On December 04, 2023, about 1250 Hawaii-Aleutian standard time, a Diamond DA-40 airplane, N691YW, sustained substantial damage when it was involved in an accident in Kapolei, Hawaii. The pilot sustained minor injuries, and the designated pilot examiner sustained serious injuries. The airplane was operated by the pilot as a Title 14 Code of Federal Regulations Part 91 flight test. The pilot reported that an engine run-up inspection was completed without any issues of concern before departure. They departed from Danial K Inouye International Airport (PHNL) Honolulu, Hawaii, about 1235. After departure they leveled off at 1,500 ft mean sea level and proceeded to the practice area to conduct an evaluation for an airplane instrument rating. The pilot configured the airplane for the first maneuver. When he moved the throttle control lever forward, they heard a “pop” sound come from the engine, followed by a rumbling noise and an airplane vibration. The pilot pulled the throttle back to its original position in an attempt to stop the rumbling and vibrations and turned the airplane back toward runway 11 at Kalaeloa Airport (PHJR), Kapolei, Hawaii, for an emergency landing. They scanned the ignition, control levers, fuel selector, and multi-function display. The pilot confirmed that the mixture control lever was full forward, the propeller control lever was full forward, and the throttle control lever was about 3/4 full forward. The ignition was positioned on "BOTH", the fuel selector knob was positioned directly over the right tank, and the multi-function display showed no discrepancies in engine instrument readings that would indicate a need for immediate corrective action. The airplane started a descent without any manual pitch adjustments. They increased the throttle lever to the full forward position, but this did not provide any additional thrust from the engine. They declared an emergency; the designated pilot examiner took the flight controls, and they began to look for a suitable area for a forced landing. They performed a forced landing to a flat area of land that was covered with dirt. After initial touchdown, the airplane continued to roll straight down the dirt path until it impacted a dirt hill, resulting in substantial damage to the fuselage and both wings. A postaccident examination conducted by a Federal Aviation Administration inspector and a certified airframe and powerplant mechanic revealed no internal failure of the engine. The fuel injection servo control was in the full throttle position and the mixture control was in the full rich position; neither cable could be moved in the cockpit due to impact damage. The fuel injection servo was removed for inspection, and it revealed that the throttle/butterfly did not move and was in the full-throttle position. The valve covers and lower sparkplugs were removed; no anomalies were noted. No obstructions were observed in the fuel system. The air filter was removed with no obstruction observed. Internal baffling in the exhaust muffler had broken loose and had blocked about 3/4 of the opening. The last 100-hour inspection was completed on November 15, 2023, with the engine total time of 2,290.5 hours