On August 14, 2020, about 1143 central daylight time, an experimental Siebel Auriga airplane, N505JS, was substantially damaged when it was involved in an accident near Rockford, Illinois. The pilot was not injured. The driver of an automobile sustained minor injuries and the passenger in the automobile was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that he departed runway 18 at Cottonwood Airport (1C8), Rockford, Illinois, and continued north of the airport where he intended to monitor engine oil temperatures after making changes to the engine cowling to increase engine cooling. The pilot reported that the engine “sputtered and quit” about 3-5 miles north of 1C8 while climbing through 3,000 ft mean sea level. He changed fuel tanks and turned on the auxiliary fuel pump. The pilot noted that the engine restarted a couple of times, but it would lose power when he increased the engine throttle. The pilot completed a 180° course reversal for a straight-in forced landing on runway 18 at 1C8. The pilot reported that the airplane was high on final approach and that he performed a forward slip maneuver to lose additional altitude. The airplane touched down about midfield on the wet turf runway and overran the end of the runway despite the pilot using maximum braking. The airplane traveled through the airport perimeter fence and onto a street where it was struck by an automobile. The airplane’s left wing and its rear spar attach point were substantially damaged during the accident. A postaccident examination conducted by a Federal Aviation Administration airworthiness inspector did not identify any anomalies or failures with the airplane's fuel tanks, fuel tank vents, fuel tank selector valve, fuel strainer assembly, P-leads, and ignition switch. The pilot reported that he departed on the flight with 45 gallons of fuel and observed fuel leaking from the airplane following the accident. The pilot estimated that 25 gallons of fuel were drained from the airplane before it was recovered from the accident site. The engine was removed from the airplane and mounted in a test cell where it was run multiple times. The postaccident engine test runs did not identify any mechanical malfunctions or failures that would have precluded normal operation. The engine-driven fuel pump functioned normally during the engine test runs. The auxiliary fuel pump functioned, but the output pressure was below the engine manufacturer's minimum specification.