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I know my 2002 Pontiac Montana has a fuel cutoff. Before I had my scanguage I would guess it was working during some of the time that the dash instant MPG meter would go to 99 mpg when coasting.

When the car is fully warmed up, Scanguage shows that the O2 sensors stay closed and do not open on coasting unless extremely decelerating with the gearbox. As a result, it takes three hail marys and a call to the vatican to get scangauge to show cutoff / 0 gph fuel usage. Its not safe for me to drive that way and I don't attempt it.

I know that the wheels are turning the engine well before this time even without touching the gear selector so I think cutoff should be working before that.

I saw the SG setting for Cutoff/TPS and was more curious about how this feature really works. I don't want to change a setting to see what I want to see vs what is really going on in the engine.

What have you seen with this feature?

Deceleration fuel cutoff shuts the fuel injectors under specified conditions programmed into the ECU. For my 2002 Nissan Sentra SE-R, DFCO occurred when I was in gear, over 2500 rpm, and decelerating. The throttle position sensor drops to its idle position, and the ECU goes into open loop. Scangauge interprets the open loop and low TPS (anything below the cutoff value you set) to mean DFCO, and displays a 9999 mpg (and/or 0 G/H) result. If you (as I've done) mis-set the cutoff value too high, the Scangauge will think you're in DFCO while your engine is merrily drinking fuel at 2500+ rpm. Scangauge instructions say to set cutoff to 4 over your idling TPS position/value. In practice, 2500 rpm was >65 mph in 6th gear, a speed I rarely reached, so DFCO didn't occur often on the SE-R. Additionally, the SE-R didn't go back to closed loop operation for a full four minutes after entering open loop, burning fuel inefficiently those four minutes.

My 2006 Scion xB goes into DFCO at engine speeds over 1500 rpm, and returns to closed loop instantly. In practice, it drops into DFCO at speeds >35 mph in 5th gear, a much more useful application than the SE-R. Since the Scion returns to closed loop instantly when throttle is applied, there's no fuel consumption penalty for using DFCO in it.

My 2006 Hyundai Elantra AT doesn't have DFCO. It never enters open loop when decelerating, despite it being a modern engine that should have DFCO.

Finally, Scangauge's calculation of DFCO is really just blue smoke and mirrors, or bells and whistles, if you prefer. Ultragauge shows 000 mpg under DFCO in my xB, but in the long run, the UG is just as accurate as the SG. I only DFCO down long 5%+ downgrades, and only long enough to scrub off speed, so that may occur for a mile once every five tanks of gas - not enough to make a difference. I can see DFCO on the xB, since the open loop display is always displayed on the UG. To see DFCO on the SG, you can watch your mpg display go to 9999, or the GPH display go to 0 (if you have the cutoff value set properly), or you can monitor the basics - the TPS and open/closed loop displays.

Hope that helps.

That is very informative and does help, and especially hearing about it on multiple cars.

If the scanguage cutoff sensor is more of a suggestion than a real reading, I really don't want to throw a curveball into my results to date. SG is helping me measurably improve my mileage right now, and I prefer not to introduce any artificial highs into it.

If my miles driven / gallons pumped is turns out to be consistently higher the SG for a few tanks, I will think about it then

Thanks Darrell for a very thorough answer. That was the answer I was looking for.

Paul

ps. The loop closes instantly on the Montana too, within 5 seconds or so is what I have observed with SG

You're welcome, Paul. Glad I was able to help.

Paul,
FWIW I had to set my 07 HAH SGII cut off to zero in order to keep the SG from "hanging" up at 999. Tech suppoort at SG said that this would not be much of a factor in the computation of mpg for the SG. I have the Honda Nav option which includes the large graphic trip computer feature and with this setting the SG and the Nav unit display instantaneous mpg very closely.

Have used the SGII for years on numerous rentals and the instruction recommendations and or default with just motor size and fuel type seem to work well.

With the cutoff set at zero in the HAH SGII puts out mpg numbers in the 200+ range where it would have previously been 999. On the plus side it reponds quickly to a throttle change instead of lingering in the nines.

Thanks EV, after reading Darrell's post I'm thinking its not going to have a lot of impact. I am going to wait for my next tank to fiddle with it. You confirmation confirms that as well.

I hadn't heard of the Solectria EV before reading your post. I did drive a Volt today at the autoshow

Paul

Recommend reading "Charging Ahead" by Paul Sherman. Interesting read on the Solectria and the EV struggle to emerge during the 90's. A few years ago Solectria the vehicle company was bought by Canadian firm Azure as in AZD. Solectria Renewables now makes PV inverters, which isn't a very big jump from BEV controllers.