On June 13, 1994, at 1743 eastern daylight time, a Beech BE18, N231LJ, operated as a Part 91 positioning flight, sustained substantial damage when the left engine caught fire during the landing roll on Runway 27L at the Willow Run Airport in Belleville, Michigan. The pilot and the copilot reported no injuries. Visual meteorological conditions prevailed for the flight, an IFR flight plan was filed. The flight originated from Chesterfield, Missouri approximately 1530. The pilots reported the flight was normal until they were on the downwind leg in the traffic pattern. At that point they observed the left engine fuel pressure gauge was indicating below normal. The fuel boost pump was activated and the fuel pressure climbed back up into the green. The pilots continued the approach and made a normal landing, with both engines running. They stated as the airplane decelerated on the runway, the left engine backfired 2 or 3 times, then caught fire. The pilots brought the airplane to a stop on the runway and exited. Crash Fire Rescue personnel arrived and extinguished the fire. The accident airplane was a one-of-a-kind modification, a turbine engine conversion using Garrett TPE 331-6 engines, as described in Supplemental Type Certificate (STC) Number SA2958WE, appended. Postaccident investigation revealed the left engine #3 fuel line (P/N 869518-1), which runs between the fuel control unit and the oil/fuel heat exchanger, failed near the heat exchanger. Photographs were taken, then the fuel line was removed and discarded by the operator. The engine manufacturer's engineering department examined the photographs in an attempt to determine how the fuel line (P/N 869518-1) failed. The examination revealed the fuel line installed on the accident airplane appeared to have the same span, but "significantly different bends" when compared to P/N 869518-1 blueprints. The engineer's report indicated due to the unique STC installation, "...a dimensional 'stack' to check the fit of the customized tube on the engine was not performed." The engineer's report is appended.