We had a simular problem a few years back up here in Kodiak. Above crossover the TD and prop did it\'s job and everything was fine but below crossover and even into reverse we had a FF and TIT flux. Changed fuel control(2), fuel pump and then the TD Valve. Still did it and we needed the plane so we had to change the engine. What finally was the problem was a 3 way elbow/valve assy thats on the bottum of the TD valve and was allowing fuel to flow through it(wrong diection). Of coarse when we did the TD valve change we took the old elbow off and put new orings and installed on new TD valve. What a waste of man hours but I will never forget that again and always put new on new if available!!