A method is disclosed for controlling operation of an engine coupled to an
exhaust treatment catalyst. Under predetermined conditions, the method
operates an engine with a first group of cylinders combusting a lean
air/fuel mixture and a second group of cylinders pumping air only (i.e.,
without fuel injection). In addition, the engine control method also
provides the following features in combination with the above-described
split air/lean mode: idle speed control, sensor diagnostics, air/fuel
ratio control, adaptive learning, fuel vapor purging, catalyst temperature
estimation, default operation, and exhaust gas and emission control device
temperature control. In addition, the engine control method also changes
to combusting in all cylinders under preselected operating conditions such
as fuel vapor purging, manifold vacuum control, and purging of stored
oxidants in an emission control device.
