ScanGauge II, ignition timing, and fuel consumption during engine braking

[theorist]

I just got my Scan-Gauge II. I love all the information it provides. It's great to see the gallons per hour, miles per gallon or L/100km, TPS, MAP (intake manifold air pressure), intake air temperature (especially if evaluating a CAI), coolant temperature, and engine load in real time while I drive. It also reads OBDII codes, sends OBD commands, and tracks trip information including MPG, but also distance, average speed, maximum speed, maximum RPM, and maximum engine (coolant) temperature.

I have two concerns or questions about information it's providing. It says that the car is using fuel even when the throttle is closed and the engine is being used for braking. Also, I'm surprised by the range of ignition timings reported.

Most fuel injected cars will cut the fuel supply (usually by disabling the injectors) if the throttle is closed and the RPMs are above idle. The scan gauge is reporting that the car is using gas under these conditions. In fact, if I'm engine braking with the engine turning at 2,500 RPM, the ScanGauge reports that it's consuming gas at a rate over 1 gallon per minute, as much as it would if I put the car in neutral and used the throttle to hold the RPMs at 2,500. Can anyone with a Scan Gauge say if they've observed this or not? Does the trip computer on the GT's report instantaneous fuel economy as extremely high when engine braking, suggesting that no fuel is being used? I'd like to know if my 2002 Elantra GLS is wasting fuel or if the ScanGauge is misreporting due to an error or imcompatibility with the ScanGauge or the OBDII on the Elantra. The ScanGauge does report no fuel flow (0 Gallons per hour, 9999 MPG) with no throttle and rpms above idle on the Civic, Prius, and Mustang that I've used it on.

I'm also surprised at the ignition timing reported by the ScanGauge. It usually idles with IGN around 5 degrees BTDC but varies from -10 to 43, with throttle position, RPM, fuel rate (gallons per hour), and engine load. IGN, ignition timing, often reads in the twenties. Is this normal? I guess I only paid attention to idling ignition timing on cars before I had this tool and never had any idea how much it varies according to needs.

[ricerrx7]

MAP (intake manifold air pressure)
MAP= Manifold Absolute Pressure

No engine cuts the injectors off while the engine is slowing down with the throttle closed. The only time the injectors stop firing is when the engine is turned off, or if you have a fuel cut rev limiter.

[matt.matolcsi]

I'd like to know if the software you have is actually reading out the amount of fuel going into the engine.

AFAIK, it probably isn't, but is actually only reading the MAF, and extrapolating from there (using ideal stoichiometry) how much fuel the ECU is actually delivering to the engine.

If I'm wrong, I would love to know it, because I've been really interested in getting an accurate, real-time fuel consumption gauge for a while now.

[04 elantra cvvt]

If you think about it, the engine needs the same amount of fuel to turn 2500RPM no matter where the throttle is (within reason). It will use more fuel to use the engine as a brake than it would if you just pushed in the clutch and let it coast to a stop.

[ricerrx7]

If you think about it, the engine needs the same amount of fuel to turn 2500RPM no matter where the throttle is (within reason). It will use more fuel to use the engine as a brake than it would if you just pushed in the clutch and let it coast to a stop.

That's not true. 2500rpm at 25% throttle will let in a lot less air than 2500rpm at 100% throttle, meaning there will be less fuel added.

[bmxdad]

Above certain limits there is a fuel cut ... I have a 2006 beta II and have seen it, where fuel flow goes to almost zero. As for the timing jumping all over the place ... yea, I've wondered about that too. Its all electronic so it can be adjusted very quickly.

[mtlelantra]

If you think about it, the engine needs the same amount of fuel to turn 2500RPM no matter where the throttle is (within reason). It will use more fuel to use the engine as a brake than it would if you just pushed in the clutch and let it coast to a stop.

That's not right... you're thinking the only way an engine can "turn" is through burning fuel. But when you're coasting/decelerationg, the wheels are now "turning" the engine instead of fuel/combustion "turning" the engine, which is why fuel cut during decel should be possible.

I thought I read somewhere that there are cars that cut fuel when decelerating...
yeah, here: http://www.autoshop101.com/forms/h43.pdf
It's a toyota manual, page 7, ECM does not fire the injector under certain deceleration conditions and will resume fuel injection at a calculated rpm...

Oh, but I think I know where ricer is coming from though- that fuel cut I think was for OBDI... now for OBDII, I think the injectors are always firing at least some fuel, so that the exhaust gas temps stay high enough for the cat to work efficiently.

Not 100% sure on a) if there are cars that will still fuel cut or b) if it's OBDII that's screwed us over, but it makes sense.

[Cypher]

Under engine braking the AFR backs all the way out to 22:1 which means the injectors are just barely firing.

[mtlelantra]

Oh, but I think I know where ricer is coming from though- that fuel cut I think was for OBDI... now for OBDII, I think the injectors are always firing at least some fuel, so that the exhaust gas temps stay high enough for the cat to work efficiently.

I take that back... VW and BMW has decel fuel cutoff, I've heard the Honda Fit has it too...

[HYTECH]

our injectors do cut out on decel.

there is no sensor to measure fuel used so the scanner guesses the fuel used based on map readings and sometimes other readings but they are almost always wrong.

[ricerrx7]

Yeah, I was wrong on that part. It had to happen sooner or later...:D

[sciphi]

The original question wasn't answered, though.

[ricerrx7]

How was it not?

[mtlelantra]

How was it not?

Cuz the questions were:
"Can anyone with a Scan Gauge say if they've observed this or not? Does the trip computer on the GT's report instantaneous fuel economy as extremely high when engine braking, suggesting that no fuel is being used?"

[ricerrx7]

That was so buried in there that it was hard to find. GT's trip computer does not show instant fuel economy.

[RotaMan99]

No engine cuts the injectors off while the engine is slowing down with the throttle closed. The only time the injectors stop firing is when the engine is turned off, or if you have a fuel cut rev limiter.

WRONG

MOST F.I. engine WILL cut fuel on decel to reduce the chance of bucking. Mazda Rx7s do this as well.

If you think about it, the engine needs the same amount of fuel to turn 2500RPM no matter where the throttle is (within reason). It will use more fuel to use the engine as a brake than it would if you just pushed in the clutch and let it coast to a stop.

WRONG

If this were true than you would run VERY rich under low throttle. Using the engine as a brake on MOST FI engines will cut the fuel injectors to increase engine braking and to reduce bucking. Some FI engines may run the injectors still though. The momentum of the car will keep the engine turning and this is why you still have an exhaust note after turning the key to OFF on a standard.

Under engine braking the AFR backs all the way out to 22:1 which means the injectors are just barely firing.

What is the highest limit your AFR meter reads?

Does the trip computer on the GT's report instantaneous fuel economy as extremely high when engine braking, suggesting that no fuel is being used?"

Its close to instantaneous if you reset the trip meter all the time it will go all the way up to 99MPG on engine braking. I have a picture of this on my phone actually. The ECU probubly uses various readings such as the odometer and maybe also calculates injector duty cycle.

GT's trip computer does not show instant fuel economy.
It can if you reset it at the time you want to know your fuel consumption, it calculate very quickly but will get slower and slower the more miles you go.

[ricerrx7]

WRONG

MOST F.I. engine WILL cut fuel on decel to reduce the chance of bucking. Mazda Rx7s do this as well.



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