Once again, I've enrolled in another debate. A fellow board member says the following (when using o2 simms to delete o2 sensors on an OBDII long-tube application) about installing oxygen sensor simms:

It may actually increase performance. If the secondary
02 sensor fails, the computer will run in 'open-loop' mode creating a protection from leaning out
by injecting more fuel into the cylinders.

The ratio will be "closer" to 12.5:1 which is best
for power. More power, poor fuel economy.

Is there any truth or validity to that statement? I've always been under the assumption (quite firmly) that rear o2s ONLY monitor the efficency of the catalytic converters, and in NO way, either deleting or modifying them, can have bearing on fuel delivery.

Any input is vastly appreciated.

 

The PCM doesn't use any O2 values at WOT.. which is where performance really matters

On that note, the rear O2's don't do a damn thing other than check the efficiency of the converters. If they fail they in no way affect the car's performance and just give you an SES light. You will not have an increase or decrease in performance either way.

 

Whoever said that is on crack. If you are removing the rear O2 sensors, install the simulators. The only reason people get rid of the rear O2 sensors is if they want to go !cat.

 

Thanks for the help guys. I've got some arguing to do.

 

In response to your statements, he states the following:

Well...one of us has a poor source of information,
and I'm not going to go-out on a limb here because
I know that your sources are credible.

My sources after studying GM PCM's states that the
primary 02 sensor is responsible for metering
A/F ratio under all conditions - except WOT

The secondary 02 sensor (post cat. converter)
uses the air pump and pulse width modulation of
the injectors to make sure that under closed loop
mode, the A/F ratio is 'stochiometric' which allows
the least amount of harmful emissions into the
atmosphere.

If the system is not balanced, the air-pump supplies
more air to regulate the emissions. If the
secondary sensor fails, or any in the fuel
system, the computer goes into protection mode
and fattens the fuel to prevent damaging the
engine (from running lean).

Any input?

 

I will be the first to admit that I'm not OBDII guru.

OBDII regulations mandate that catalytic converter effeciency be measured on the vehicle which is the main reason for the rear O2's. Secondly, most fall-back safety systems in the PCM are there to protect the catalytic converters first and foremost. By engaging the AIR system with cats up to temp you risk melting them down. This will further aggravate the system as the exhaust becomes restricted. The only cat protect schemes that I have seen run the car open loop with a rich AFR to keep the catalytic converters cool (by eliminating excess O2), the SES light will be triggered which should prompt the owner to service the vehicle which is ultimately which eliminates excess emissions while also protecting his/her bank account. AFAIK on OBDII systems, as long as the primary O2 sensors remain online and switching at a reasonable rate the PCM will remain in closed loop, it has absolutely nothing to do with the rear "monitoring" sensors. I may be wrong, what are your friend's sources?

This "argument" is a moot point however. To tell someone to force their car into open loop is an all around bad idea. I personally like to do things correctly and tune the PE tables and in my case the VE tables as well. The only reason to run open loop is if you have a radical cam with excessive overlap, which isn't a true closed system and doesn't work well with closed loop fueling. Even in that case you are wise to do it correctly by enabling open loop operation only, not by setting eroneous trouble codes. Since these cars typically run too rich as it is at WOT, running open loop without other changes typically leads to reduced performance, always worse fuel economy, and sometimes masks drivability problems that you should at first try to "tune" out, if it's possible.

 

I just reread your friend's third paragraph a little more closely. The AIR system is never used to regulate closed loop fueling. All closed loop fuel corrections are done with injector pulsewidths that are modified by the block learns.

The AIR system is used to help light off the cats more rapidly on cold startup. Introducing fresh O2 in an exhaust system with available fuel and a cat at operating temp can destroy the cats.

He may be thinking of older systems which route O2 from the AIR system into the catalytic converter before the oxidation catalyst. In this configuration the air is not regulated and any excess O2 after the oxidation reaction blows out the exhaust. Introducing this air before the O2 sensors would result in a massive "fake" lean condition as seen by the PCM and will result in an excessively rich AFR.

 

Thanks a million Buttercup. I'm headed to tell him exactly what you said.

 

I'm no computer expert by any means, but I can tell you what my car is doing. The rear o2's are shot, not to the point of throwing a code but one side stays low (around .2-.3) the other side stays high about (.6-.7). What caused them to go bad was the opti crossfiring/missfiring which has been fixed, but ever since then none of the block learns will reset (they all show 0 on my auto xray scanner). The long term trim % value also starts at 0 even after the car goes into closed loop and uses only the short term trim for adjustment. Eventually some of this value transfers to the long term trim. Funny thing is one side will go to say -4st and +8lt while the other side goes to say +4st and +4lt (guessing at numbers but they are close). Every time you start the car it has to relearn fuel adjustments. Also the computer is not losing power when you shut it off.


Sorry this is long but I've always read the rear o2's don't affect on fuel delivery but I'm real interested in what you find out on this as it seems to in my case or I have something else wrong.