Air/Fuel Question

[jdbubb@ix.netcom.com (Dan Bubb)]

Just to add alittle more info;
The O2 sensor output curve has a very sharp transition right around
stoicheometric. i.e it jumps from ~.1 volt to ~.6 volt output in the
range of 15.2:1 to 14.2:1. Particularly on the leaner side of stoich the
voltage drops very quickly over a very small A/F range.
The cheaper meters assume every .1 volt corresponds to .5 change in A/F
ratio. e.g. 14.7=.5v, 15.2=.4V, 15.7=.3V. Actually, 15.2 is closer to
.1V, so if you're cruising at 15.5 A/F, which is not out of line for a
non-cat engine, then the cheaper meter will show no voltage. Dead lean
when in fact it's not.
On the rich side of stoich the voltage transition is less abrupt, but
it's still not useful over a very wide range and it is sensor/exhaust
temperture dependent. A relatively cold sensor (just warm enough to have
an output) will show .9V at just richer than stoich. If the sensor is
really hot the same A/F ratio will only put out .7V. So, if you're
driving the piss out of your car (with a one wire non-heated sensor) it
will appear to be running leaner at WOT than if you're just getting on
it occasionally.
Dan


Cory Langford wrote:
> 
> Thanks to all those replies.  It looks like the list is in agreement that
> there is no real problem with lean with no load.  Lean with a load is what
> you should watch out for.
> 
> I did richen it slightly at idle, and it made no difference under load so I
> will leave it as is.  (the little richening seemed to give me a little more
> oomph off the line)
> 
> Thanks again
> 
> Cory
> 
> At 09:32 AM 10/17/2002 -0400, Dan Bubb wrote:
> >As long as your engine runs fine without missing while cruising I
> >wouldn't bother trying to richen it up.
> >With CIS (i.e. NOT lambda or E) there really isn't any way to change the
> >mixture aside from messing with the control pressure and from my
> >experience that is a real hairball.
> >Changing the idle mixture will have a minor effect on the mixture once
> >the throttle is opened even a small amount. eg. if you change the
> >mixture 10% at idle then doubling the airflow (say doubling the idle
> >speed) reduces the % change to 5%, with 5 times the airflow the % change
> >is only 2%. So, the effect of adjusting idle mixture dies out quickly as
> >the throttle is opened.
> >Personally, I think your a/f readings are interesting but not indicative
> >of a problem.
> >Dan
> >
> >Cory Langford wrote:
> >>
> >> Okay, I have small conundrum I am working on and maybe someone here can
> >> throw some info at me to help out :)
> >>
> >> I installed a cheap autometer air/fuel meter into the '79 with a 1 wire o2
> >> sensor.  At start up everything seems normal. I get a rich reading, as I
> >> expected due to the cold start injector kicking in.
> >>
> >> When it is warm and under load it runs just shy or rich.
> >>
> >> What I have noticed is that when I am simply cruising along a 2,0000-2,500
> >> rpm and the engine has no real load, it leans out right to the end of the
> >> scale.
> >>
> >> Step on the gas, back up to stoic just before rich.
> >>
> >> I can see it getting lean when you take your foot off the gas as engine
> >> winds down, but once at a steady rpm I thought it should go back to stoic
> >> again.
> >>
> >> I am going to try and richen the mixture a bit, but I can't seem to think
> >> how to get it richer just at the lower rpms (unless I am thinking wrong
> >> here) with out richening out the upper rpm range.
> >>
> >> Any ideas?
> >>
> >> Thanks
> >>
> >> Cory Langford
> >> '86 turbo project - running, just needs tuning :)
> >> '79 daily driver - Restore in progress...
> >> '65 Ghia - The next project in the garage
> >>
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