legacycentral bbs

1) An oxygen sensor placed after a high flow cat - totally useless? Should it be showing a lean condition whenever I'm on the gas, but correct readings at idle? I may be able to turn the cat piece around, which would move the O2 sensor in front of it. Or are cats only designed to flow in one direction. (Okay, maybe that should be a separate question)

2) Tinned copper wires - okay to use them to extend the oxygen sensor wiring to reach this new position in the midpipe? I soldered both junctions, but is there any chance the different metals are interacting in the same way a thermistor does?

3) Is it possible to damamge an oxygen sensor if I accidently checked the resisitance of the signal wire? I was hooking up an A/F gauge to see what was going on with the sensor, and accidently checked the signal wire while I was checking my ground connection.

I really hope I don't have to replace the new oxygen sensor I got with this exhaust.

on a first gen legacy there is only one oxygen sensor. It's purpose is to tell the computer how much oxygen is in the exhaust so it can adjust the a/f ratio. If it is behind the catalyst...high flow or not, the reading will be off, and the reading it's giving to the ECU will be wrong.

You MUST have it before any catalyst.

Thought so Josh. Hopefully I can just turn the catalyst around.

1) Yeah, what Josh said.

2) Soldered copper wire should be okay. Make sure it's well shielded from moisture and exhaust heat. Whenever you have two dissimilar metals you do get a little bit of stuff going on, but in this case the signal is strong enough that it's okay.

3) Some people say it can damage the sensor and some say it can't. There's only one way to find out if your sensor's still good...

My brother said the same thing about the soldered connection Vikash. I was looking for confirmation that the problem didn't lie there.

I am getting what looks like good readings out of it at idle, but it goes to 0V when I get on the gas. This is even after I fully reset the computer (left it unplugged for 3 hours). That's what makes me think I've damaged it. Of course, the cat relies on exhaust heat to do its job, so maybe its cleaning the exhaust properly whenever I give it gas.

I'll make sure I take a look at whether I can turn the cat around. Hopefully that's what the problem is. I hope I don't have to spend any money on this. My newish wheel bearing has started the warble of death.

I don't think you can conclude you damaged the sensor if you're reading after the cat. You also shouldn't connect the sensor to the ECU if it's after the cat, since that'll mess with fueling; you'll likely be better off with no oxygen sensor signal going to the ECU at all.

I think it makes sense that the post-cat sensor voltage would be low when you're accelerating; when the engine is working hard it produces nitrogen oxides, and the catalytic converter reduces the oxides, so there's probably a bunch of free oxygen afterwards.

I agree Vikash. I've stopped earmarking money for a new O2 sensor until I can get it moved in front of the cat.

I guess all of this is the problem with using an exhaust originally intended for the WRX, but modified to fit the Legacy. Although I don't see a bung anywhere for the WRX's wideband sensor.

The WRX's front sensor is threaded into the passenger side exhaust manifold.

The way to go might be for you to find a passenger side exhaust manifold from a WRX owner on NASIOC who has upgraded to aftermarket headers. Matt (legacy92ej22t) was gonna do that before he got his Legacy-specific exhaust.

Or you could just get aftermarket headers meant for a WRX.

Too much work.

Now I remember reading a post that mentioned that. That explains why I only saw the one bung then.

Provided the cat has flanges on both sides, I'm just gonna turn it around, putting the O2 sensor before the cat.

mikec wrote:Thought so Josh. Hopefully I can just turn the catalyst around.

ok, this is gonna sound really anal folks, but it's a catalytic convertor, not a catalyst.

i only say this, cause mis use of terms by the more advanced , confuses newbies.

You might still have problems if the sensor is much further downstream than the stock position at the turbine outlet.

vrg3 wrote:You might still have problems if the sensor is much further downstream than the stock position at the turbine outlet.

Yeah, I was thinking about that too. Hrm...

And it turns out I can't just turn it around. I'm gonna call Brullen themselves, and see what they say about it.

Got them to put a new bung in just ahead of the cat, but now the O2 sensor is dead.

Vikash's scantool is showing a constant .32V, and my A/F gauge agrees. No voltage is present when I check it with a multimeter, but its definitely grounded properly. The heater wires show a resistance, so they must have exposed the sensor to something when they moved it. Or maybe they didn't even pull it out before they welded the new bung in. I don't know, they wouldn't let me into the back.

So now I have to see if they're willing to cover the cost of a new one. And I have to leave for 4 weeks of work this Sunday, which means I have no time to be dealing with this crap.

No return phone call today when I called about the sensor. Bought a 22mm wrench and pulled it out. The ring the socket / wrench grabs (what's that called, BTW?) shows signs of them having to force the sensor out. Why they didn't tell me they had trouble getting it out and just left it for me, I dunno.

But I also found the problem with the sensor. It makes noise when I lightly shake it. So they either REALLY forced it, or dropped it. And I need a new one before Sunday, because I've got a 2.5 hour drive and then no way to get packages shipped to me for 4 weeks. So I don't see a way to get a cheapie universal from oxygensensors.com.

You can drive without the sensor connected for extended periods of time if necessary. Matt's been doing it for several months.

True, but with the price of gas these days, I didn't want to. I'd like to extract every dollar I can out of the car, especially now that I've had to put so much into it.

Besides, I shouldn't have to pay for a new one! Hopefully they'll get a hold of me today and we can straighten everything out.

Reviving old thread

Good info in this thread. The wideband has to be located before all cats.

BUT, I was talking to a guy at Zeitronix who said that putting it immediately after the turbo (the stock location!!!) is too hot. This info pertains to a a typical Bosch wideband sensor.

True or BS?

If true, that doesn't leave many options. Luckily I have an aftermarket downpipe with no cat, so I could theoretically weld a new bung near the end of the downpipe but before the cat pipe. That would be annoying, though, if I don't have to. Is the stock bung really unacceptable?

Temperature is only a concern if it's going to force sensor temperature above about its operating temperature (something like 1400 degrees if I remember right). A few inches downstream of the turbo should be okay in that regard.

With turbos the issue is actually often pressure. High pressures do cause the sensor to read richer than it's supposed to... you're supposed to put the sensor where exhaust gas pressure is fairly close to atmospheric.

So in principle you'd stick it just far enough from the tailpipe that reversion (fresh air being sucked up the tailpipe) is not a problem.

But you've got a pretty free-flowing exhaust with fairly low backpressure. Even the stock location's probably okay. I'm using the stock location even with my stock midpipe and muffler and it seems okay.

All wideband sensors work on more or less the same principle, including the Denso one that the WRX puts in the passenger side exhaust manifold.

As usual, VRG is my hero!!!

Ok, well I guess I will give the stock location a shot and see what happens.

Just curious, what is the specific concern with overheating the sensor? Would overheating hurt the sensor life-span? Or would it throw off the measurement accuracy? (or both?)

Thanks!

I don't fully understand how these amazing devices we call oxygen sensors work, but from my meager understanding I would say that if it overheats, the reference cell (which is basically a narrowband sensor itself) would appear to read rich because the Nernst equation says voltage is proportional to temperature. In the range of air/fuel ratios where the reference cell reads rich but the actual mixture is lean, the sensor's controller would send pump current the wrong way and completely fail to consume the residual oxygen. I don't know if that would trigger a fault in the controller or if it would just hit a rail and be read as an incredibly rich condition or if something else would happen.

As for life-span... I guess a lot of heat could damage the sensor, just because heat can damage anything. And I also suppose that a badly designed controller could respond to the failure of the pump current to consume fuel by just shutting down, which would allow the sensor to quickly become hopelessly contaminated with carbon (which is normally burnt off by the regulated heater). I would hope that it would at least keep running the heater, though, even if it gives up on the sensor.

How about cleaning the o2 sensor out the car, I bought a used one and it looks pretty black, is there any way to clean it, with solvent or air ?

Sometimes some heat can burn the carbon off... if you can make the engine run lean for a while with the sensor installed that might do it. Avoid solvents or highly pressurized air.

Would the sensor read richer and lean out the mixture? I put my O2 sensor right after the turbo when I had problems with one that was further down by the transmission (wires melted together while driving). I've been having problems with the engine running really rich since then.

dzx wrote:Would the sensor read richer and lean out the mixture?

When?

When its directly after the turbo. Like a couple inches. I would post a pic, but nasioc is down.