Idle fuel mixture setting

[Bohdan.L.Bodnar at att.com]

Jonathan,

You are correct that GM stuff tries to run closed loop at idle.  Note that
I wrote "some engines" (I had Ford's 2.3 liters engine in mind...) run
open loop.  Hence, this was not a "blanket statement."

The statement about running richer at idle is, to the best of my knowledge,
correct.  This is done for several reasons (I mentioned the increased
scavanging time -- roughly, this can be viewed as an increase in EGR).  The
14.7/1 a/f ratio is required for NOx reduction, not HC, CO oxidation.  At
idle, there is little NOx formation and therefore the need for tight operating
bounds is not as great.

Cordially,

Bohdan