Date:         Wed, 8 May 2002 23:14:08 -0500
Reply-To:     John Clavin <jc@AUSTIN.RR.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         John Clavin <jc@AUSTIN.RR.COM>
Subject:      Re: A damn OS question again, yea!!! now i need some help,
              it's my turn....
Content-Type: text/plain; charset="us-ascii"; format=flowed

At 02:12 PM 5/7/02 -0400, you wrote:
>I have two problems with this; first, you really cannot tell this with a
>regular meter that takes 2-3 readings a second -- the ECU changes the
>mixture on its own schedule that does not correspond to the averaging time
>of the meter.  If you want to know what the actual values are you have to
>use a scope.  Second, the feedback loop between the O2 sensor and ECU is
>very simple: if the ECU sees > 500 mV it leans the mixture; if it sees <
>500 mV it richens the mixture.  It does not attempt to hold exactly at the
>500 mV mark but rather is constantly flipping back and forth past it,
>therefore the mixture is constantly going from *very* slightly lean to
>*very* slightly rich and back.

David,

I have been trying to puzzle out exactly what is happening in the ECU-O2
sensor loop for some time now. Do you have any idea as to the frequency at
which the ECU attempts corrections? Recognizing that it is an electronic
device, it is inherently capable of updating the fuel injector pulse width
faster than the engine can respond. So it seems to make sense that the ECU
not change the fuel pulse width quicker than, say, once every N rpm, where
B is possibly as low as 1 rpm. Or does it?

The reason I ask, is that I see some inconsistent readings from my O2
sensor which I think might be caused by the DVM sample rate and ECU update
rate synchronizing or beating together for an interval. For example, at
times I see something pretty close to the expected crossovers, something
like H-H-L-H-L-L-H-L-H-H-L (where H>.5v and L<.5v). But other times, the
readings seem to tend to the high side and run more like
H-H-H-L-H-H-H-H-H-H-L-H-H-H-L. The latter suggests that the ECU really is
updating to keep swinging back and forth over the crossover point, but that
the DVM's sample rate is running at, say, half the ECU rate, so only
catching the readings on the high side.

At first, I attributed this to the sampling rate/clock rate differences.
But more recently, I have suspected that the latter readings are indicative
of a problem. First, it seems that I see more readings biased to the high
side, than the low. I almost never see L-L-L-H-L-L-L-L-L-L-H-L during
normal cruise or idling. Second, I see the bias to the high side mostly
when I have been cruising a while at 50+ mph. In some cases it seems to
have been followed by a bout of hesitation/coughing behavior. I am not
convinced as yet, however, that it is not just a coincidence or my
imagination.

I would like to catch this on a scope as you suggest. Unfortunately, I can
only do that in my garage, where not much exciting ever happens ECU or
O2-wise.

John