On July 23, 1994, at 0815 central daylight time, a Cessna A188B airplane, N9140G, operated by Harold Thomsen 4 Aerial Application of Wymore, Nebraska, was destroyed during a forced landing shortly after takeoff at Thomsen Field near Wymore, Nebraska. The commercial pilot sustained minor injuries. Visual meteorological conditions prevailed for the aerial application flight conducted under CFR 14 Part 137. No flight plan was filed. The pilot reported he was about 100 feet AGL when the engine lost power. He made an emergency landing in a plowed field about one half mile from the departure end of the runway. He said the landing gear collapsed during the landing. The wreckage was examined by an FAA airworthiness inspector who reported no evidence of preexisting aircraft malfunction. The engine was installed in a test cell by Central Cylinder Service Inc., of Omaha, Nebraska. The engine started uneventfully but lost power at 1600 rpm due to a loss of fuel pump pressure. A serviceable fuel injector pump was installed and the engine performed normally. Inspection of the fuel injector pump revealed one spring retaining plate was not installed. The model and serial numbers on the faulty pump were different from those specified in the engine log book. Investigation failed to reveal the source of the faulty pump.