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I just got this racing downpipe off ebay, basically it replaces the cat and goes from where the header ends to where the catback exhasust begins, well I emailed the guy and he said it has 2 o2 sensors, so i bid on it. I even got off work early today because It was coming, I raced home, opned up the box, and gues what? only 1 o2 sensor, it has the one right before the header, but not the one on the cat, this really pissed me off because I asked him before hand!!! What exactly is the point of the o2 sensors?? what if I just use 1, what will happen? i don't want to mess up my car, I should have waited but I always get anxious when i see new parts. Any help, suggestions would be greatly appreciated , and here is the link from the auction so you know what it looks like:

ebay racing downpipe

 
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D'Oh!!!!!![IMG]i/expressions/face-icon-small-mad.gif[/IMG][IMG]i/expressions/face-icon-small-mad.gif[/IMG][IMG]i/expressions/face-icon-small-mad.gif[/IMG] Lyin rat bas tard [IMG]i/expressions/face-icon-small-mad.gif[/IMG] Write him and kill his User rating on ebay.. demand a refund..he pays shipping..

 
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I've been doing some reading and now i am really confused. basically the o2 sensor reports voltage to the computer, if its a low voltage its running lean and it adjusts by adding more fuel, if its a high voltage its running rich. Ideally you want like .45 volts for the 14.7 a/f ratio. I read somewhere that you could ground the 2nd sensor. The first sensor is the one that is most important. I've been reading about oxygen sensor simulators that let you adjust the voltage. I might build one. I guess I have a couple options.

1) drill and tap a hole in the new downpipe and use the 2nd o2 sensor.
2) ground the 2nd o2 sensor just not sure if i short the wires or what???
3) just let the o2 sensor hang under the car, but then it would probably think it was running too lean because of so much oxygen.

if anyone has any ideas or comments please let me know, thanks!

 
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here is some interesting info:

Cars with multiple O2 sensors:
Starting around 1994, and definitely by 1996, all cars have multiple O2 sensors. There's one in the normal position between engine and catalytic converter, and another behind the catalytic converter. The engine control module monitors this extra one to verify the efficiency of the catalytic converter. With normal operation this oxygen sensor produces almost no voltage because the catalytic converter gobbles up all the good stuff left in the exhaust.
I got a message from a fellow in Venezuela who had to remove his catalytic converters because they burned out, unleaded fuel being unavailable where he lived. The O2 sensors also failed (leaded fuel is bad for them, too). His car noticed, lit up lots of dash lights and ran poorly. He built an O2 sensor simulator, connected it to the front O2 sensors(I didn't say, but cars with dual exhaust have them in both pipes, so he had a total of four) and to the rear sensors. This didn't completely work, of course. The car thought the catalytic converters were bad since the signals were the same "after" as "before". I suggested he hook the O2 simulator only in place of the front sensors, and ground the rear sensors connections. I think that worked. You could also attenuate the O2 simulator signal with a couple of resistors and feed that to the car instead of the rear sensors, the front signal coming directly from the simulator.

 
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Hey man,

I think the easiest thing to do is drill and tap the O2 sensor in. If you try tricking the ECU, you will probably just end up spending more time on it then needed. I've seen friends spend weekends upon weekends in trying to trick the ECU. Well they didn't have 7th gen civics but ECU's are basically the same. Hope I help. I doubt that few people know what your talking about.

 
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Uh I looked at the downpipe after I posted. But ya, definitely put in the second O2 sensor.

1) What happens if your engine really starts to run lean. The ECU will have no idea
2) What if you ground the O2(which I imagine you will cut out the sensor). Then in 6 months decide to sell the civic and get an SI for example

What I would do is try not to modify the car in the electrical sense. I am a elec. engineer major in my senior year, and believe me electricity is a funny thing.

Peace

 
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This is what I've came up with... In my stock system the 2nd o2 sensor is there to make sure the cat is doing its job. Now that I'm going to remove the stock cat, I'm not sure if its a high flow cat or just another resonator that i'm replacing it with. Anyways, If I just leave the o2 sensor hangning under the car, it will report to the computer a low voltage value since there will be no gases, which is what the cat is supposed to do. The first o2 sensor should take care of business. I'm thinking if I drill a hole, It might confuse the computer since it might not be controlling emissions and therefore make the car run even leaner, since the original cat is not there. I don't know if this makes sense. I guess the o2 sensors don't even work until they reach a certain temperature, so Maybe if I don't use the 2nd one it will never reach the proper temp, who knows... Doesnt the new HP header replace the cat, whats the deal with that????? How does that work....

BTW I emailed the a$$hole who lied to me and still no response [IMG]i/expressions/face-icon-small-sad.gif[/IMG]

 
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I remember when the original thread was posted about that thing and I asked if it had both O2 sensor holes and the response that I got was yes it does in fact have both holes. Just goes to show that you cant always trust the people on Ebay. Let us know what you end up doing, and if it works.

 
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you should get a check engine light if you gut the cat. nothing bad should happen by not using the 2nd sensor.

 
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edited out.

 
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quick question: If you turbo the civic, Could you just disconnect the 2nd O2 sensor to get more fuel instead of having to try to rig a returnless fuel system? Just a thought.

one more question: I saw a sc 4.6 mustang (returnless fuel system also) with 3 fuel pumps. Could adding another fuel pump supply enough fuel to make some power in our civics?

 
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Hey all, I finally put the downpipe on last weekend. Upon looking, the 2nd o2 sensor had a plug so I just unplugged it. Everything installed easily, pefect bolt on. It bolted right to my DC header and to my OBX cat back exhaust. I took it for a spin and I definitly noticed increased power and torque, it was pretty sweet! Then after driving for a while the check engine light came on, probably took the computer a while to update. The engine ran fine. Last night I plugged the o2 sensor back in the plug, and used some ties and tied it down under the car. Guess what, the check engine light went off and it still runs smooth. I am happy. The stock cat was supposed to remove emissions, so the 2nd o2 sensor would not be picking up much emissions, since I have it just under my car it should report similar results, kinda fooling the computer. I have my old cat just in case I ever have to put it back on. I didn't notice any stong emission smells or any black smoke so it seems like it might work fine. It sucks because the same guy I bought it off ebay is selling them like 20 bucks cheaper now! It also made my car a little louder, I gotta fix some rattles that I keep noticing at different RPM's...