Variable displacement engine diagnostics

A method for monitoring a cylinder valve deactivation mechanism coupled in an engine, the cylinder valve deactivation mechanism coupled to at least a cylinder valve of a cylinder of the engine, the cylinder having a plurality of cylinder valves with variable cam timing, the method comprising:

• monitoring a signal indicative of intake manifold pressure of the engine;
• sampling the signal at least over a movable crank angle window, the crank angle window adjusted responsive to adjustments in cam timing;
• and
• indicating degradation of the cylinder valve deactivation mechanism in response to a magnitude of the sampled signal at or around a characteristic resonant frequency of the intake manifold pressure.

A system, comprising:

• an engine having a crankshaft and a plurality of cylinders, each cylinder with a plurality of cylinder valves;
• a deactivation mechanism for deactivating at least one cylinder valve;
• a variable cam timing mechanism for adjusting a timing of at least one cylinder valve;
• an intake manifold;
• a sensor providing information indicative of manifold pressure;
• and
• a control system configured to receive a signal from the sensor, sample the signal indicative over a movable crank angle window, the crank angle window adjusted responsive to adjustments in cam timing and including an intake stroke of a cylinder of the engine, indicate degradation of the valve deactivation mechanism in response to a magnitude of the sampled signal at or around a characteristic resonant frequency of the intake manifold pressure, and to differentiate valve degradation among the plurality of cylinder valves of the engine based on the signal content and a crank angle at which the signal content is greater than a threshold amount.