Re: Emissions

[Mark Shaw <autox@inficad.com>]

Chris Coracini wrote:

> >If your controller and oxygen sensor are working properly
> >there is no adjustment required to lean it out.
>
> not true during idle, you can adjust the CO with a 3mm allen tool.

Yes, you can lean out the mixture with the 3mm hex screw, but youare
overriding the controller and O2 sensor in the process.  This
limits the ability for the controller to adjust the fuel mixture back
to correct ratio for all conditions.

> >I suspect they "leaned it out" be over adjusting the idle
> >air mixture (hex head screw in air flow meter) to limit
> >the fuel flow at idle despite what the controller was trying
> >to do based on a bad O2 sensor.  This will work for
> >passing an idle test, but makes the mixture too lean in
> >some cases.  This is not a good thing to do.
>
> 16V's have an idle switch that switches the FI into open-loop (no O2
> sensor feedback) during idle.  this allows you to adjust the mixture
> using the 3mm tool.  when you do adjust the idle, the O2 sensor isn't
> going to do a thing to change it.  during normal operation, neither
> throttle switch is closed and the system runs closed-loop.  this is
> when
> the O2 will control the mixture at 14.7:1 no matter what the CO is set
>
> for (unless it's WAY outta wack and the O2 cannot compensate for it,
> but
> then it would barely run anyway).  other open-loop situation is during
>
> WOT for full-load enrichment.

This does not match with my experience (or the Bentley manual).  It
alsodefies a significant reason for the O2 sensor in the first place.
The idle
switch is for the idle stabilization control (which is driven by the
controller
on the 16V, rather than a separate plug-in relay module) and also
controls
fuel cutoff when you are decelerating.  The system stays in open-loop
during idle as evidenced by the fluctuating differential pressure
regulator
current.

The proper way to set the idle mixture is to monitor the differential
pressure regulator current and "center" it during idle so that the
O2 sensor can compensate for variations in fuel, air density,
temperature, altitude, etc.

Also, the engine will run just fine with the fuel regulator "pegged"
to one side so that the O2 sensor control cannot compensate.  May
idle weird, but it will run down the road fine.  I have driven cars
with the adjustment pegged and felt little difference except for
unstable idle speed.

Since most engine faults can be cured by running richer mixtures,
the common mistake is to adjust the mixture too rich to smooth out the
engine and then adjust the idle speed.  The net result is the controller

has no adjustment range and both emissions and fuel economy take
a hit.

I am not aware of any engine control design which will allow
open-loop operation during idle and still meet the EPA test cycle
required for it's initial manufacturing certification.

Bottom line... do not adjust the idle mixture outside the O2
sensor compensation range.  If the engine exhaust is out of
spec, go track down the fault with the O2 sensor, controller,
the other sensors, or the harness.

regards,

Mark



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