What happened
While operating in cruise flight, the pilot reported that the engine began running roughly and losing power. Smoke entered the cockpit, accompanied by a clanking noise originating from the engine. The pilot subsequently elected to perform a forced landing on I-20 West. During the descent, the pilot failed to extend the landing gear, resulting in the aircraft striking the guard railing separating the eastbound and westbound lanes. The impact caused substantial damage to the fuselage and wings.
The investigation
The engine was removed and sent to Textron Lycoming for examination. An inspection of the number one and number four spark plugs revealed oil and heavy carbon debris. Heavy carbon debris was also identified in the exhaust valve guide area. Investigators found that the number four exhaust valve was stuck in the open position. The tip of the number four exhaust valve exhibited damage signatures consistent with operating without the rotator cap; the cap was later located at the bottom of the number four rocker cover.
Upon examination with an optical comparator in the Lycoming Metrology laboratory, the number four exhaust valve stem appeared straight. However, the number four piston dome showed a circular mark consistent with an impact from the exhaust valve. Additionally, the number four cylinder exhaust push rod and shroud tube were found broken and separated from the engine upon receipt. The hydraulic lifter for the number four cylinder exhaust was also found to be below minimum leak down specifications.