Using DFCO to increase your MPG

[BailOut]

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.