[s-cars] Motronic musings: ECU guru's take a look (long)

[Sandy Sligh]

With regards to out of wack emissions at idle, I've
tried to imagine the senario's that would create an HC
rich exhaust:

First:

Especially at idle, isn't the A/F ratio monitored in a
closed-loop fashion by the 02 sensor? if the sensor
sees anything richer than the 14:1 mixture deemed as
optimal for converter efficiency, shouldnt the ECU cut
back on the injector timing?  Unless there is a
minimum injector pulse width that the ECU is butting
up against in attempts to lean out the mixture.  Could
this be happening?

Now, what about this senario:

There is a vacuum leak after the MAF. The ECU takes
the MAF input data, calculates the proper injector
timing to match that value and then the engine runs.

However, the 02 sensor, seeing an oxygen rich exhaust
due to more air being let in via the vacuum leak gives
the ECU a reason to up the injector timing in order to
compensate for the vacuum leak.  But, what happens in
the ECU when the O2 sensor reading doesn't mesh with
the data the MAF sensor is providing. The MAF says
there shouldn't be a lean conditio, the O2 sensor says
there IS a lean condition. Which component does the
ECU yield to?

I would guess that the MAF wins that fight, or else
the ECU would continue to correct for the vacuum leak
and no one would ever know. The ECU renormalizes
itself, and the car runs LEAN at idle, leading to a
"lean misfire" condition.

Third Senario:

In the case the fuel pressure was higher than it
should be, for whatever reason (AAN vs RS2): This
would have the result that for a particular injector
opening time, more fuel would come out than the ECU
would expect. This would be sensed by the O2 sensor as
a RICH condition. Wouldn't the ECU dial back the
injector timing to compensate. Then again, there is a
conflict in that the ECU see's no reason for the
injectors to be outputting more fuel than necessary so
perhaps it doesn't dial back the timing (instead using
the new rich version of the injector output as the
baseline?) creating a RICH condition at idle.

Fourth Senario:

There is a misfire in the ignition system caused by
faulty boots or faulty PSO or whatever. The misfire,
preventing combustion, creates oxygen rich exhaust.
The O2 sensor senses the O2 rich exhaust and informs
the ECU, who in turn sees no out of wack signal from
any of its other control inputs, thus continues to
run. The ECU renormalizes the baseline O2 sensor
reading and the engine continues to run unoptimally
resulting in high HC emissions.


Anyway, for anyone who understands the control systems
present in the Motronic software, let me know where my
reasoning is faulty or is correct.


Sandy
Scratchin his noggin

__________________________________________________
Do you Yahoo!?
The New Yahoo! Search - Faster. Easier. Bingo
http://search.yahoo.com