Scangauge questions

[bdc101]

I finally took the plunge on a Scangauge II after several years of thinking about it. I've been using it for a week now and have a few questions that I haven't been able to find answers to.

1. Under deceleration the manual says to change the setting for decel fuel cutoff to something like 4 less than the TPS reading when the throttle is closed. Otherwise it says it will read "9999MPG" under decel in gear. When I installed it for the first time, it would read 9999 under deceleration (which makes perfect sense to me, if it can't display infinity). So I change it as instructed and now it tells me that the car is getting 120-250 miles per gallon when decelerating in gear, even at higher RPM. That reading doesn't make any sense -- if it's using zero fuel and moving, why would it not read infinity, or 9999? Should it say something else when in fuel cutoff?

2. When installed in my other car (primary car), the MPG reading is erroneous. I log mileage religiously and lately have been averaging 26-28 miles per gallon and have never recorded a tank less than 25. But on my normal commute today it tells me that the trip average is just a hair over 20. The instantaneous MPG reading supports this and even at low highway speeds in top gear it only reads about 24. I'm not interested in the "tank" logging features on the Scangauge, but do I have to calibrate the MPG reading this way to make it agree with the MPG I calculate with the odo and receipt? In the first car it agreed with the gas log very closely.

[gone-ot]

...on "some" engines DFCO does NOT actually shutoff ALL fuel flow; instead, it simply "reduces" the amount to a very small amount, which is indicated by the 200-400 mpg numbers.

[bdc101]

It's kind of strange that neither of my vehicles actually go into full fuel cutoff. I'm not sure if I really trust that.

I found that by changing the engine displacement, my MPG readings in car 2 became realistic. They came set to 3.0l by default and car 2 has a 2.0l engine. Unfortunately, during the week that I was using the scan gauge on car 1, the readings were spot on (in fact the average was almost exactly what I recorded when I filled up the tank) but car 1 has a 2.4l engine and was also using the 3.0l setting. So that must mean that the scan gauge is reading low on that car... unless it is using a different protocol (car 2 is CANSF whereas car 1 is older) that doesn't need to be scaled. Very strange. The OBDII protocol must output a characteristic or normalized value that requires scaling. The Scangauge manual did not say that inputting engine size is required, so I do find this very strange.

Overall, I am disappointed with how useful the scangauge is. It has satisfied a lot of my curiosity, but has provided very little useful data. Either simple datalogging/dumping to USB drive, or a five-minute average MPG, either of those things would make this thing seriously useful. I'll give it another couple of weeks before I craigslist it.

[SentraSE-R]

By default, the SG cutoff setting is 24, which means the SG registers 9999 mpg any time your throttle position sensor is <24 and your engine is in open loop. That's amazingly unrealistic, since your cars likely aren't in deceleration fuel cutoff then. Your current settings are more realistic.

One of my cars idles at TPS 15, and the other 2 at TPS 11 (one of these doesn't have DFCO). So of the two with DFCO, I set SG cutoff on the first at 19, and the second at 15. (The one that doesn't have DFCO never enters open loop on deceleration, so the SG never thinks its in DFCO.)

Car 1 is/was my Sentra SE-R. It entered DFCO when decelerating (TPS <19) in gear with rpms >2500. That effectively meant DFCO didn't happen in 6th gear below 65 mph! When it did happen, I paid dearly for it. The SE-R stayed in open loop for the next 4 minutes, running a rich fuel mixture the whole time.

Car 2 is my Scion xB. It enters DFCO when decelerating (TPS <15) in gear with rpms >1500. That means DFCO happens in this car in 5th gear at any speed above 35 mph. Not only is it DFCO-friendly, it stays in efficient closed loop as soon as I kick it out of DFCO by pushing the gas pedal.

Give yourself some time to understand your SG. You haven't learned how it works yet, and you haven't calibrated it.

[bdc101]

So what does it say when you are in DFCO? Does it say 9999? I have a hard time believing that both of my cars do not have DFCO mode. Car 1 maybe, it has an electronic throttle, but car 2 is a 2003 and has a cable throttle.

A large part of my disappointment stems from the fact that the scan gauge does not seem to read an actual volumetric fuel flow rate. If it did, setting the TPS cutoff wouldn't matter.

So what else do I need to calibrate? I am not going to use the fuel fill-up feature as I already log my mileage independently, so I don't care to calibrate my fuel tank size, nor my horsepower or any of that silly stuff. All I really cared about the scangauge for (besides a bit of curiosity) was to get a better idea of how factors such as cruising speed affect steady-state mileage, but I'm finding that simple tools that would make that very easy (such as a short-term rolling average) have not been included, and after two weeks of using it daily I've only found that it's very difficult to make a distinct conclusion about anything.

[SentraSE-R]

Mine says 9999 when I'm in DFCO. If you set your cutoff 4 below your idle TPS, it'll never reach that, ergo you'll never see 9999 mpg in normal driving. You need to set cutoff 4 above your idle TPS.

It's indeed possible that both of your cars do not have DFCO. It depends on your car manufacturer and year. My Toyota-made 2006 Scion has excellent DFCO. My 2002 Nissan had very primitive DFCO. My 2006 Hyundai didn't have DFCO at all.

I own both Scangauge and Ultra-gauge. Neither read an actual volumetric fuel flow rate. Both measure fuel injector pulse duration instead.

You need to calibrate your fuel consumption and speedometer accuracy, IIRC. The former because it's obvious. The latter because it affects distance traveled. To do the former, you have to log your fuel consumed, at least for the first tank or two.

[gone-ot]

...I had to back my setting down to 12 from the original 24 setting. Took awhile to get it finally 'dialed-in' as I would decrease the number by 2 and then drive for awhile to see if that worked...it didn't. Took 5-6 iterations before I found that 12 worked best.

[bdc101]

So what did you actually learn by doing these iterations? You got it to read 9999 when it wasn't before? What is the reasoning to think that it's actually using no fuel now after you've dropped the TPS cutoff to 12 below the closed-throttle TPS reading instead of 4 below?

Injector PWM is very easy to translate into a GPH figure (hence why GPH is displayed as a standard gauge). Injector PWM = zero should translate very clearly into DFCO. But the TPS cutoff setting determines whether the fuel consumption is zero (DFCO) or nonzero (MPG = less than infinity) and that makes no sense to me at all. I can understand if it doesn't want to say infinity MPG when it thinks the throttle is not fully closed, but if the fuel flow is zero it seems like any MPG number less than infinity would be imaginary.

[gone-ot]

...I now get BIG numbers like 280-500+ MPG, rather than 9999, which makes sense *when* fuel isn't being 100% shut off.