Abstract: A diagnostic method for a reciprocating internal combustion engine is disclosed. The diagnostic method includes determining a cylinder pressure relationship between a periodic or continuous pressure of a combustion chamber of the engine and a crankshaft angle of the engine for a combustion cycle including a fuel injection. The method further includes determining a heat release rate relationship between the heat release rate in the combustion chamber and the crankshaft angle as a function of the pressure relationship. The method includes estimating an end of fuel injection crankshaft angle for the combustion cycle as a function of the first derivative of the heat release rate relationship.