Variable valve timing for cylinder deactivation

An engine method comprising,

• operating a first group of combusting cylinders on a first bank with valve timing adjusted to provide a first, higher flow of charge from a first intake manifold to a first exhaust manifold;
• operating a second group of non-combusting cylinders on a second bank with valve timing adjusted based on an exhaust air-to-fuel ratio at the second bank to provide substantially no flow of charge between a second intake manifold to a second exhaust manifold;
• and
• wherein operating the second group of non-combusting cylinders on the second bank with valve timing adjusted to provide substantially no flow of charge includes, in response to the exhaust air-to-fuel ratio sensed at the second bank being leaner than stoichiometry, adjusting the valve timing to a first timing to reduce flow of charge from the second intake manifold to the second exhaust manifold, and in response to the exhaust air-to-fuel ratio sensed at the second bank being at stoichiometry, adjusting the valve timing to a second timing to reduce flow of charge from the second exhaust manifold to the second intake manifold.

An engine method, comprising:

• operating a first group of cylinders on a first engine bank to combust and exhaust a larger amount of gas to a first catalyst and then to an exhaust junction at a first, higher flow rate;
• while operating a second group of cylinders on a second engine bank to not combust and alternate flow direction of a smaller amount of charge between a second catalyst and the exhaust junction at a second, lower flow rate including reversing the flow;
• and
• indicating a post flange exhaust leak based on an exhaust air-to-fuel ratio of the second bank being leaner than an exhaust air-to-fuel ratio of the first engine bank during reverse flow through the second bank.