Could someone explain to me why an exhaust leak could affect the reading of a O2 sensor?
I cannot picture how a leak out of a pressurized pipe could make a difference in the AFR reading inside the pipe.
Or am I not understanding something here.

I'm no expert but I expect that the pressurized pipe is really filled with pulses of exhaust gas pressure interspersed with pulses of vacuum. The vacuum (less than ambient pressure) draws in outside air.

This might get into reaction time for O2 sensor response to rapid pulsations of different concentrations of air/fuel. I have not got a clue how rapidly the sensor can read changes like we are discussing. But, I bet Matt can shed some light on it.

I do know from 1st hand experience that when the system is in closed loop, that very tiny changes, like a misfiring plug or an injector that sprays odd patterns occasionally due to foreign stuff in the fuel stream, will cause fluctuations in rpm. So, is the computer playing catch-up with erratic information from sensors? I dunno.
Jim Hupy
Salem, Oregon

I agree. Even reversion of gasses or reverberations in sonic pulses in the
gas stream can affect the O2 sensor. Placement of it upstream or downstream
is also critical, but nowhere near as critical as an exhaust leak.
2 stroke exhaust design is almost as critical, particularly on
expansion chamber type exhausts. Pipe shapes, twists and turns, even
welding slag, pipe diameter and length also have an effect.
Jim Hupy
Salem, Oregon

On Tue, Oct 29, 2019, 1:14 PM Matt Colie via Gmclist <
gmclist@list.gmcnet.org> wrote:

> Tom Lins wrote on Tue, 29 October 2019 13:07
>> Could someone explain to me why an exhaust leak could affect the reading
> of a O2 sensor?
>> I cannot picture how a leak out of a pressurized pipe could make a
> difference in the AFR reading inside the pipe.
>> Or am I not understanding something here.
>
> ljdavick wrote on Tue, 29 October 2019 13:34
>> I'm no expert but I expect that the pressurized pipe is really filled
> with pulses of exhaust gas pressure interspersed with pulses of vacuum. The
>> vacuum (less than ambient pressure) draws in outside air.
>
> This is exactly the correct answer and it is why when running a closed
> loop engine in a laboratory setting, we still needed an absolutely tight
> exhaust system.
> Yes, even a little leak can matter.
>
> Matt - the refugee from DynoLand
> --
> Matt & Mary Colie - '73 Glacier 23 - Members GMCMI, GMCGL, GMCES
> Electronically Controlled Quiet Engine Cooling Fan
> OE Rear Drum Brakes with Applied Control Arms
> SE Michigan - Twixt A2 and Detroit
>
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Slightly off topic but related:

Given the above discussion about pulses, are 02 sensors/gauges useful as a tuning aid on two stroke engines, or would EGT be more useful?