My L4630, four cylinder diesel started knocking real loud today and emitted blue smoke under power. I only have 880 hrs on the engine and this happened once before. I am guessing it is an injector that is the problem but I can't remember how to test to see which one it is? I let it cool off for several hours and also put additive in the fuel tank to dry up any water that might have formed in the tank. After I restarted the tractor I took it out to put a load on it as before and it was fine for the first few minutes and then after it warmed up it started knocking again. When it is knocking I experience a lack of power.Any help on diagnosing this problem would be greatly appreciated. Thanks......