Hey guys, I'm gonna be doing some custom exhaust work and I would like to know before hand if the after cat O2 sensor on our cars (if it has one) is strictly a cat efficiency sensor like most other OBDII vehicles? or is it a MAP sensor for fuel trim adjustments? Important to kow because i would be modifying its housing to get rid of a inefficiency CEL or putting a CEL fix in the wiring and I would like to not screw up my fuel mixtures. Any experts on here, please let me know Thanks

Cheers,
Tim Quinn

 

On every OBD-II car I've seen the downstream sensors were there primarily for checking for the existence of the pre-cats. The two upstream O2's are the ones that monitor the air/fuel ratio. If the voltage drop between the the up and downstream sensors was within spec, then no errors were thrown. If the voltages were too close (or the same) then something was amiss with either pre-cat and needed fixing.

Now, this doesn't necessarily mean that you can just do away with the downstream sensors and call it a day. The ECU will still be looking for them to be installed and will still be wanting to see proper voltages. Therefore, while the primary function of the downstream sensors is just to monitor catalytic efficiency, if the voltages are out of spec, the ECU might complain and cause the car to dump fuel (run rich) to prevent any kind of engine damage.

If you would like to provide more information about your project I might be able to tell you more about where you could potentially run into trouble.

 

Well essentially, I would like to re-create the rear O2 mod that i made for my WRX when i removed three cats. Its basically a spacer for the rear O2 bung that removes the sensor tip from the direct flow of the exhaust (the sensor is in fact still in teh system, but because of its new location outside the main exhaust flow, causes it to read a slightly different level then the front O2's. Worked like a charm on the WRX so I was figureing it should work well on basically any car with a strict cat eff. O2 sensor).

The spacer is essentially a pair of 18mm sparkplug anti-foul adapters, drilled out to about 9/16th an inch. The O2 sensor screws right into the adapter, and the pair of adapters put together would pull the sensor just outside the main exhaust pipe, but is not a big enough change in exhaust flow to cause any problems. Of course some vehicles do not have enough room near the exhaust pipe to move this sensor out 1.5-2" in which case ill have to fabricate a 90 degree elbow and mount the sensor, essentially, parallel to the mid pipe with the sensor facing the rear of the car.

Hope that makes sense I've seen it work on many other vehicles, but I am not completely familiar with the Crossfires yet, so I just want to confirm that this rear sonsor is there for cat eff. reasons only

Thanks for your replay, any and all information is greatly appreciated

Tim

 

Just to clarify - your intentions with the Crossfire are to leave the catalytic converters in place, but move the downstream O2's out of direct exhaust flow by spacing them out 1.5"-2"?

That being the case, I can see where you're going with it - you're trying to open up the exhaust piping as much as possible by remove extra blockages (O2's) within the pipe, correct? I don't think that would cause an enormous improvement on the Crossfire. The O2's themselves don't create all that much of a restriction. Yeah, they stick into the pipe, but the hot end of the sensor is drilled out to allow the exhaust to flow freely through it.

On the WRX you were able to see a nice gain because (at least, I think) you did this mod in conjunction with removing the 3 catalytic converters. Removing the cats is what would have caused such a nice gain with moving the O2's having little effect.

If I'm completely off base with what you're planning, let me know. The above thoughts are just based off what I think your goals are with your mod.

I do know, however, that adding a 90* bend in the exhaust in which you can mount the O2 sensors would be a bad idea. You'd end up causing more turbulence and could throw off the whole rhythm of the exhaust system.

 

Ahhh thanks for the replies. I will be "removing" the cats. Hence the need to relocate the O2 sensor to lower its readings from the exhaust to give an illusion of a cat being in place. The bend is a last resort if the spacers dont work, (the spacers will only leave a space about 1/16" wide for xhasut to get to the sensor, so there is not much turbulence with that plan.) the 90 degree adapter (which several manufacturers include in their "CEL free catless systems" is actually pretty good at causing minumal turbulance. as it is not a modification to the exhaust itself, just a little 1/2" round adapter for the o2 bung mount.

Well, when i get to work on my exhaust ill have to just try the first idea and see how it goes, as long as that O2 is cat eff. and thats it, then no harm can be done (other then a CEL.)

Tim

Thanks for your info

 

Ah, I see now. I wasn't sure if you were removing the cats or just relocating the O2's. That makes sense now.

You might want to try replacing the two main catalytic converters with straight pipes before tampering with the pre-cats and O2 sensors. The two pre-cats are already of the high-flow variety. The two main cats. are high flow as well, but don't have any sensors monitoring them that you'd have to worry about.