On May 16, 2021, about 1545 Pacific daylight time, a Cessna 210 airplane, N7350E, was substantially damaged when it was involved in an accident near Lake Arrowhead, California. The pilot and passenger were seriously injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that, while climbing through an altitude of about 8,500 ft mean sea level (msl), while enroute to Big Bear Airport (L35), Big Bear City, California, he noted a partial loss of engine power. He contacted air traffic control and was provided with alternate airports and current weather information. While over mountainous terrain, the engine lost total power. Concerned that he did not have sufficient altitude to clear rising terrain, the pilot initiated a forced landing to a clearing on sloped terrain. The airplane subsequently collided with vegetation and came to rest upright on a steep slope. Postaccident examination of the airplane revealed that both wings and the fuselage were substantially damaged. A subsequent examination of the airplane revealed the upper and lower spark plug electrode areas were black in color. Additional examination of the fuel flow manifold valve, throttle body/fuel control, and engine-driven fuel pump revealed a malfunction of the fuel flow manifold valve plunger, which prevented normal operation. The fuel flow manifold valve was connected to a test stand, and the valve failed to open when fuel pressure was applied. The fuel flow manifold valve was disassembled, and tooling marks were observed on the plunger. The seal near the plunger was deformed and not seated flat on the manifold body. The diaphragm spring was longer when compared to a normal spring. Tooling marks were observed on the fuel flow manifold plunger, as seen in Figure 1. The plunger assembly and diaphragm were substituted in the fuel flow manifold valve. The fuel flow manifold valve was reattached to the test stand and performed within manufacturer specifications when tested. A review of the airplane’s maintenance records could not determine if any maintenance had been performed on the fuel flow manifold valve. Figure 1. View of tooling marks on fuel flow manifold valve plunger