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Relocating down-stream o2 sensors with header install -- will the PCM like it?

Kresnik

CEG'er
Joined
Feb 12, 2008
Messages
212
Since having installed headers a couple of months ago, there is no pre-cat to make the rear downstream o2 report a pleasing reading to the PCM. There is also no bung for a downstream o2 sensor on the front header or y-pipe. I've read around some and saw people making mention to welding in a couple of bungs past the catalytic converter and simply extending the wiring harness from the downstream sensors to make this work.

Has anyone done this, and what were your results? My 2000 SVT is pretty much stock, save a clutch, flywheel and exhaust system. Would I be able to simply weld in a couple of bungs past the CAT and place my downstream sensors there and not have any more check engine light and poor fuel economy as a result? Will the main cat work well for this, or will I also need to mil eliminators spliced in to this?

Thanks,
 
So I am guessing that you didn't use MSDS headers .... there is a bung on the front header for the lower O2 sensor.

I would install O2 sensor bungs in the correct locations then use mil-eliminators to prevent the CEL for the pre-cat codes.
 
So I am guessing that you didn't use MSDS headers .... there is a bung on the front header for the lower O2 sensor.

I would install O2 sensor bungs in the correct locations then use mil-eliminators to prevent the CEL for the pre-cat codes.

Yes, I have weapon R headers -- though I wish for my emissions control stuff to work as well as possible. I also don't want the trouble of removing the front header and needing to use mil eliminators if it's possible to simply relocate the o2 sensors and have everything work well that way. Much less hassle, time and cost that way if it works out.

So.. has anyone done this and had results, either negative or positive? Personal experience would be most helpful. =)

Thanks,
 
I'll take a shot at this.

Putting the downstream O2's after the cats may cure you CEL problem but it may not.
The pre-cats are supposed to "light off" quickly after engine start up, that's why their part of the exhaust manifold.
This means that the exhaust cleaning process starts almost as soon as the engine settles into a steady idle after start up.
The downstream O2's are supposed to check the operation of these pre-cats and report back to the ECM after a certain time period or once the engine has
reached certain operating conditions. Putting the downstream O2's after the downstream cat/s could still result in a CEL as the engine may come up to "temp" before the downstream cat
is hot enough to operate correctly. The ECM then asks the downstream O2's how's the pre-cats are doing and because the downstream cat is not hot enough the downstream O2 report back that "Catalyst System Efficiency Below Threshold" and you get a CEL.
 
Now this my friend, is indeed a helpful post! I wonder if the trigger is an event of switching to and from loop modes. I'm replacing my coolant temp sensor today as well, which was constantly illuminating a code. I wonder if that might have been related to how quickly I used to use up o2 sensors in the past, when I had an OEM exhaust system.

Thanks for the insight, I'll probably do this pretty soon here and I'll respond back to let everyone else know how it did or didn't go. =)

~



I'll take a shop at this.

Putting the downstream O2's after the cats may cure you CEL problem but it may not.
The pre-cats are supposed to "light off" quickly after engine start up, that's why their part of the exhaust manifold.
This means that the exhaust cleaning process starts almost as soon as the engine settles into a steady idle after start up.
The downstream O2's are supposed to check the operation of these pre-cats and report back to the ECM after a certain time period or once the engine has
reached certain operating conditions. Putting the downstream O2's after the downstream cat/s could still result in a CEL as the engine may come up to "temp" before the downstream cat
is hot enough to operate correctly. The ECM then asks the downstream O2's how's the are pre-cats doing and because the downstream cat is not hot enough the downstream O2 report back that "Catalyst System Efficiency Below Threshold" and you get a CEL.
 
What about getting a tune and just having the downstream o2's turned off? To get the most out of a set of headers you'd want a tune anyways. With the downstream o2's turned off the PCM will no longer look at them meaning you could just leave them out.
 
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