I have recently hooked up my rear O2's and I'm getting better driveability and fuel economy. OBD2 cars have 2 rear O2's and OBD1 cars have 1 rear O2. My car is OBD2. My rear O2's are placed in my pacesetter y pipe directly behind the 2 3 inch magnaflow cats. I've noticed that the rear O2 sensors are not so "jumpy" and since I reinstalled them after deleting them 3 yrs ago I've noticed several things.

1. Short term trims are more accurate and stable
2. Light throttle 1700-2100 cam surge areas are better
3. Fuel economy appears to be slightly better.
4. AFR is steadier at idle and when decelling

Judging by these observations I believe the rear O2's actually play a part in fine tuning the fueling, prevent "spikes" in the short term fuel trims, and increases the accuracy of long term trims. I'm interested to see if anyone has experienced these things on OBD1 cars.

 

"OBD2 cars have 2 rear O2's and OBD1 cars have 1 rear O2"


this is not true, OBDII cars are REQUIRED to have one O2 sensor after the cat converter to monitor its efficency. If there is only 1 cat in a OBDII car there will only be one o2 sensor. Now, if its a V configuration engine with 2 banks that mirge into one cat. chances are there will be 2 front o2 sensors, and one rear.

if its a true dual, and there is 2 cats.. then yes there will be 2 cats

On OBDI car there is no requirment to monitor catalyst efficiency (wich is why a obdI can needs to be emission tested with a sniffer/roller and obdII does not).. so in 98% of OBDI cars there is no o2 after the cat.

having said that, yes.. your right, there is a small bit of input from the rear o2 that come into play on a OBDII vehicle.. but its not going to happen on a OBDI car since there is no rear o2.

 

i wasnt aware that OBD1 cars had rear O2's???

 

I stand corrected about the OBD1 cars. Thanks for clearing that up. My repair manual shows that on single cat OBD2 cars GM places one rear O2 in front of the cat and one after. On my WS6 car which had two cats both rear O2's are after the respective cat.

The repair manual also shows that OBD1 cars don't even have rear O2's. Ok then all you OBD1 guys can get out of here! LOL

 

lol.. nice. hey, My f-body is ODBI, but to be honest I was trained ALOT more on OBDII. So-no, I wont get out. lol. :-). (plus all my other cars are OBDII)

 

Well I'm just very pleased that I picked up some mileage and driveability. I had a hot rod shop put in my exhaust and they assured me that the rear O2's were just to check the catalyst efficiency. There is no doubt in my mind when I do datalogs with my wideband and autoxray scanner that the car is being fueled more accurately in closed loop due to the fine tuning the rear O2's provide.

 

The rear O2s are used in some (all?) OBD2 cars to determine if the front O2s are going bad (set point drifting) and to make corrections for that drift.

So, if your rear O2 is having an effect on fueling, that tells you that your front O2 needs to be replaced.

 

If you want to know for sure, get under that car and scope the front o2 sensor.

 

Thanks for the reply. I have a wideband with a guage in my car 100% of the time. My car is a daily driver with over 30,000 miles on my current blower setup and engine. I would know if the front O2's were in need of replacement. They are fine. Also don't you think it would throw a code if I had a bad front O2.

I can't tell if you have an OBD2 car because you have no sig. Do you have your rear O2's hooked up?

 

What do you mean by scoping it? I have an autoxray scanner that plugs into the OBD2 port(and I don't even have to get under the car...LOL). Are there other tools that would give me a more accurate scan?

 

ohh yea. anything you get thew the obd port of the car is very delayed, slow to respond, and second hand info. (giving you the info is not your cars first priority, telling the rest of the car whats going on is, then it lets you know.) and since it updates so so slow you miss ALOT of stuff!

its called a oscilloscope (people say scope for short). Its basicly a very high resolution multimeter. Pico scope is my fav so far. Tho on a quick job I dont mine using a Snap-on Modis.

you make a direct connection with the output and ground of your o2 and you can very acuratly diagnose it.

http://www.picotech.com/auto/automot...illoscope.html

 

its got a million and 1 uses on a car to. especiall for ignition work.

a cool thing you can do with it. it hook an amp clamp to your battery cable. then disable spark or fuel, crank you car and it measures the ammout of current it takes to compres each cylinder, your result, a relative compression test.

the more amps it takes to compress a cylinder the betters compression. you get a nice graph showing compressing all in about 2 mins.

 

If your readings are offset slightly, there's no way for the PCM to detect it, and you are not necessarily going to get a code. Pre-cat sensor codes are set for things like the sensor voltage staying above 775mV, below 200mV, no activity (352-552mV), slow transition (switching) response, and excessive time to heat up (display activity). If the output voltage is shifted 50 or 100mV (or more), there's no way for the PCM to detect it.

Post-cat sensors are checked for similar malfuntions, but the high voltage limit is 930mV and the low voltage limit is 40mV. "No activity" is defined as staying in the narower range of 391-491mV.

All pre-cat and post cat sensor functional checks are made continuously, once the PCM enters closed loop.

The cat activity (O2 storage) check is correlated to a comparison of the post-cat sensor signal amplitude as a percentage (90%) of the pre-cat sensor amplitude. That check is only made once per "trip".

 

Thanks for the post Injuneer. When I had O2 sims on my rear O2's they constantly gave a steady reading of .450 millivolts. Why didn't that set the no activity code? I tune my own car and I know the codes were not deleted.

Do you believe that the rear O2's fine tune the fueling? Judging by the activity of my wideband I'm convinced.

 

I have no way of knowing what the after-cat sensors do, other than what I read. Ray Bohacz (tech columnist in several of the GM performance magazines) did an article on OBD-II and indicated that they were just checks on the catalytic converter performance. I never ran an OBD-II setup, and haven't even been running a stock PCM since 1999.

450mV out of the "sim" would indicate it wasn't functioning. You could achieve the same results simply by leaving the harness connector disconnected. Not sure why you didn't get a code. My info comes right from the GM online OBD-II code guides.