Narrative:

I took off [and] departure was without incident and all instruments were in the green. At 400 ft. I pulled the prop lever to 2500 rpm; proceeded to turn on course and called departure for IFR clearance. I received clearance via radar vectors. I did my cruise climb out at 120-125 kts. And performed checklist. Everything was running normally. At 5;000 ft. I switched the fuel pump to low.while I was climbing through 8;000 ft.; I noticed that the aircraft began moving back and forth. Trying to ascertain what was going on; I looked at the engine analyzer I noticed that cylinder 5 and 6 were running in the red and that the oil temperature was in the red as well. I concluded that I needed to figure out a way to cool the engine and confirmed that cowl flaps were open. I turned on the fuel pump on high because I noticed the fuel flow was now between 20-21gph and it needed to be 35gph in the climb. I also decided to not continue and find the nearest airport. I checked on the GPS and saw the nearest airport 5+ miles south from me. I contacted departure informing them that I need to divert due to a hot engine. I was instructed to descend to 3;000 ft. And expect vectors. I proceeded to make a steep descent to aid in the cooling of the engine and pulled the power to 20inches [and] the propeller to 2000 rpm to ease off on the performance a bit while descending.departure asked me if I needed assistance and I told them that I did not need any at the time. I was cleared left turn direct to [diversion airport]. I notified [ATC] when I had the field in sight and was cleared for the visual for. I set up the RNAV as a backup to the visual. For the approach I brought the props back to full forward and the throttle at 17'-18'. The temperatures had dropped during the descent and remained normal during the approach. I landed without incident and taxied to the ramp where I kept the engine running. I called the owner to inform him of what had taken place. He asked if possible and if comfortable if I could bring the airplane back to [home base]. I told him that I could try as I saw that the temperatures had gone back to normal and that I would do a run up before take off and make sure that everything was performing normally. Cylinder 5 and 6 indication was back to normal. I was at gph for 15 min and taxied back and did a run up. Everything read normally; nothing was overheating and departed VFR. [Distance enroute] is about 20 miles and the plan was to go at 2500 feet to stay below the bravo. I departed from on rwy 18 and there were no issues during the take-off roll nor were there any issues in the initial climb out. When I got about 4 miles out I noticed that the temperature of cylinder 4 began to rise not as high as cylinder 5-6 earlier however it was significantly above all the other cylinders including cylinder 5-6 at the time. I was approximately at 1200-1300 ft. I continue to climb and reached approximately 1700-1800 feet however the moving back and forth of the airplane from earlier started up again [and] the aircraft began to behave sluggishly. I began to look for a place to land at which point the airplane began to shake and I was unable to hold altitude. I was descending 300fpm. My initial thought was to land in the fields however due to the rains and the possibility of high corn stalks I surveyed the light poles and powerline poles and thought from the position I was in that the next best option I saw was to land on [a highway]. As I came in on a right base to the road the engine gave out; I was at about 90 kts. And dropped the gear. I tried to line up with the road as much as I could however ended up coming in at maybe a 10-20 degree angle. Due to low airspeed as well as how fast things were unfolding I did not want to get into a stall situation by banking more so my concern at that point was to get the aircraft on the ground as straight and level as possible. As soon as the plane touch the road; I lost directional controlof the aircraft as it skidded into grass area on the side of the road. On the ground I turned masters; mixture; tanks and exited the aircraft.

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Original NASA ASRS Text

Title: Pilot reported executing a forced landing on a highway after experiencing power loss associated with high cylinder head temperatures.

Narrative: I took off [and] departure was without incident and all instruments were in the green. At 400 ft. I pulled the prop lever to 2500 rpm; proceeded to turn on course and called Departure for IFR clearance. I received clearance via radar vectors. I did my cruise climb out at 120-125 kts. and performed checklist. Everything was running normally. At 5;000 ft. I switched the fuel pump to low.While I was climbing through 8;000 ft.; I noticed that the aircraft began moving back and forth. Trying to ascertain what was going on; I looked at the Engine Analyzer I noticed that cylinder 5 and 6 were running in the red and that the oil temperature was in the red as well. I concluded that I needed to figure out a way to cool the engine and confirmed that cowl flaps were open. I turned on the fuel pump on high because I noticed the fuel flow was now between 20-21gph and it needed to be 35gph in the climb. I also decided to not continue and find the nearest airport. I checked on the GPS and saw the nearest airport 5+ miles south from me. I contacted Departure informing them that I need to divert due to a hot engine. I was instructed to descend to 3;000 ft. and expect vectors. I proceeded to make a steep descent to aid in the cooling of the engine and pulled the power to 20inches [and] the propeller to 2000 rpm to ease off on the performance a bit while descending.Departure asked me if I needed assistance and I told them that I did not need any at the time. I was cleared left turn direct to [diversion airport]. I notified [ATC] when I had the field in sight and was cleared for the visual for. I set up the RNAV as a backup to the visual. For the approach I brought the props back to full forward and the throttle at 17'-18'. The temperatures had dropped during the descent and remained normal during the approach. I landed without incident and taxied to the ramp where I kept the engine running. I called the owner to inform him of what had taken place. He asked if possible and if comfortable if I could bring the airplane back to [home base]. I told him that I could try as I saw that the temperatures had gone back to normal and that I would do a run up before take off and make sure that everything was performing normally. Cylinder 5 and 6 indication was back to normal. I was at GPH for 15 min and taxied back and did a run up. Everything read normally; nothing was overheating and departed VFR. [Distance enroute] is about 20 miles and the plan was to go at 2500 feet to stay below the Bravo. I departed from on Rwy 18 and there were no issues during the take-off roll nor were there any issues in the initial climb out. When I got about 4 miles out I noticed that the temperature of cylinder 4 began to rise not as high as cylinder 5-6 earlier however it was significantly above all the other cylinders including cylinder 5-6 at the time. I was approximately at 1200-1300 ft. I continue to climb and reached approximately 1700-1800 feet however the moving back and forth of the airplane from earlier started up again [and] the aircraft began to behave sluggishly. I began to look for a place to land at which point the airplane began to shake and I was unable to hold altitude. I was descending 300fpm. My initial thought was to land in the fields however due to the rains and the possibility of high corn stalks I surveyed the light poles and powerline poles and thought from the position I was in that the next best option I saw was to land on [a highway]. As I came in on a right base to the road the engine gave out; I was at about 90 kts. and dropped the gear. I tried to line up with the road as much as I could however ended up coming in at maybe a 10-20 degree angle. Due to low airspeed as well as how fast things were unfolding I did not want to get into a stall situation by banking more so my concern at that point was to get the aircraft on the ground as straight and level as possible. As soon as the plane touch the road; I lost directional controlof the aircraft as it skidded into grass area on the side of the road. On the ground I turned masters; mixture; tanks and exited the aircraft.

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.