways; and known traffic. His vectors to the airport had me perfectly aligned with the active runway when I descended out of IMC. During the descent; cylinder #2 returned to service and the aircraft engine became smooth and responsive to the throttle. I was able to make an uneventful landing. However; cylinder #4 was still inoperative as I taxied to the ramp and as I shut down the engine.An inspection by an FBO Mechanic the next day revealed no mechanical failure of the engine or airframe. The Mechanic suspected a sudden intake of moisture through the alternate air intake manifold had fouled plugs in two of the cylinders. I 'test flew' the aircraft at ZZZ for a few landings; then flew back to its home base two days later in VFR conditions with no mechanical issues.
An M20J pilot operating at altitude in rain suffered the loss of two cylinders as evidenced by his engine monitor and accompanying power loss and rough running. ATC assisted in diverting to the nearest suitable airport where a safe landing was made with; now; three of the engine's four cylinders functioning. Maintenance found no mechanical issues and suggested water ingestion may have been the culprit. Test flights and a return to base reinforced that assessment.