model PB337 need help black smoke shut down

engine PX-7 have a truck here it if you try to rode test the truck it gives off black smoke no power and dies, but if you keep it in natural the trucks runs fine no black smoke and can run at high idle. I have check the turbo also check and the exhaust to make sure that was clear there is no restriction I've change the fuel filter and the air filter and also have change the fuel rail pressure sensor. has anyone run across this problem before