Piggie Pipes and O2 Sensors... How to avoid a CEL & FAQ

[Krueger]

I've got a Milltek non-res catback and APR downpipes. Like others, I didn't have any luck with the VAST O2 spacers. They would throw a CEL after ~50 miles. I've had much better luck with the Wayne angle block.

Incase somebody doesn't know what it looks like:



Email Wayne at wayne92SLC@yahoo.com and he'll hook you up.

I've only gotten a CAT efficiency CEL maybe a dozen times and I've had the angles blocks on for about 10,000 miles.

I invested in a code scanner when I had the VAST eliminators in order to delete the recurring CAT codes. It is absolutely worth it. I bought one from summitracing for $70 and I keep it in the bin under the driver's seat incase a code pops up. Here is the one I bought:

http://www.summitracing.com/parts/AXY-AX700/

[Blackcoog]

Are the O2's in the S4 the same as any other Vehicle? On other cars I've worked with (Fords) you can wire a resistor and a capacitor into the O2 wiring which delays the signal back to the ECU as if the precats were still in place. I'd imagine that would work on the S4. Just a thought for you having issues with spacers. I can post up a schematic for those interested.

[WinterRunner]

Wayne's are def better than VAST's but I would still get CEL's sometimes.

[durrmcdanks]

Wayne's are def better than VAST's but I would still get CEL's sometimes.

How often compared to Vast? I would get a CEL every 20 to 30 miles with my Vast spacers.

[WinterRunner]

How often compared to Vast? I would get a CEL every 20 to 30 miles with my Vast spacers.

I'd say 2-3 times a month so maybe every 500-1000 miles. Not really sure, but it def works better than VAST's.

[Old Baldy]

OK, so I have a few Q's on this whole setup.

I have some (little!) experience with wide band O2 sensors (mainly the universal Bosch LSU4 pump series O2 sensors) and data logging of lamba, RPM, TPS and temp values, for my track Ducati motorcycles - used to build my own Magneti-Marelli ECU maps onto blank 512KB EEPROM chips, usuing a Moates chip burner. Data logging is via the tiny Zeitronix WBO2 controller with serial logging enabled to drive a Palm III PDA logging function of Lambda, RPM, TPS, EGT/Temps.

My questions related to this O2 sensor, CEL and piggy DPs are:

1. Are the Audi sensors not simply O2 Wide or narrow-band O2 sensors that measure the level of O2 in the exhaust?
2. If so...what on earth does this have to do with "air speed?"
3. If not...these sensors are actually not O2 sensors, but some tpe of "air speed" or velocity sensor? Is there any information or link to these sensors, which describes how they work? (My apologies, I have little car exhaust experience....!!!
4. If they are indeed simple, normal O2 sensors, then how does the "spacer" work in this context? USUALLY....an O2 spacer is used to isolate the delicate O2 sensor from the overly hot exhaust gasses and so reduce the heat input into the O2 sensor when it is expected to be subjected to overly hot exhaust gasses (either located very close to the exhaust valve, or from turbo/supercharger streams, etc.....and nothing to do with actual air flow volume.
5. If these are simple O2 sensors, are they wide or narrow band?
6. Do they feed back into the ECU for actual fuel/spark map updates (Closed loop mode?)
7. If wide band...are they in constant closed-loop mode, or partial closed loop for cruise and economy / light throttle feedback only?

My assumptions (if these sensors are indeed simply O2 sensors)....

1. A spacer is never going to be a permanent solution to avoid a CEL if the O2/lambda values are out of ECU expected range. The spacer MAY delay the signal from the sensor pump, but I would expect that after some time, the out-of-range O2 (due to missing or relocated cat) will be highlighted with a CEL. It's kinda like assuming that you could blow SLOWLY into a breathalyzer after 10 beers and not eventually blow a red light. If you keep on blowing, even less than full volume...you're gonna get caught with the sensor.

2. If there is ECU closed-loop feedback....which sensor provides this data? The second sensor at the main cat? If the 1st sensor simply triggers a CEL and does not provide lambda/AF feedback to the ECU, then could you not simply replicate the expected sensor output voltage and remove that first sensor entirely - and avoid the CEL? Can't recall the normal Bosch WB02 output voltages but as they are 0-5V range sensors, would expect that stoichiometric range is in the 2.4- 2.6V range, from a constant 5V input signal? That should be fairly easy to replicate, to avoid a CEL?


Excuse my ignorance on this topic....but I see one or two guys that seem to know this stuff on this thread, and would really like to understand how the Audi system works, and also how this "spacer" deal is supposed to work.

Appreciate the advice/info!

[coltonk]

if you get a catback system does that attach to piggies or replace piggies?

[joeycuccaro]

if you get a catback system does that attach to piggies or replace piggies?

Attaches to the downpipes. Baldy when i get a little bit of time i will comment on your questions.

[Old Baldy]

Joey....just a friendly "bump" here, in case you've forgotten about the advice/follow-up (I know you're probably busy as hell, so excuse my little nudge here :) )

[AJ88CAB]

Old Baldy - I think you are correct in your thinking. As I understand with the "dual O2" Sensor system...the first O2 sensor (before the pre-cat) does the majority (all?) of the exhaust gas sensing and signaling to the ECU for engine management. That is why it is ahead of the CAT.

The second O2 sensor located before the primary CAT is provided to check for a differential between the two O2 sensors. If it sees a differential it assumes the CATs are working. It is just checking for the proper function of the pre-cat...the assumtion being...if the pre-cat is working, the primary is probably still working too...I guess. Questionable engineering logic if you ask me but who am I to question the Audi engineers

In any case, I believe the use of the spacers basically moves the second O2 sensors away from the exhaust flow enough so in some cases there is enough differential that the system thinks there is a functioning CAT between the sensors. It seem to work fine on some cars, but not on others. It probably has to do with the condition of the O2 sensors, but I am guessing a bit now.

The only 100% sure way to not throw a code with piggies is to relocate the primary CAT sensor bung to after the primary cat as shown on the image of the VAST piggies. I can't imagine a muffler shop charging much to do that if you are thinking about doing the piggies yourself. I have gutted my pre-cats that is what I am going to do with mine...Just haven't checked with any shops yet. I just don't want to take the chance of reinstalling and having problems.

The other 100% sure option is modified programming to remove the main CAT O2 sensors from the mix...since they do not really contribute to engine management anyway.

The disclaimer... my S4 is a B5 series and my rambling comments are based on that car's systems. I am assuimg it is similar for the B6 platform...