Dual O2 Sensors and how they work?

[Fast355]

Wow, thats a pretty significant difference between trucks with the same engine.
If they have the same engine, and basicly the same truck, why not make them both have the same exhaust to cut down on production costs? Thos 1/2 ton pipes are tiny!

My guess is the needed to get the cats to light off quicker in the 1/2 tons for emissions reasons. Some of the early 1/2 ton Express models had the 3/4 ton exhaust size setup.

[EagleMark]

That will get you in trouble on Nissan......Bank 1 on a V6 Nissan is the firewall or passenger side, RH side of the engine.

Well I guess we need a new chart for engines mounted in car sideways? Wonder if that is still true for American sideways engine?

My guess is the needed to get the cats to light off quicker in the 1/2 tons for emissions reasons. Some of the early 1/2 ton Express models had the 3/4 ton exhaust size setup.

I wonder if it had to do with gearing too? Smaller exhaust does help with low end torque... for example if you want to screw up a perfect running Scout II take off the single exhaust and put dual exhaust on... they had heads that flowed worse then TBI heads so there was no need anyway.

[Fast355]

Well I guess we need a new chart for engines mounted in car sideways? Wonder if that is still true for American sideways engine?

I wonder if it had to do with gearing too? Smaller exhaust does help with low end torque... for example if you want to screw up a perfect running Scout II take off the single exhaust and put dual exhaust on... they had heads that flowed worse then TBI heads so there was no need anyway.

That line of engines started as school bus/medium duty truck engines. No need for flow when you have a 3,500-4,000 rpm governor and a 2bbl.

Nissans RWD V6 have bank 1 on the RH side too.

[Playtoy_18]

Bank 1 is always the bank that cylinder 1 is on.
front is sensor 1,after cat is sensor 2.
sensor 3 is usually LEV,ULEV,PZEV etc on the stuff I was trained on and it is generally the post cat sensor and sensor 2 will be installed in or directly before the cat.

not sure on the sensor 3 in gm offhand though.

years ago I read that GM does use the rear 02's to account for approx 1% of long term trims over time and deleting can cause strang transients.
i've deleted at least 50 over the past 6-7yrs and never had an issue though.

as far as 1/2 vs 3/4 ton cat position it prob has to do with different emissions standards and if it has an AIR system?

[daleulan]

Bank 1 is always the bank that cylinder 1 is on.
front is sensor 1,after cat is sensor 2.
sensor 3 is usually LEV,ULEV,PZEV etc on the stuff I was trained on and it is generally the post cat sensor and sensor 2 will be installed in or directly before the cat.

not sure on the sensor 3 in gm offhand though.

years ago I read that GM does use the rear 02's to account for approx 1% of long term trims over time and deleting can cause strang transients.
i've deleted at least 50 over the past 6-7yrs and never had an issue though.

as far as 1/2 vs 3/4 ton cat position it prob has to do with different emissions standards and if it has an AIR system?

The rear sensors are indeed used for fuel trims. There are three functions of those sensors in 'typical' software. I don't have a disassembly of GM's code in front of me but the most obvious is for catalyst diagnosis. The second function is for a continuous fuel trim adjustment to basically adjust for catalyst efficiency and desired feedgas composition. This won't affect fuelling much but to put it in perspective the front switching lambda sensors have an accuracy of around +/- 0.02 lambda; UEGO sensors are generally a bit worse. But put a switching lambda sensor behind the catalyst and the accuracy is about +/- 0.0005 lambda, with a significant improvement in emissions while allowing the engine calibrator to tune the engine for reasonable efficiency and power without sacrificing as much to get low NOx. The third function is one of active NOx recovery after fuel cut or deceleration recovery. This software basically starts when a lean transient is noted. Then the air-fuel ratio is richened up and the spark timing may be retarded (my software does it and I've needed spark retard here of up to 12 degrees to meet emissions) until the postcatalyst sensor snaps up to its normal voltage. In some versions of this algorithm the ECM determines the front-to-rear delay as a learned value so it can do the jumpback proactively. In any case the enrichment and the spark retard can affect drivability and certainly can affect emissions.

[skandolis]

i could be wrong, but when i was going to school(in the early nineties) we were taught that bank 1 was always the leading(frontmost) cylinder head (cylinders heads are always offset front to back) which makes cylinder 1 match up with bank 1 ect.........this has always proved to be the case to me.

[EagleMark]

Here's one that I'm not sure about and may change with systems?

Is the post cat O2 sensor supposed to switch? Or have steady reading?

[RobertISaar]

pre vs post cat O2 sensor signals. the post-cat is simulated, but that's about what you could expect to see with a well functioning system.

[daleulan]

What you see out of the postcatalyst sensor can depend on the design OSC (cerium dioxide) loading of the catalyst. Early OBD-II catalysts used lighter OSC loading and so you would see an attenutated oscillation out of the rear sensor like above. Newer low-emissions catalysts have very high OSC loading and another interesting algorithm that I forgot about. The generaly target of most late model controllers is to maintain the postcatalyst sensor at a very steady target voltage - for gasoline it is commonly about 0.55 to 0.6 volts but may vary based on load and speed. Definitely the target changes with the fuel in use and also the NOx production of the engine. So on newer controllers you should see the postcatalyst sensor stay very steady. After a deceleration the voltage should return to steady-state quite quickly - within 5 seconds or so, maybe even quicker. You should see an enrichment in the front catalyst. The other sneaky algorithm is one that monitors the catalyst oxygen storage capacity and adjusts the front O2 sensor step-and-ramp to ensure that the postcatalyst sensor doesn't wiggle much.

[Playtoy_18]

Does anyody have any insights into the single bank front 02 on fwd 3800's versus the dual banks on the rwd's?

[Hog]

Playtoy is the FWD kinda like the Vortec 4.3/305?
I notice on the Vortec 4.3 L35 and Vortec 305 L30 that they use a 3 Precat O2 sensors, 1 for Bank 1, 1 for Bank 2 and then a 3rd O2 sesnor which reads BOTH Bank 1 and 2 mixed together just forward of the cat, then the aftercat O2 sensor.
The Vortec 350 L31 and Vortec 454L29/L21 use the conventional B1S1, B2S1, B1S2,B2S2 config with 2 precat and 2 postcat.

I remember something from Monodax about the post cat sensors affecting fueling. The acronym was FOASCO or something like that. I really wish we could access that info on Monodax, it was a great resource.

peace
Hog

[EagleMark]

I always wondered what that third O2 sensor was for in Vortec...

From what we have gathered and I partially found in a 98 Vortec Dissasembly the post cat O2 sensors do effect fueling up to 10%. That said I just did a 97 Vortec and both post cat O2 sensors were swinging! Both cats were shot. Disconnect and remove rear O2 sensors from cal and man did it run better.

[Playtoy_18]

No, except for the camaro most 3800's I've seen have a single 02 sensor 1, and single 02 sensor 2.
So it has only a single 02 sensor for fueling both banks and one after cat for monitors.

Most vortecs I've dealt with have 4 02 sensors.
Precat/fueling is bank 1 sensor 1 and bank 2 sensor 1
Postcat/monitor is bank 1 sensor 2 and bank 2 sensor 2

Low emission ulev, pzev etc would have like 3 sensors total for say a 4cyl.
Precat/fueling would be bank 1 sensor 1 then there will be another installed in cat towards the front and another that is post cat.
These would be bank 1 sensors 2 and 3, but not necessarily in order "I THINK".
I suspect sensor 2 might be always post cat monitor and sensor 3 is in the middle designated for whatever makes it a pzev.
Confused me everytime, had to look in book and verify etc every time they rolled into shop.
There is an order and it is not necessarily IN order I think.
I took a class long ago but forget it honestly, not a lot came thru our place.