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Old 03-10-2007, 08:21 PM   #1
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Post Using DFCO to increase your MPG

What is DFCO?

It is an abbreviation for "Deceleration Fuel Cutoff", and the Yaris' engine (1NZ-FE) has it.


How does it work?

When your Yaris is moving forward with zero throttle and is in gear (engine braking) the ECU (your car's computer) cuts off the fuel flow to the injectors. This means you are experiencing forward movement with no fuel cost.


Does this work on both the Liftback and the Sedan?

It is not dependent on body style.


Does this work on both the manual and automatic transmissions (MT and AT)?

Yes, but it is much more efficient with the MT. The MT can achieve DFCO in any gear as long as the clutch is engaged (meaning you don't have it pushed in) and your foot is not touching the throttle. Early versions of the AT (Including ones sold in the U.S. until around January 2007) can only achieve DFCO with a gear selection lower than (D)rive, i.e. 3 or L, but since half way through the 2007 model year the AT can achieve DFCO in all gears..


Is the Yaris the only car that does this?

Not by any means. Many cars manufactured since 1999, even a few as early as 1995, have this feature.


How can I tell if a vehicle other than the Yaris has DFCO?

Be careful with this test as you may lose Power Steering and Power Brakes during it! Choose the right time and location to perform the test.

On any level or slightly downhill road get up to at least 40 MPH, then start engine braking by keeping the vehicle in gear and letting all the way off the throttle while keeping the clutch disengaged (MT) or keeping the transmission in a gear lower than D (AT). Notice the deceleration force and the sound of the engine.

Now tun off your ignition (This is where you may lose your PS/PB, so be wary!) and watch for any harsher deceleration or change in the pitch of the engine for a few seconds.

If nothing was any different with the ignition turned off then your vehicle uses DFCO. If it decelerated at a different rate or pitch with the ignition off then it does not use DFCO. Do not forget to start your engine back up!


How long does it take the Yaris' 1NZ-FE engine to enter and leave the DFCO mode?

The transition takes between 500 and 750 milliseconds (ms) depending on several other ECU measurements.


What will make the engine leave DFCO mode?

1) Touching the accelerator.
2) Dropping below 1,100 RPM.
3) Putting the transmission in neutral/disengaging the clutch.


Does changing gears make the engine leave and then quickly re-enter DFCO mode?

If you take <= 1 second to change gears (as most experienced drivers do) then no, you will remain in DFCO the entire time. However, if your shift takes longer than 1 second there is a chance you will leave DFCO for a few seconds.


Will the ScanGauge II show a DFCO state in the Yaris?

Yes, but it does it best if its firmware has been upgraded with the XGauge feature. Hit the "Menu" button, Select SETUP, then FUEL, then CUTOFF and set it to 21. Once done the instant MPG display will show 9999 MPG when DFCO is occurring.

If your SGII has not been upgraded with the XGauge feature you can verify a DFCO state by correlating several other readings:

1) LOD (Engine Load) will drop well below a reading at idle. My 5MT Yaris shows an LOD of 43 at idle but only 34 in DFCO.
2) GPH/LPH (fuel rate) will bottom out at 0.2 - 0.4 GPH or 0.75 - 1.5 LPH.
3) LP (fuel system loop) will be solidly Open.
4) MPG/LHK will go to the high side (it will take a few seconds but it will go quite high, i.e. 100+ MPG).


How can I use DFCO to increase my MPG?

Let me count the ways...

- When traveling down a grade get into DFCO mode instead of racing from corner to corner or riding your friction brakes, etc.

- As soon as you hit an exit ramp off of the highway go into DFCO all the way to the end of the ramp.

- When approaching a stop or turn use DFCO to decelerate down to just a few MPH before applying the friction brakes.

- When you're stuck in heavy traffic use the well-known technique of finding the right speed to keep a few car lengths of distance ahead of you and maintain forward momentum, then use DFCO to decelerate when needed instead of your friction brakes.

- Use DFCO to maintain controlled forward movement while you're trying to stay moving while approaching a traffic light that you're waiting to turn green.


How can I maximize my speed control when using DFCO?


By selecting the right gear for the purpose. Select a higher gear for faster DFCO movement and a lower gear for slower movement (this is where the MT really outshines the AT). Both transmission types can have their DFCO movement speed adjusted by tapping the fuel when a higher speed is required than gravity can achieve, and by light friction brake usage when some speed needs to be bled off but the next lower gear would be too much.


Does using DFCO hurt my vehicle?

No more than maintaining an idle, or maintaining a fuel-based engine load on it would be, as normally happens in these instances.


Is DFCO more efficient than using the clutch and/or friction brakes to stop?

When you push the clutch in or the AT enters its lowest power state your engine idles (the AT idles a bit higher than the MT in order to produce the pressures needed to operate the torque converter). Idling requires fuel while DFCO does not so DFCO is more efficient. However, you will not be able to use DFCO to completely stop all forward movement as you will eventually drop below 1,100 RPM, so the idea is to use DFCO as much as possible before engaging the friction brake system in the normal manner.


Is using DFCO like this legal?

Absolutely. Even those rare Interstate passes or other roads where engine braking is prohibited don't apply here because those areas have banned engine braking due to emissions controls (notice how semi trucks spew acrid black smoke when they are engine braking), but we are producing zero tailpipe emissions because we are not burning any fuel at all.


What kind of MPG increase will I see by using DFCO?

The answer will vary from person to person, vehicle to vehicle and according to driving conditions but you can rest assured that you will never see an increase in fuel consumption due to using DFCO. Remember that when in the DFCO mode you are using absolutely no fuel.

DFCO is also symbiotic with other high-mileage techniques. For example, ignoring tailgaters when you are doing the speed limit (or the safest speed for the scenario) applies to using DFCO on a downhill grade as much as it does when you're driving using the friction brakes (don't use fuel to speed up just to please a tailgater because they'll still be tailgating you anyway). Another example would be the one I gave above of using DFCO to control your momentum while waiting for a traffic light up ahead to change to green.

Here is one example of the mileage you can achieve in a Yaris without doing anything drastic:

As of this writing my 2007 Yaris Liftback is not quite 3 weeks old yet and will not even hit 1,000 miles until at least next weekend, so I am most definitely within the break-in period. Local temperatures have been 25-40F. My daily commute is over a mountain from 4,500ft. to 8,900ft. and then down to 6,300ft. over a total of 27.2 miles (one way), and I start that climb 1.3 miles after I leave my home. We are still on winter gas (which robs anywhere from 3-10% MPG) and I am running on Mud + Snow tires. In other words I am by no means in an optimal position to achieve high mileage.

My efficiency on that tank was 43.67 MPG.



Please feel free to post any questions or corrections you may have.
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Last edited by BailOut; 08-26-2010 at 02:12 PM.
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Old 03-11-2007, 12:24 AM   #2
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Wow... That's a lot to take in...

So your telling me, that since I have a 5spd Liftback... That if I simply take my foot off the Gas Pedal every so often...I will save gas?

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Old 03-11-2007, 12:28 AM   #3
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Hmm... good to know.... I think I am going to stop using Neutral so much then, my car sounds better slowing from 3rd anyway
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Old 03-11-2007, 12:59 AM   #4
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hmmm very informative.... Gonna have to experiment with that!
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Old 03-11-2007, 01:29 AM   #5
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Quote:
Originally Posted by Nutzoids View Post
So your telling me, that since I have a 5spd Liftback... That if I simply take my foot off the Gas Pedal every so often...I will save gas?




I'm telling you that if you take your foot off the gas pedal while your engine has forward rolling resistance that you'll save gas.
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Old 03-11-2007, 08:22 AM   #6
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Let me get this straight....
You're saying that if we drive normally (lifting foot off accelerator while we want to slow down) then we will save gas. How is that different from how we hve been doing it all along?
Or... are you about to tell us that the Yaris doesn't cut the fuel while we do this, but your $XX DFCO chipset will allow it to? This just HAS to be a sales gimmick, since the Yaris either has this feature or it doesn't. My money's on it NOT having it, so there is something to sell.
Of course, I'm quite prepared to be labelled an old cynic as well as an old phart, and have to apologise. I do that a lot
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Old 03-11-2007, 08:55 AM   #7
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While it does sound like a prefab sales gimmick, if my reading skills don't fool me, it would seem this "DFCO" is a Yaris' ECU feature.
In any case, that;s kinda the way I drive (slowing down taking the foot off the gas, slowing down till I almost stall it, etc. The only difference is that I like to coast to stops in neutral when I see a red light kinda from far a away...)
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Old 03-11-2007, 09:15 AM   #8
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yes, that's what I understood too, a little-known feature of the car.
The idea makes sense, but it is confusing, I mean, if you're engine-braking, and the fuel is supposedly not being supplied, there should be no internal combustion, and hence, not much sound coming from the engine and exhaust, right?
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Old 03-11-2007, 10:01 AM   #9
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I found a fairly detailed explanation here:

http://www.autoshop101.com/forms/h43.pdf

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Old 03-11-2007, 10:01 AM   #10
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You ever hear a car that has an exhaust that pops and crackles when it is decelerating? That is because fuel is still being dumped during decel. When I got the Mopar stage 1 turbo upgrade for my PT one of the selling points was more fuel was being dumped to create louder pops and gurgles. LOL

Usually if you are coming to a stop light you will not have enough time to shed enough speed using this method.

If you are going down a grade I wouldn't recommend engine braking because of the obvious added wear to drive train components. Popping the car into neutral and riding the brakes will use very little fuel.

I have always been told that brakes are cheaper than clutches.

Also, large diesel trucks that use engine braking is a completely different setup than a gas car.
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Old 03-11-2007, 11:50 AM   #11
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@SailDesign:

lol - This isn't a SaleDesign. Our Yarii (and many other cars on the road) already have this feature. I'm just trying to help folks understand how to maximize the advantage they can get from it simply by altering their driving style at a few key points.


@TrancosRt:

Try "coasting" in a high hear instead of neutral while approaching a light.


@eTiMaGo:

You still hear some engine sounds while in DFCO because your engine is not inactive but is acting as a compressor rather than a combustor. I doubt you would hear much at the tailpipe other than a soft whoosh of air, though, as air is still let into the cylinders to provide pressure (the resistance from compressing and ejecting air is what provides the engine braking in DFCO).


@acrbill:

The engine in the PT Cruiser does not use DFCO and the cracking and popping you hear on some turbo vehicles is a feature of the car's ECU that throws higher amounts of exhaust output through it when the car is at low RPM (i.e. idling in a corner) which keeps the turbo spun up at a higher RPM for its next use (i.e. shooting out of the corner). To maintain those pressures even with the turbo's waste gate open (which it has to do in order to dump excess energy) some combustion is purposefully taking place after the cylinders to create more pressure inside the early parts of the exhaust system. You can see and hear this in action on some rally cars.

If you spend some time on the technique of waiting for a light to turn green you'll soon learn to anticipate stops well ahead of time, mostly by looking further down the road than most folks do. Once you get to that point you will find that, more often than not and with the exception of some heavy traffic scenarios, you have more than enough room to maintain forward momentum before the light changes.

While the DFCO mode does produce engine wear so does idling, and since no combustion is taking place during DFCO there is that much less stress placed on the engine (again, in DFCO the engine is acting as a compressor rather than a combustor).

Using neutral and riding the brakes is specifically what you are *not* supposed to do on a grade as brakes experience fade or even failure (hence why there are "runaway vehicle ramps" on just about every long, steep grade in the U.S.). You should always use engine braking on a grade which will already put you into DFCO (as long as an AT is not left in overdrive)... I'm simply trying to show folks that they can maintain DFCO on grades for longer periods of time if they use gear selection and light, intermittent braking effectively.

While brakes are cheaper than clutches we're not adding any additional stress onto the clutch when striving for longer DFCO periods unless we are downshifting on a downhill grade to dump momentum. However, this is no harder on the clutch then coming into a new gear at high RPM during hard acceleration (which most folks are wont to do at least once per week). Remember that a clutch does not experience any stress when it is in its disengaged state.

You're right that large diesel trucks use engine braking in a different manner but it's a moot point to our discussion. The only reason I put that into my original post was to allay concerns some folks might have when they see a "No Engine Braking Allowed" sign.
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Old 03-11-2007, 11:59 AM   #12
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Does this come with that Ebay +20 HP speed module?
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Old 03-11-2007, 12:08 PM   #13
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Does this come with that Ebay +20 HP speed module?


Only USD $1.99 + $19.99 shipping, and I promise it's not just a $0.50 1 ohm resistor!
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Old 03-11-2007, 12:09 PM   #14
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BailOut… I believe you 100% that this DCFO thing is on our car… I am just a bit skeptical that it makes all that much of a difference...maybe for automatic cars it does, but there is a pretty steep grade near my house that I take almost everyday…and normally I just pop my car into Neutral and coast down it… It’s almost a full mile lone… and if I start the grade at 60mph I don’t need to use my breaks until I need to turn off…

On my next fill up, I will force the car into DCFO mode and see if it makes any difference in MPG…

Just for the record I have kept a running total of my MPG ever since I have bought the car… So rest assured it will be accurate!

Any hints or pointers for me?


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Old 03-11-2007, 12:21 PM   #15
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@Nutzoids:

Testing this out for a tank or two and getting a feel for it is all anyone should do, and having a grade like that to play with is nice. However, if it is the only place you induce DFCO at then you won't see that much of a difference in your overall MPG as it's just one mile long and you should be getting at least 300 miles per tank. As such please see where else on your daily commute you can employ DFCO - even if it's just for a few seconds each time - in order to get a more accurate picture of it.

As I mentioned in my original post MT owners actually have some advantages over AT users because we have 5 gears to use DFCO with while they have only 2. This usually lets us pick the perfect gear for the need.

I commend you for tracking your MPG from tank to tank because this is the only way you'll get a feel for how your vehicle is behaving over time. I do the same thing for all my vehicles.

I can't give you many pointers beyond what I put in the original post (although it's a lot of reading this concept isn't rocket science) except that if you normally transition that grade in neutral and you gain no speed then it has to be quite a light grade, so try DFCO on it at the highest gear possible so that you have the least amount of engine resistance.
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Old 03-11-2007, 04:51 PM   #16
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Bailout,
Well, i guess I have to 'fess up and apologise. But.... ya gotta admit that it came off as a sales gimmick. You join on the 9th, and on the 10th post this. No "Yaris has this" but a "you can test your car like this".

As I said - I am used to apologising.
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Old 03-11-2007, 05:54 PM   #17
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Thank you for that, SailDesign, and I've made some small changes to the guide to make it more clear that the Yaris has this feature.

My original post certainly didn't sound like a sales pitch to me but you're not the only person that had an initially negative reaction, so that means that either I did indeed do a poor job with the presentation or you folks on this forum have been hit with some spam-type things here in the past and have become hyper vigilant about it. heh

By the way, joining on the 9th and posting this on the 10th happened because of two things. I didn't find yarisworld.com until someone on priuschat.com pointed it out, and the reason I was discussing my new Yaris on priuschat.com is because the first forum I found was yarischat.com which I chose to not work with as it is an abandoned site that's been overrun by spammers and scammers.

Once I figured out the DFCO thing I felt I just had to share, and eventually ended up here.
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Old 03-11-2007, 09:01 PM   #18
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Thank you for that, SailDesign, and I've made some small changes to the guide to make it more clear that the Yaris has this feature.

<snip-de-dip>

Once I figured out the DFCO thing I felt I just had to share, and eventually ended up here.
No worries - welcome aboard (says he, only being a member about 2 weeks longer than you...)
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