Date: Tue, 4 Jul 2006 20:20:16 -0400
Reply-To: David Beierl <dbeierl@ATTGLOBAL.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@ATTGLOBAL.NET>
Subject: OOPS TYPO Re: 88 digifant O2 readings off
In-Reply-To: <C0D04C12.65C3%msutts@earthlink.net>
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At 04:47 PM 7/4/2006 -0400, Matt Sutton wrote:
>Thanks David!
You're welcome. :)
>Super helpful, as your posts always seem to be. As far as marking the AFM
>position before fiddling...... well my shame is no longer secret.
Do as I say -- f'heaven's sake don't do as I do. BTDT.
> I know
>it's a no-no; my only justification was that with the target open loop O2
>sensor voltage being .5 volts, it seemed that the high reading indicated a
>wrong setting, and not worth noting. Not much of an excuse, really.
My reasons for screwing up always look better before I get bitten.
>It is back where it started now, more or less. Between the idle adjustment
>and the AFM, it seems like there's only a small window where the idle sounds
>right, smells right, etc, in open loop mode.
Messing with the AFM is scary. SOMEBODY knows the expected curves of
voltage output per flap displacement and flap displacement per
rpm. The people at Bournes obviously know what values to shoot for
when they're laser-trimming that ceramic resistor plate, and the
people who assemble them know what the calibration procedure is to
make the output correct on a new assembly. But they all seem to be
related to Dick Francis's solicitors who "would make a clam look
sloppy." And anyway you haven't got a new resistor plate, you've got
an old worn-out one. And you've got two things you can fiddle with,
the resistor array and the spring tension. And of course a belief
that something in the AFM has changed so it's no longer doing its job
correctly. Is the spring losing tension? Every time I look at that
spring I ask it "Are you the kind of lily-livered overworked spring
that sneaks aat t'the blower at all hours 'stead of keepin' steadfast
to your duty?" And it just grins a little and says "Stiddy on,
Sarn't. Not even worth switchin' 'ands fer this lot. Naow there's
*some* we could put a name ter..." and it sort of shoots its eyes
over to the corner where the resistor pack seems to be in a muck
sweat, jittering back and forth with little bits of carbon pouring
off it, looking pale and...translucent almost. It suddenly notices
me and jumps, and looks resentful. "Doin' m'duty Sarn't." it
grits. "But that wiper's got bleedin' cold 'ands." and it runs off
muttering under its breath. Well everyone knows the wiper's been in
a permanent snit ever since they wired its nose to its shirt-collar
and took away its carbon button. Not the slightest use talking to
*it* about anything except its bleedin' button which it can do no
bleedin' wrong and how *dare* they and it's perishin' crool and on
and on 'til you want to sick up. Can't blame the lad entirely, I
'spose it was a bit of a shock, but bleedin' get over it, mate. We
all 'ave our lot.
<ahem> Well as I was saying...I'd try to find the spring setting
that comes closest to stoichiometric mixture (half-volt output from
the O2 sensor) at all rpms with the engine at normal running
temp. I'd hope that the spring tension would primarily change the
slope of the curve and the idle screw would primarily walk the whole
curve up and down. Span and zero, in other words. And I'd prepare
for a tedious time because the whole reason that the ECU uses the
"bang-bang" style of control is that the O2 sensor is too sensitive
to do much more than tell you that you just overshot. Whatever the
size of each incremental adjustment that the ECU makes in the
mixture, one of those steps is enough to move the sensor from near
full-scale one way to near-full the other way. A correctly set-up
engine and AFM will operate perfectly correctly with no oxygen sensor
-- for every purpose except giving the catalytic convertor the
starting conditions it needs to do its job correctly.
The idle plug is just a calibrated leak that lets air get to the
engine without being measured. For a first guess I'd assume that
it's already in turbulent flow at idle speed and is more-or-less
linear, 2x rpm --> 2x air. If the initial starting position is lost
I'd begin by setting it to the middle of its range.
Good luck. Trying to figure this stuff out with so little
information about *correct* operation makes my head hurt.
> I guess I'm still concerned
>about the high o2 reading because of the (lucky)correspondence between that
>voltage number and CO output. Seems like I'm pushing it as far as passing
>emissions goes.
I don't understand your concern here. Although maybe my typo caused
it. When I said
>> > but the ECU knows that an ice-cold engine will never run without a
>> > very rich mixture, so it goes back into open-loop and pours on the
>> > gasoline. The ECU yells RICH RICH RICH and maybe PTUI! GAS YECCHHH!
I should have said the O2 sensor yells RICH etc. Because by
unplugging the TempII sensor you told the ECU that the engine hadn't
even begun to warm up, forcing it to richen the mixture to keep the
engine from stumbling. This is normal operation, I don't see why you
view it as a problem. Try driving down the road that way, your
mileage will be horrible assuming it will even run ok. You're trying
to fix what isn't broken, so far as I can see. Am I missing something?
cheers,
david
--
David Beierl - Providence RI USA -- http://pws.prserv.net/synergy/Vanagon/
'84 Westy "Dutiful Passage," '85 GL "Poor Relation"
