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Discussion Starter #1
This past weekend I had the cat cut out of my exhaust and replaced with a test pipe. I also had a 90 bend welded on the test pipe. I am obviously getting a CEL (will have o2 bung shortly), but I am more worried about the idle. It is fluctuating a few hundred RPM's but hasn't stalled out. Attached is a picture of the decat. I have a feeling I need a tune to correct the idle, but thought i'd ask. Thanks in advance.
 

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Unless you supercharge or turbocharge the car you shouldn't have to tune it for a decat. I can definitely change the rear O2 settings to stop a P0420 (cat efficiency below threshold) code but it generally won't pass smog like that as the monitor will always show up as incomplete. Some customers will re-install their factory cat to pass that. In that case you'd have to flash it back to stock before getting the car smogged.

support.uvtune.com • View topic - P0420
This gives the criteria and how we disable them.

I have yet to write up tutorials on totally disabling the rear O2 and all that but I like to test things out fully first.

-Michael
 

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Discussion Starter #3
Unless you supercharge or turbocharge the car you shouldn't have to tune it for a decat. I can definitely change the rear O2 settings to stop a P0420 (cat efficiency below threshold) code but it generally won't pass smog like that as the monitor will always show up as incomplete. Some customers will re-install their factory cat to pass that. In that case you'd have to flash it back to stock before getting the car smogged.

support.uvtune.com • View topic - P0420
This gives the criteria and how we disable them.

I have yet to write up tutorials on totally disabling the rear O2 and all that but I like to test things out fully first.

-Michael
Thanks for the help. I discovered an exhaust leak which I believe was causing the o2 to be so far off. I'm going to take care of the leak this week and hope it fixes the problem. If not, i'll definitely be contacting you. I'm not worried about smog testing, I just don't like the light being on constantly.
 

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FYI Kyle, the 90 deg bend that you had welded in is what is causing your poor running condition. Pulling the up stream O2 sensor out of the direct flow of the exhaust is a BAD idea. The ECU uses the up stream sensor to control the fuel delivery to the engine in closed loop operation. With the way you have it mounted it is not getting the proper feed back and this is whats driving your bad idle. That 90 deg bend would better serve you in the down stream position as you are trying to fool the ECU into thinking that the cat is still there and functioning.
If left the way it is you may also develop codes for lean condition under steady state driving or fuel trim as the ecu enriches the mixture trying to maintain the target A/F ratio in the fuel tables.
I would get this corrected ASAP.
 

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Good eye mikelr...

He is exactly correct. The cat defeat thing is only good for the rear O2 sensor. Although the front O2 sensor can be turned off in the ECU it's not a great idea. Lotus (EFI) did a pretty good job of the O2 stuff in the factory ECU.

-Michael
 

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Discussion Starter #6
Thanks for the help! I took the 90 bend off and moved it to the second sensor. The idle has returned to normal and everything looks ok. I'm going to have the CEL cleared soon and hopefully with the 90 bend in the right place, it won't come back.
 
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