A/F ratio fluctuates around 3000 rpm, not in boost
 

Title pretty much says it. When cruising well out of boost at 2800-3200 rpm the a/f ratios will swing wildly between approximately 14 and 16. I'm not transitioning into boost, just cruising along at part throttle, at about 10 in hg.

I'm running the stock ECU with a piggyback. Someone claimed this is normal, but the car surges a bit, just enough to be annoying, and I'd like to be able to get rid of that. I cleaned the MAF and tightened anyplace that might get air leaks, don't know what else to do.

Do other people's widebands do this? Is this normal, or did that guy have his head up his butt? Thanks for any and all help.

I've seen this as well and have also been told its "normal." I have a second bung in my midpipe pre cat with an AEM UEGO wideband just for visual reference. With the stock ECU, I see it bounce between 14.2 and 15.6 +/- 0.2. I can hear it in the exhaust but I dont really feel it as a "surge" per se. The frequency at idle is much slower but I get it under load too, frequency of the oscillation increases with RPM. I also only get this when the factory ECU is in closed loop. When the ECU hits open loop, the oscillation disappears and everything goes rock steady.

I believe this is caused by the ECU seeing the narrow band o2 sensor going past its top end, then cutting fuel back for a bit until it goes low, the adds until it goes high... wash, rinse, repeat, almost like its trying to get an average. I've got a really shitty cell video of my wideband + scangauge I can upload somewhere if you'd like to take a look-see.

I just tuned an MS2 car last month with an M45 blower that had the same issues on the Powercard they were using before. We switched to bigger injectors and the MS2 and they can't believe how well the car drives compared to the old setup.

It's not normal for a well-tuned car, but apparently it's pretty normal for piggybacks.

Yep, that behavior matches up with what I've seen on my super/power/voodoo card + stock ECU.

Sav: ...and that's why I'm still bummed I missed you guys on Thursday. How many cars did you guys end up with? Hopefully I'll have my isht together before the next time you are in town running a tuner day :vash:

Thanks for the responses. I'd assume it's really the fault of the stock ecu, rather than the powercard, because the car isn't in boost when it does it - partial throttle around 3000 rpm. I'm guessing this is something a stock car normally does, but I feel surging on my car because of the added volume between the throttle body and intake manifold. If the car didn't surge I wouldn't care that the a/f ratio varied the way it does. It's the surging that's annoying.

I really don't think the stock car surges or varies the AFRs at 3k and partial throttle, seeing as how 99% of city driving is done under those conditions.

I guarantee that a standalone won't surge at any RPM.

I believe the stock ECU does it.
This NA 1.8 appears to do it even at WOT, <4000 RPM (hi FM altitude tho, maybe < 3500 at sea level)

Behold the torque wiggles:

http://flyinmiata.com/tech/dyno_runs...ebb_082099.pdf

Here's a bit of a follow-up. I did a some more research, and it is normal for the a/f ratio to fluctuate as I described, because under those conditions the ECU is in closed loop and averages the a/f ration, constantly adjusting it, trying to keep it at 14.7.

Out of desperation I decided to risk $70 and get a new O2 sensor, even though I had no CEL and the car has only 48k miles on it. I did this because I read in a couple of places that rather than being simply good or not good, the O2 sensor can get sluggish. I couldn't find any test for sluggishness, so I decided to replace it and see what happened, though I had only a small bit of hope that it would solve the problem.

Much to my surprise I've put about 60 miles on the car and so far it's reduced the problem by at least 80%. There is much less surging, and when the a/f gauge needle swings back and forth it does so much less wildly. Time will tell, it was one of those "comes and goes" kind of problems, but I think it would have shown up again by now if it was going to get worse.

*Sigh* Drove another 75 miles or so of mixed condition driving and it's back to it's old ways. Must have jinxed myself.