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I have a Black 97 VR4. So far I have turned the boost up to 15psi with an HKS mbc and installed a K&N FPIC forced injection kit. For my next upgrade I was hoping to drill out the precats and main cat , and maybe add a downpipe. I have read that sometimes this causes problems with 96 and newer VR4's because of our extra o2 sensors. Is this true?? If so, how can I remove my cats without freaking out my ECU???
 

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Yes it's true. You might get a check engine light...then again you might not! :D The truth of the matter is a check engine light for the cats doesn't have any affect whatsoever on performance. You know what I did? I just cut the wires to my post cat O2 sensors and left them in my old downpipe. Sounds rough & I got a check engine light but everything runs fine. No big deal.
 

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Have you guys thought about getting O2 simulators? It's a resistor that plugs into the O2 sensor socket and fakes the computer into thinking everything is fine with the sensor. The DSM guys do it all the time.
 

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So is there any reason for leaving any O2 sensors at all then? This winter I'm having a custom exhaust done (including DP) with no cats or mufflers. Should I even install anything besides EGT sensors?...or are the O2 sensors still somewhat needed?
thanks in advance,
Mark
 

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i gutted all 3 & my computer didnt do a thing. no check engine lights or anything.


hey mark, i am planning on doing the same thing(kinda). do you have a price for this, or havent you found out what this will cost yet? i was just gonna get some pipes bent for the DP back with no mufflers but i'd rather do what you are doing if it aint to much $$$...
 

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The reason you dont get a check engine light from your O2 sensors is , the sensors are before your cats , i gutted my cats no lights , the computer doesnt know you gutted the cats beause no sensors detect it . And you needsome type of O2 sensor or a fake O2 ( i dont know if the make the plug in type for our cars ) but the computer needs some type of sensor to know how to run te car .
 

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mitsu3kgt311:
A guy I know owns a garage, his price for me he said would be about $400 if I buy the piping (which isn't too much, $100 max). The whole exhaust will be mandrel bent....2.5" DP to dual 3" to the back.

Brian:
There are post cat O2 sensors.
Mark
 

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Casper Electronics O2 sensor sims don't work. The reason why I just cut the wires to my O2 sensors was because I was wiring simulators in. I still got a check engine light with them installed. I went through two sets of O2 sims just to make sure.

You will DEFINITELY get a check engine light if you just don't re install the post cat O2 sensors with a new downpipe. If you get the exhaust shop to weld O2 bungs onto the new downpipe you might not get a check engine light.

And BTW everyone, this is only applicable to 96-99 3/S's.
 

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I'm curious... what in the hell is the need for so damned many O2 sensors?! I can understand the original two just after the turbos, but three or four? WTF? O2 levels aren't going to change from the manfold to the muffler. Not unless you've got a pretty bad exhaust leak, but even then, with flow going out, your O2 levels won't be any different.
 

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The second pair that are in the downpipe check to make sure the cats are doing their jobs. What's funny is you can gut the pre-cats and alot of times it won't trip the check engine light!
 

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I see why they're called Idiot lights. Personally, I'd be curious when I hear a really loud rattling (like a marble bouncing around) from my exhaust. :confused:
 

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I wouldn't run without that second O2 sensor. The second O2 sensor detects the amount of oxygen after the cat and leans or richens the fuel mixture. If it doesn't show an increase in oxygen the will set fuel mixture to a base setting. This could leave you running lean. That's the way I understand it's operation.:)
 

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MrAlex714 said:
The second O2 sensor detects the amount of oxygen after the cat and leans or richens the fuel mixture.
Actually, no it doesn't. The air/fuel mixture is not affected by the post pre-cat O2's.
 

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I was under the impression that 1st gens had only 2 O2 sensors, both before the cat. (with the exception of Cali cars)
 

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No problemo!

I gutted out my cats about a year ago and have never had any problems except, now my cars faster!....When I did the EGR blockoff thats when I got a light!.....Jc
 
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