O2 Sensor reading with misfire?

[Jerryl]

NB and WB sensors read the amount of free oxygen in the exhaust system, correct?
So, if you have a misfire, will the O2 sensor read rich or lean?

 

[pacecarta]

lean o2 (low MV)

lean WB (high A/F)

if your running closed loop (cruise) then the 02 sensor usually picks up the lean , drives the blms up to add fuel and the result is an a/f -O2 that appears normal .. but the blm will not be

 

[Jerryl]

lean o2 (low MV)

lean WB (high A/F)

That's what I thought as well.
Read a few opposing statements, which is why I posted.

 

[pacecarta]

added while you posted ^^^^

 

[RmvBfrFlght]

NB and WB sensors read the amount of free oxygen in the exhaust system, correct?
So, if you have a misfire, will the O2 sensor read rich or lean?

There really isn't a solid answer to this. Part of the reason is that it depends upon the type of misfire and the engine RPM.

For the most part the NB O2 sensor will lie about what is in the exhaust gas during misfires. An example would be a mis-firing spark plug at low RPM. In this case the NB sensor won't show anything out of the ordinary.

While a WB sensor will show a lean spike each time that cylinder exhausts.

Another example which happened while tuning an engine, was that the AFR went lean under load at about 4K RPM. The engine was misfiring and breaking up.

In this case the NB O2 was flat at someplace in the 800 to 900 mV range (can't recall exactly).

While the WB reported AFR was bouncing off the rails. From 10:1 to 20:1 back and forth. It was interesting looking at a graph of that. A flat NB O2 value with the WB AFR bouncing like a basket ball.

One thing to remember about O2 sensors is that they respond to both free O2 and combustion-able products in the exhaust. With the NB sensor being slower to respond then a WB sensor.

RemoveBeforeFlight

 

[DailyDrifter]

A misfire should read leaner. Due to the fact no oxygen was used up to burn the air/fuel mixture, so the O2 molecules are still in the exhaust on the way past the O2 sensor.

RmvBfr is right also with which type of misfire it is.

 

[Jerryl]

Thanks guys.
So, now here is O2 reading question #2.

Exhaust leak, before O2 sensor.
Many will say/agree that the O2 sensor will read “lean” (Low NB, High AFR) with an exhaust leak before the sensor.
Most of our cars run exhaust back pressure in the 1.8:1 to 2:1 range.
So, I always wondered (albeit facts) ; How would the free air oxygen make it into the exhaust system with a 30-40 PSI exhaust pressure?

 

[pacecarta]

at low rpm the exhaust leak allows air to be pulled in between the exhaust pulses (like a vacuum) ..o2 sensor sees this drives blms up car runs richer than should be ...result is slow spoolup due to leak (like having the puck open) and also becuase of the overly rich mixture because of the ecm correcting the lower blm cells .. fix the leak car will perform better,
at full throttle there really isnt much difference in O2 readings but et already suffers because of the above problems caused by the leak but the crack if long enough does open up under high temp and you might find you're not able to build to the boost you wanted to or prviously ran at ..et and mph suffers

but i thought we were talking misfires not leaks .. a misfire sends unburned O2 into the exhaust stream and with leak or not 2.1 or not , that o2 is going to get to the sensor at idle or WOT and it will affect overall horsepower wether the o2 sees it as rich or lean beacuase now your down 1 maybe two cylinders ... so fix the misfire which io beleive most people have and arent even aware of at wot , a good working ignition system is key and for me the casper tester find many issues most people were never aware and it can be used test the wires and plugs to some extent but cant simulate a plug under load.

 

[Jerryl]

at low rpm the exhaust leak allows air to be pulled in between the exhaust pulses (like a vacuum) ..o2 sensor sees this drives blms up car runs richer than should be ...result is slow spoolup due to leak (like having the puck open) and also becuase of the overly rich mixture because of the ecm correcting the lower blm cells .. fix the leak car will perform better,
at full throttle there really isnt much difference in O2 readings but et already suffers because of the above problems caused by the leak but the crack if long enough does open up under high temp and you might find you're not able to build to the boost you wanted to or prviously ran at ..et and mph suffers

but i thought we were talking misfires not leaks ............

I concur.
Yes, the FIRST question was about misfires.
The SECOND question was about "exhaust leaks".

What you stated, is the way I understand it.
One variable; Open loop idle/low rpm chip.
BLM's wont change, but spool will be affected, but the combination of misfire AND exhaust leak will very likely raise EGT's as the fuel burns in the exhaust with the oxygen.

Anyway, THANKS for the reply!
Just checking my sanity ................. :tongue: