[urq] O2 sensor output, WAS: I'm SO close to giving up - Part 2?

Craig Cook craigcook at rogers.com
Wed Apr 20 21:49:22 EDT 2005

Hi Steve,
yes you're theory is absolutely correct. As the atmosphere contains @20% oxygen then a 5 cylinder engine with one cylinder in full misfire is going to send 4% oxygen out the tailpipe. The O2 sensor as you mentioned only looks at oxygen. The problem with this is that the computer senses a lean condition and rich ens the mixture up via the OXS frequency valve. The loser in all of this, is the poor catalytic converter which is at this point trying desperately to burn of the excessive HC's. from the misfiring cylinder.Never mind trying to deal with the extra fuel that the computer is commanding  for.
So, no spark or a misfire condition is going to show lean on the O2 sensor and a rich command from the computer on the OXS frequency valve to compensate for the excess oxygen in the exhaust.
I hope this helps.
Craig Cook.

"Buchholz, Steven" <Steven.Buchholz at kla-tencor.com> wrote:
... why is it that it seems folks seem to be dropping the relationship
to the OXS Frequency Valve duty cycle? I don't know for a fact that it
is the culprit, but it should be at least as valid a theory as the fuel
pump petering out or an engine misfire. I looked to the Bosch handbook
to see what the OXS signal does in a misfire condition and found nothing
... but it has been my experience that a misfiring cylinder reports a
rich signal via the OXS. 

Funny, I've had grandiose plans to instrument the crap out of my engine
... pressure transducer to monitor fuel pressure (and/or control
pressure), duty cycle on the OXS Freq Valve, Wideband OXS and EGT on
each cylinder ... all that to monitor the stock CIS! If I decided to go
Megasquirt or EFI332 I was thinking it would be great to have "ION"
feedback for knock sensing and mixture evaluation ... 

Steve B
San Jose, CA (USA)
> > If it were spark, you'd see rich condition. So
> > must be fuel.
> Would it? I've always wondered about this. The O2 sensor simply
> reports the difference between oxygen content of the exhaust stream
> oxygen in the ambient air, right? Assuming this is the case, and you
> have a spark problem, you would have unburned fuel but also *unburned
> oxygen* in the exhaust. Wouldn't the sensor report this as an
> excessively lean condition, provided that the unburned mixture isn't
> ignited in the manifold or turbo?
> I've got a little A/F meter that reads the O2 signal, and I've often
> wondered about how to interpret it when troubleshooting a problem.
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