Stupid question? Search reveals little info, so if this is a "duh, you should know better" thing, I'm all ears.

I bought a Cobalt stainless cat back exhaust from Moss. It's got a sensor bung on the midpipe, just after the flange for the cat. Is this only for OBDII cars or can I run my wideband there? Not having to remove the downpipe and weld on a bung will save me some time/effort, but I don't want to do it wrong. If this location should just be plugged, how far in front of the cat should the new bung be located?

Also, do I remove the stock sensor (in the downpipe) & plug the hole or just leave it there? I will be installing a MSPnP, so the stock sensor will be unused.

 

Seems to me that the logical solution here is obvious: Remove the stock sensor from the downpipe and install the wideband in that location.

 

No. Post CAT O2 will not read AFR correctly, since its supposed to burn whatever is left in the exhaust stream.

 

ive done it and didn't have any issues.
have you experienced big problems? I know people say its off, but haven't experienced this.

but either way just do what joe said cause its super easy and simple

 

No, ive always just put it before the cat. So i guess it could be one of those theory doesn't really cause the reality kind of things.

 

Yeah perhaps some cats affect it more than others, but Iv'e done it on a couple scoobs and evos, and it was dead on each time.

 

Everything I've read says this is too close to the manifold, and will get too hot. Don't want to fry the wideband.

 

At least 18" from turbine IIRC. should in the manual for whatever WB you are using.

 

If you tune for cruise I assume you will see differences. For the fat mixtures you tune there is no (very little) leftover O2 to do conversion.

 

Yes, there will be issues, I've seen it happen twice. The mixture will read rich, stay rich for a few seconds, then within milliseconds switch to lean, stay at that lean reading for a few seconds, then again rich. Repeat. Untuneable.

 

that's weird cause marcello's (thumpetto) car didnt' do this.
stable idle, stable cruising, etc.

 

Wiki something something something:

A three-way catalytic converter has three simultaneous tasks:

Reduction of nitrogen oxides to nitrogen and oxygen: 2NOx → xO2 + N2

Oxidation of carbon monoxide to carbon dioxide: 2CO + O2 → 2CO2

Oxidation of unburnt hydrocarbons (HC) to carbon dioxide and water: CxH2x+2 + [(3x+1)/2]O2 → xCO2 + (x+1)H2O.

Under lean engine operation, the exhaust contains excess oxygen, and the reduction of NOx is not favored. Under rich conditions, the excess fuel consumes all of the available oxygen prior to the catalyst, leaving only oxygen stored in the catalyst available for the oxidation function.

Three-way catalytic converters (all cars since 1981) can store oxygen from the exhaust gas stream, usually when the air–fuel ratio goes lean. When sufficient oxygen is not available from the exhaust stream, the stored oxygen is released and consumed (see cerium(IV) oxide). A lack of sufficient oxygen occurs either when oxygen derived from NOx reduction is unavailable or when certain maneuvers such as hard acceleration enrich the mixture beyond the ability of the converter to supply oxygen.

 

From my former life as an auto mechanic when obd2 started coming out, the front o2 was of the switching variety and the rear o2 was not. Of course the sensors were not wide band. The rear o2 was supposed to stay at a certain range. When the ecu saw the rear o2 act as the front for extended periods of time it would throw a CEL and the likely fault was a bad catalytic converter. I actually saw this happen at least once. Memory serves me right it was on a couple year old Honda Accord and the dealer replaced it under warranty since emissions devices have a longer warranty with obd2 compliance.

But in any case most people's perception of an o2 sensor is misunderstood. An oxygen sensor measures free oxygen content in the exhaust. Most people think it measures the fuel content in the exhaust.

Then there are various types of cats. Back in the day when I stopped working on cars they were mainly three way cats for NOx, HC, and CO emissions. Don't ask me what they were catalyzing these gases into but obviously the cat was trying to change these into something less harmful. However, was it turning them into free oxygen? I highly doubt it and would think that being the case, a wide band o2 sensor would read the same before the cat than after the cat. Just conjecture on my part as I haven't read any papers on what cats can do to a wbo2.

Then add to the fact that after 10+ years of service, there is a good chance that a catalytic converter isn't doing any catalyzing.

Add all these things up and maybe it would explain the varying results people see putting the wbo2 after the cat.

 

Thanks for the input. After searching the web for this, I found wildly varying opinions, and some very heated discussions. Seems people agree that it will change the readings, but there is no clear cut answer as to by how much, or even if it is significant. Plenty of people seem to have done it with no ill effects.

As I have to remove the DP to swap out the clutch, I have decided to play it safe and weld a bung on. Other two bungs will be plugged.

BTW, Moss website states that the second bung is for 96-97 (OBD II) only. Then they want $17 bucks for a single plug! I bought 2 on eBay for less than $7, shipped.

 

Plenty of people have done it, not realizing the effects it had upon their readings. My post about clearly outlined the chemical reactions that would alter the amount of free oxygen present. There would be no reason to sample oxygen levels both before and after the catalyst if they were expected to be the same.

I know you will be happy with your decision to place it upstream.

 

I would be happier if the damned downpipe wasn't rusted and the stupid factory O2 sensor would come out.

Partsgroup to the rescue, yet again.