Ultragauge DFCO observations
 

I have had erratic results from tanks in which I used DFCO extensively, so I reset my displays to allow me to observe both the instant FE reading and the primary O2 sensor. The FE readings are, of course, a calculation by the UG, while the Primary O2 reading is the voltage coming from a dedicated sensor. The sensor reading would have to be a better indicator of DFCO than the UG calculation.

Results: the UG will read 999.99 mpg almost anytime you have it in gear and release the throttle but the O2 sensor often will still indicate fuel being burned. This is sometimes quite a lot of fuel, it would seem. In third gear descending a hill for 10 seconds with a thoroughly warmed engine, the UG showed 999.99 instant MPG while the O2 readings was .6 volts, slowly sliding down to 0.135 volts and then 0.015 volts. Very lean... but not "cut off" ...

Why do you feel the sensor reading is a better indicator?

Does the .015 reading indicate fuel passing through, or fuel still left in the line?

Because the ultragauge is dependent on the sensors and there is no FE sensor. So therefore the 999.99 mpg number is a UG calculated guess. The sensor is directly reading the exhaust and reporting out voltage according to what it finds. Given how many times the engine turns in a second at 1500 or 2000 rpms, I can't imagine residual fuel would last more than 1 or 2 seconds. But this can go on continually over 10 seconds, and not at 0.015, but at numbers like 0.6 and such. There's no way that at a 0.6 O2 voltage reading from the sensor the FE could actually be 999.99 mpg, is there? What do you think?

I don't know. I'm not sure how that works, but am curious.

Me too.

I always coast in gear rather than in neutral because my Camry has DFCO. My UltraGauge reads 999.9 MPG when I release the throttle.

I can say for myself that it does save gasoline, I just averaged over 37 MPG on my last trip cycle from lots of in gear coasting and other techniques in my Camry :) I hope this helps!

-SC

o2 sensor & lean burn
 

According to the manual:

The lower the O2 sensor voltage, the more lean the car is. I saw a graph at one point but can't remember where I saw it. 0.600-0.855 is normal range though.

[Edit]: I don't have experience with DFCO as I can't get my UG to detect when my insight is in DFCO, and I highly doubt the insight doesn't have it.

There's a good explanation here, the best air to fuel (atf) ratio for economy being 16:1, which they say is the target for closed loop operation.

http://www.endtuning.com/images/airfuel.jpg

There it is. That's the graph I was talking about.