Methods and systems are provided for detecting degradation of a positive crankcase ventilation (PCV) system valve and a cylinder valve. In one example, a method may include comparing a rate of pressure bleed-up in a PCV system during each of a cold-engine condition and a hot-engine condition to differentiate between the PCV valve and the cylinder valve.

A control method for an internal combustion engine which includes a variable compression ratio mechanism: controlling the compression ratio by using a control target compression ratio whose a variation amount per unit time is limited to be equal to or smaller than a predetermined variation amount threshold value; and setting a first variation amount threshold value which is the variation amount threshold value of the control target compression ratio per unit time toward a high compression ratio side, to be smaller than a second variation amount threshold value which is the variation amount threshold value of the control target compression ratio per unit time toward a low compression ratio side.

A method for operating an internal combustion engine includes providing a vehicle having an internal combustion gasoline engine including multiple cylinders and wherein the engine is capable of running on at least one of a plurality firing fractions, providing a vacuum offset (Offset.sub.vac) to adjust airflow capacity for each of the plurality of firing fractions, determining a torque capacity of each of the plurality firing fractions and a plurality of available firing fractions that provides at least enough torque capacity to accommodate a current torque requested (T.sub.req), determining a plurality of viable firing fractions of the plurality of available firing fractions, and determining and implementing an optimal firing fraction of the viable firing fractions if the optimal firing fraction provides enough fuel economy benefit over a current firing fraction.

A method for operating an internal combustion engine includes providing a vehicle having an internal combustion gasoline engine including multiple cylinders and wherein the engine is capable of running on at least one of a plurality firing fractions, providing a vacuum offset (Offset.sub.vac) to adjust airflow capacity for each of the plurality of firing fractions, determining a torque capacity of each of the plurality firing fractions and a plurality of available firing fractions that provides at least enough torque capacity to accommodate a current torque requested (T.sub.req), determining a plurality of viable firing fractions of the plurality of available firing fractions, and determining and implementing an optimal firing fraction of the viable firing fractions if the optimal firing fraction provides enough fuel economy benefit over a current firing fraction.