On April 13, 1997, at 1600 central daylight time, a Pzl-Mielec M-18A, N6872, owned by American & European Aircraft Sales and operated by ASAP Airspray, under Title 14 CFR Part 91, sustained substantial damage during a forced landing following a loss of engine power near Sinton, Texas. The airline transport rated pilot was not injured. Visual meteorological conditions prevailed for the local work use flight and a flight plan was not filed. The flight departed the San Patricio County Airport about 10 minutes prior to the accident. During a telephone interview, conducted by the investigator-in-charge, the pilot reported servicing the airplane with 190 gallons of fuel and the hopper with water. A GPS unit had been installed on the aircraft and the pilot was practicing with the unit prior to beginning his summer spraying season for hire by Burkett Flying Service at Sinton. The pilot had completed several practice sessions that included flying the airplane back and forth to the airport where he talked with the GPS manufacturer's representative about the utilization of the unit. At the last departure, the pilot recalled that the fuel gauges indicated 1/4 full for each tank, which was enough fuel for 30 minutes of flying. The pilot further stated that the fuel gauges were accurate. Subsequently, during a swath, the engine lost power and a forced landing was initiated to a soft field. During the landing roll the airplane flipped over and came to rest inverted. Structural damage occurred to the propeller, vertical stabilizer, and rudder. The pilot reported that workers in the field stated that "one of the fuel caps was off" the fuel tank. The FAA inspectors examined the site and found only one of the two fuel filler caps. Both of the fuel filler points were open; however, there did not "appeared" to be any major fuel spillage under the wings at the fuel filler points when the airplane was "put back onto it's landing gear." Fuel flowed from the low point fuel drains, the fuel filter strainer bleed valve had a positive pressure, and the fuel filter was not contaminated. During an on site examination, a mechanic found that the engine driven pump had failed; however, the reason for the pump failure was not determined. The pump was replaced and the engine operated at full power without discrepancies. The mechanic repaired the vertical stabilizer, rudder, and propeller prior to the pilot flying the airplane from the field.