The pilot of an amateur-built Sonex airplane was initiating a test flight to check a new propeller. After advancing the throttle, the airplane accelerated with normal power. However, at an altitude of 300 feet, the experimental UL Power 260i engine lost total power. The pilot turned back to the departure runway, and landed on soft ground to the south of the runway. The landing gear collapsed, substantially damaging the firewall. Examination of the airplane revealed that continuity existed throughout the engine drive train, and the engine had no visible damage. The fuel was sampled and no contamination was observed. Examination of the fuel filter revealed no obstructions, the fuel pump operated normally, and the air filter was clean. The spark plugs were determined to be functional; however, no electrical energy was being delivered to the spark plugs. Further examination revealed that the electronic control unit (ECU) was not transmitting voltage to the dual coils. The ECU was manufactured to automotive specifications, and was supposed to be installed in an area of the airplane where temperatures do not exceed 65 degrees Celsius (preferably on the cabin side of the firewall). The ECU on the accident airplane was installed on the engine side of the firewall.