ScanGauge shows fuel consumption during engine braking

theorist · #1 10-13-2006, 01:55 PM

Hello,

I recently got my ScanGauge II and am loving it. One piece of information really surprised me and I don't know how reliable it is. When the throttle is closed and the engine RPM is above idle, I expected that no fuel would be consumed. Not so according to the ScanGauge.

The ScanGauge showed the absence of fuel flow I expected with all cars with a 2006 Prius. (This is at speeds above 42 when the engine rotates without fuel flow.)

With a 2002 Elantra (MT) and a 2001 Civic (AT), on the other hand, if I coast in gear with no foot on the accelerator pedal, no cruise control, and the engine fully warmed up, the ScanGauge reports that a fuel flow rate in GPH (gallons per hour) that is as high as what's needed to idle in neutral. I never see 0 L/100km or 9999 mpg on the Elantra or Civic while the ignition is on. In fact, I'm using the engine to brake with higher RPMs, the reported fuel rate increases to about what would be needed to hold the RPMs at that level in neutral. Yet I can clearly feel the engine braking is causing the car to slow down and didn't imagine that fuel is being injected.

Is this a bug in the ScanGauge, the OBDII of the cars, or an incompatibility? If it is, can I only trust the ScanGauge's reported L/100km or MPG if I never take my foot off the accelerator with the car in gear? (One possibility I could imagine is perhaps Scan Gauge is using the injector pulse duration to estimate fuel flow but the fuel flow is cutoff before reaching the injectors and the ScanGauge is unaware of this?)

Have I learned something new about how many cars function? Was I wrong to think that I would use less gas by keeping a car in gear while decelerating rather than putting the transmission in neutra and letting the engine idle?

Is there something wrong with both cars if their really burning fuel while engine braking?

xcel · #2 10-13-2006, 06:59 PM

Hi Theorist:

___Unfortunately, SG-II has no way to measure Fuel cut so it estimates a very high FE reading but not Fuel cut. This is the same in every car I have connected an SG-II into. It will measure 9999 mpg w/ ICE-Off travel after a data stream update but not fuel cut

___Good Luck

___Wayne

hobbit · #3 10-13-2006, 11:36 PM

This discussion just came up again in prius_technical_stuff, with
the conclusion [again] that the SG lies about this. If it was
reading actual injector milliseconds, it might be a different
story, but it's doing its math over the wrong parameters.
.
_H*

theorist · #4 10-16-2006, 02:09 PM

Thank you for the references. I was surprised by the the discussion at Prius_technical_stuff. On our 2006 Prius, the ScanGauge appropriately reads a fuel flow rate of 0 gph and 9999 mpg when the car is coasting at speeds faster than 42 mph without a foot on the accelerator. This is with a fully warmed up 2006 Prius and a ScanGaugeII with the fuel type set to Hybrid.

I did a little experiment in our 2002 Elantra while going down a long steep hill engine braking with the manual transmission in first gear and the engine turning around 2000 RPM. When I keyed off the ignition, the car slowed down a little more. When I keyed the ignition back on the Elantra sped up a little. Especially the fact that the keying off the ignition but leaving the car in gear slows down the car a little make me suspect that perhaps the Elantra really is supplying some fuel while engine braking.

Can any cars keep the valves closed under cylinder deactivation? (I thought I read that a few cars with V6s and V8s are designed to do this with half their cylinders under lights loads, but I'm not certain.) If an engine could keep the intake and/or exhaust valves closed for all cylinders it could reduce pumping losses (good for fuel efficiency but not for engine braking) and reduce any concern about cold exhaust cooling the catalytic converter.

brick · #5 10-16-2006, 04:26 PM

HCH-II seals all cylinders closed under zero or negative load conditions. I'm willing to bet that would read 0 GPH/9999MPG since it wouldn't register any airflow. It's that or it would read 7mpg/ lots of GPH when it finds the engine spinning and the manifold at atmospheric. Toss of the coin?

This was one of my first gripes about the SG but I have learned to take it on faith that fuel cut is active when I know it should be.

yesplease · #6 10-17-2006, 10:30 AM

That's because your engine is idling in gear. Most FI cars start idle injection from about 1200-2000rpm depending on coolant temperature. I suppose they could link this to some kind of transmission sensor, but they didn't, probably because a malfunction could result in an automatic not idling at stops, so the fuel saved wasn't worth the extra complexity and potential for failure.

If the car's idling whether you have it in gear or not, it makes sense to have it in N with the engine idling since you will coast farther than you would in gear. Since the engine's also idling at ~700rpm in N, then less fuel may be injected since the engine is idling much slower, compared to something like 1000rpm or more when coasting in gear. You can completely shut off the engine, but in an automatic that's hell on the bearings since most, with the exception of some Saturns and older domestics, rely on having the engine turn a pump to lubricate the transmission. Here's some more info on coasting in N with the engine off.

The trade off is that shifting will result in more wear on the clutch/es, and shutting off will result in more wear on the engine. Depending on how much doing these things will increase your fuel efficiency, how much additional wear they place on the components, and how much it increases the cost per mile of these components, it may or may not be worthwhile.

Compaq888 · #7 10-18-2006, 02:57 AM

It really doesn't matter if the scan gauge can't calculate the engine cut. It still helps you achieve incredible mileage.

All you need is TPS, MAP, and MPG, the rest is a light show.(for the GAUGE) readout.

1)The TPS sensor helps me figure out if I'm pressing the gas too hard or not enough.
2) The MAP helps me figure out if I'm close to the sweet spot.
3) The MPG let's me see the mpg for a current speed.

I placed them in the order of importance for myself.

diamondlarry · #8 10-18-2006, 07:01 PM

Quote:

Originally Posted by Compaq888

It really doesn't matter if the scan gauge can't calculate the engine cut. It still helps you achieve incredible mileage.

All you need is TPS, MAP, and MPG, the rest is a light show.(for the GAUGE) readout.

1)The TPS sensor helps me figure out if I'm pressing the gas too hard or not enough.
2) The MAP helps me figure out if I'm close to the sweet spot.
3) The MPG let's me see the mpg for a current speed.

I placed them in the order of importance for myself.

You have yours set up pretty much like I have mine. For the 4th gauge, may I suggest the open/closed loop gauge? This will tell whether the ECU is in control or not.

diamondlarry · #9 10-18-2006, 07:33 PM

Quote:

Originally Posted by Compaq888

It really doesn't matter if the scan gauge can't calculate the engine cut. It still helps you achieve incredible mileage.

All you need is TPS, MAP, and MPG, the rest is a light show.(for the GAUGE) readout.

1)The TPS sensor helps me figure out if I'm pressing the gas too hard or not enough.
2) The MAP helps me figure out if I'm close to the sweet spot.
3) The MPG let's me see the mpg for a current speed.

I placed them in the order of importance for myself.

You have yours set up pretty much like I have mine. For the 4th gauge, may I suggest the open/closed loop gauge? This will tell whether the ECU is in control or not. Back when I was experimenting with an EFIE, it would tell me when the ECU went into open loop from me adding too much offset.

Compaq888 · #10 10-19-2006, 04:14 AM

Quote:

Originally Posted by diamondlarry

You have yours set up pretty much like I have mine. For the 4th gauge, may I suggest the open/closed loop gauge? This will tell whether the ECU is in control or not. Back when I was experimenting with an EFIE, it would tell me when the ECU went into open loop from me adding too much offset.

That's some useful information. I was using GPH as my 4th gauge but I'll try the open/close loop now.

I don't know if you know this but if you are idling and waiting for a green light just have your foot on the brake and not touching the clutch at all. Before I waited for a green light in 1st gear but with the clutch pedal pressed all the way(not engaged).

The MAP bounces around 3.9-4.1 with the old way.
With the foot on the brake and not touching the clutch pedal at all it bounces around 3.6-3.9 MAP.

If you know this then disregard this info, maybe some noob will read it and get a .1-.2 mpg increase.