Goes super rich at zero throttle while cruising

Just like the title says, I'm having an issue where when I'm cruising at pretty much any speed, and I let completely off the gas, my wideband reads afr as 11-12, narrowband locks at 900mV, but my fuel trims read very close to zero.

I'm not sure if this is something tune related, or if it is mechanical. I've been told possibly exhaust leak (which doesn't make sense to me since the narrowband and wideband do the same thing), vacuum leak somewhere, or bad fuel pressure regulator.

Any help is appreciated! Thanks!

Decel has always been difficult to tune, for me anyways. But my fuel trims usually show rich. Not sure why yours doesn't agree with WB sensor.

Maybe just appreciate that fuel is being dumped at the right time - enjoy the post fire crackles when you lift off at high-RPM!

Can you view cross counts on the O2 sensors? That will tell you if you are getting any readings at all. It just counts how many times the O2 crosses Lambda - it's always going 'rich lean rich lean rich lean' for each exhaust pulse. It could be no signal at all.

I can't remember if our typical scanners can show O2 CC, but if so that will tell you if you are getting a false reading.

I mean, if it's not doing any harm then I guess I don't mind, I'm just confused as to what would be causing it. It does backfire lol.

Derek, if we're talking about the same thing, I know that my o2 volts are usually oscillating between 50-950mV or so, it just flatlines at 900mV or so when this happens. I would assume that means my o2 sensor is working fine?

Yes, I believe the value is 600mV where it's considered "crossing" from high to low. (but it could be 500... I'm not exactly sure) If the number of cross counts drops to a very low value (or 0) when this is happening, then there are no exhaust pulses hitting the sensor and the actual mV reading is meaningless.

Cross counts is also the ECMs way of keeping track if the O2 sensor is being "lazy". One of the O2 codes is for "low cross counts" - this means it's time for a new sensor. Other O2 codes mean lean or rich, but often people mistake those for a bad sensor and replace it. The O2 sensor is the most frequently mis-replaced part. Very rarely do you actually need a new O2 sensor, and only if you have no exhaust leaks and a loss of cross counts - that's when it's become "lazy" and will throw a specific code for it.

When there are too many exhaust pulses the narrowband sensor appears to give a steady mV reading. It's kind of false, because a narrowband sensor just can't output properly under load. That's why wideband sensors are expensive... they can still give accurate readings under a very high number of crosscounts (exhaust pulses).

Do you have readings from both sensors? Do they match _most_ of the time? (heavy throttle aside)

How would they match? Stock o2 reads out in mV and the wideband reads out a ratio. How would I match this up to make sense of it?

Sorry, I am extremely new to tuning.

The wideband also reports in mV, but what are you using to read it? If it's a gauge it's interpreting the mV for you. You should be able to get the raw mV reading somehow.

What are you using to scan the narrowband? They won't match up exactly, but you should see the same number of crosscounts.

I'm reading the narrowband in HP Tuners, and I'm currently just reading the wideband on the gauge as I try to figure out how to pipe it into the tuner.

There are a few things that I have noticed that I think could be contributing...

1. I have a p0441 evap incorrect purge flow code.

2. I noticed the plate in my throttle body has a tiny hole drilled in it.

3.When my scan tells me I'm at 100% throttle, I am NOT at 100% throttle. It feels like I'm around 80-90%, but it's not to the floor. It reads 0% correctly.

Once again, thanks for the help. I'm completely clueless.

What are your MAF Hz readings on decel?

Can you see your IAC position at idle?

MAF goes to around 2700 Hz as soon as I let of the gas.

I don't know how to view IAC.

I have the same problem. at idle in park I have 900+mv on the narrow band and a afr of 11-12 on my wideband. But my fuel trims are only a little off zero i don't understand why the ecu is NOT compensating for the richness.