A method of diagnosing a malfunction of a tuning valve of an intake manifold coupled to an internal combustion engine is provided. The method includes: commanding the tuning valve to at least one of a closed and an opened position; computing an average deviation of manifold absolute pressure; determining an average deviation threshold using fuzzy logic based on engine speed and airflow per cylinder; comparing the average deviation to the average deviation threshold; and diagnosing a malfunction of the tuning valve based on a comparison of the average deviation and the average deviation threshold.