Idle fuel mixture setting

[Mark Shaw]

> Bohdan.L.Bodnar at att.com writes:
> > Idle actually runs richer than 14.7:1.  There are several reasons for
> > this (such as longer exhaust scavanging time at idle than off idle).  I
> > also know that some engines which do not use heated oxygen sensors run
> > open loop at idle;  this is because the O2 sensor will cool off and present
> > incorrect readings to the computer which will then cause the engine to
> > "hunt" as the air/fuel mixture varies wildly.

Jonathan Lusky writes: 
> Whoa, be careful with the blanket statements.  I don't know about Fords, but
> GM stuff DEFINATELY tries to run closed loop at 14.7:1 at idle.  I think
> your statment applies more to earlier attempts at closed loop fuel
> control than it does to modern systems.  From an emissions standpoint,
> you'd want to be lean if anything at idle.

Double Whoa!  The whole point of most post-1981 systems is to maintain
the air-fuel ratio at stoichiometric (14.7) regardless of the engine's
steady-state condition (transients usually violate the rule).  Otherwise,
the three-way catalyst will go out of it's narrow band where it will both
reduce NOx (slightly rich of stoich) and oxidize CO and HC (slightly
lean of stoich).  A few percent either way and the catalytic converter's
efficiency falls rapidly.   Only the more recent systems with UEGO
sensors will try to operate to the lean side of stoichiometric.

Operation at 14.7 is a necessary condition for 3-way cats.  The problem
with unstable idle is usually the result of a deteriorated O2 sensor
whose response time is too slow at the lower idle temps or the sensor
has been located too far from the exhaust ports.

The only open-loop operation I have seen is during the warmup phase.
At least this is the Bosch approach to fuel control.

Mark