Narrative:

We had filed an IFR flight plan at 10;000 ft. Along the route we encountered favorable winds; so while in flight we changed our destination to [an airport further along the route].about 10 miles northwest of ZZZ; we were in the clouds at 10;000 ft westbound when I felt the plane slow down somewhat and noticed that the temperatures (cht & egt) for cylinder #5 were low. Normal temperatures are about 360F/1400F and #5 was reading about 290F/1100F. Within a few seconds they had fallen to 270F/900F and then fell off the digital bar-graph scale on my JPI830 engine monitor.I can't recall which ATC facility we were talking to; but I keyed the mic and said that we had lost a cylinder and we needed to go to ZZZ right now. ATC first suggested a right turn to 120 and I replied that I wanted a left turn (as it was a shorter distance). ATC gave us the left turn and cleared us down to 8;000 ft followed by a correction and cleared to 8;400 ft.prior to starting the descent we had opposite direction traffic showing on ads-B maybe 5 miles in front of us at 9000 ft. My [passenger] (in the right seat) advised me of this and I remember ATC advising the other pilot of our situation. I'm not sure if ATC advised me of the other plane or not.after coming out of the clouds between layers; we realized we were still west of the small mountains that are west of ZZZ. We were cleared direct to the airport and were given a vector over those jagged mountains. While we were above them and the engine was still producing power; I was hesitant to go over the tallest peaks. After one or two miles we found a nice gap and crossed to the east side of the hills. We were cleared for [landing] but were above a thin layer and could not yet see the airport. After getting down to perhaps 6500 ft; we found ourselves in VFR conditions but too high to land. I did a large south turn to the south and was then in a position to land. As I pulled the throttle back it began to run quite rough and I moved the throttle back up a bit. I also advised tower that it might quit on the runway. We made an uneventful landing and on roll-out the engine quit and we coasted to a stop about 150 feet short of taxiway U.the fire department met us and; using my tow bar; we moved the plane to taxiway U past the hold-short line.after two days of maintenance it was discovered that we likely had a stuck valve on cylinder #5 but it was intermittent. The shop performed lycoming's service bulletin for cleaning the valve guides and we flew home uneventfully.

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Original NASA ASRS Text

Title: PA-28 pilot reported an inflight engine malfunction due to a stuck valve. The pilot was able to successfully divert to a suitable airport.

Narrative: We had filed an IFR flight plan at 10;000 ft. Along the route we encountered favorable winds; so while in flight we changed our destination to [an airport further along the route].About 10 miles northwest of ZZZ; we were in the clouds at 10;000 ft westbound when I felt the plane slow down somewhat and noticed that the temperatures (CHT & EGT) for cylinder #5 were low. Normal temperatures are about 360F/1400F and #5 was reading about 290F/1100F. Within a few seconds they had fallen to 270F/900F and then fell off the digital bar-graph scale on my JPI830 engine monitor.I can't recall which ATC facility we were talking to; but I keyed the mic and said that we had lost a cylinder and we needed to go to ZZZ right now. ATC first suggested a right turn to 120 and I replied that I wanted a left turn (as it was a shorter distance). ATC gave us the left turn and cleared us down to 8;000 ft followed by a correction and cleared to 8;400 ft.Prior to starting the descent we had opposite direction traffic showing on ADS-B maybe 5 miles in front of us at 9000 ft. My [passenger] (in the right seat) advised me of this and I remember ATC advising the other pilot of our situation. I'm not sure if ATC advised me of the other plane or not.After coming out of the clouds between layers; we realized we were still west of the small mountains that are west of ZZZ. We were cleared direct to the airport and were given a vector over those jagged mountains. While we were above them and the engine was still producing power; I was hesitant to go over the tallest peaks. After one or two miles we found a nice gap and crossed to the east side of the hills. We were cleared for [landing] but were above a thin layer and could not yet see the airport. After getting down to perhaps 6500 ft; we found ourselves in VFR conditions but too high to land. I did a large S turn to the south and was then in a position to land. As I pulled the throttle back it began to run quite rough and I moved the throttle back up a bit. I also advised Tower that it might quit on the runway. We made an uneventful landing and on roll-out the engine quit and we coasted to a stop about 150 feet short of taxiway U.The fire department met us and; using my tow bar; we moved the plane to taxiway U past the hold-short line.After two days of maintenance it was discovered that we likely had a stuck valve on cylinder #5 but it was intermittent. The shop performed Lycoming's service bulletin for cleaning the valve guides and we flew home uneventfully.

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.