Deceleration Fuel Cutoff (DFCO) (originally implemented for emissions, not mileage)

[markweatherill]

I had an aftermarket DFCO device on my 1987 Corolla which hooked a vacuum sensor up to the 'anti-run-on' fuel line solenoid. It worked quite well!

[gone-ot]

On my '70 AAR 'Cuda, I used an "anti-dieseling" solenoid to shut off vacuum to the two outboard 2BBL carburetors, turning my 6BBL into a 2BBL for highway cruising at 55 mph...for fuel economy.

[t vago]

Yesterday and today, I've noted that my truck actually goes into DFCO on a couple of downhill stretches on my work/home commute. The truck would not previously go into DFCO the last time I used it to go to and from work last year. I think it has something to do with the fact that my truck now has an electric power steering pump instead of the traditional belt-driven one.

[WD40]

I'm wondering if the 2000 Insight even has DFCO .. the Scangauge never shows 9999 like it did in my VX

[ennored]

Original thread is old, but since it has been brought up, a couple points.

Scangauge infers DFCO, there's a setting for the throttle angle it uses to show 9999 MPG. Looks like it assumes it's in DFCO if it's open loop and at a low throttle angle. From the manual:

Set the Fuel Cutoff Level
Some vehicles will turn the fuel injectors off while
coasting – this is known as Fuel Cutoff. ScanGauge
attempts to detect the fuel cutoff condition by
comparing the open/closed loop indicator and the
throttle position.

Remember the Scangauge does not know fuel flow. It takes airflow and multiplies it by the stoichiometric air fuel ratio. (Don't know why they do it this way, fuel flow data is in the data.)

In my experience (big 3, last 25 years) DFCO is used for fuel economy. We had to be careful with it early on as it could make emissions worse. We carefully control to stoich to keep the cats working at high efficiencies, then DFCO kicks in and pumps them full of oxygen. We've gotten better about returning from DFCO, but it's still primarily a fuel economy tool.

And, as to whether a certain car, on a certain day, during a certain drive cycle, is doing DFCO. I have a phrase I use: "calibratable beyond explanation". That is, there's a zillion inputs that can and are used to determine if the fuel should shut off. It's not just pedal position. There's throttle position, airflow, engine temp, battery state of charge, gear position, torque converter state, trans temp, engine speed, road speed, ambient temp, ambient pressure, purge state, AC state, phaser position, codes present, tests running, phase of the moon, and more. Blipping the throttle and watching for the fuel to shut off isn't going to tell you much of anything.

[t vago]

Originally Posted by ennored

And, as to whether a certain car, on a certain day, during a certain drive cycle, is doing DFCO. I have a phrase I use: "calibratable beyond explanation". That is, there's a zillion inputs that can and are used to determine if the fuel should shut off. It's not just pedal position. There's throttle position, airflow, engine temp, battery state of charge, gear position, torque converter state, trans temp, engine speed, road speed, ambient temp, ambient pressure, purge state, AC state, phaser position, codes present, tests running, phase of the moon, and more. Blipping the throttle and watching for the fuel to shut off isn't going to tell you much of anything.

I knew it!

[Xist]

Strange, I cannot edit my previous posts, the .PDF I linked disappeared, and I am unable to load the Wayback Machine. Putting Deceleration Fuel Cutoff into Google shows:

1. This thread!
2.

DFCO - Wikipedia
https://en.wikipedia.org/wiki/DFCO
DFCO may refer to: Dijon FCO, Dijon Football Côte d'Or; Deceleration fuel cut-off, see Fuel economy-maximizing behaviors (section: Burn and coast) ...

However, clicking on Wikipedia sends me to This page, where "Burn and Coast" was fortunately renamed "Pulse and Glide," with the following statement "Most modern petrol vehicles cut off the fuel supply completely when coasting (over-running) in gear, although the moving engine adds considerable frictional drag and speed is lost more quickly than with the engine declutched from the drivetrain," although there is not a reference, and it does not mention Deceleration Fuel Cutoff.

I found two people in forums quoting that, but not any direct source, although this is similar:

While coasting with the engine running and the transmission in gear, most cars' engine control unit with fuel injection will cut off fuel supply, and the engine will continue running, being driven by the wheels. Compared to coasting in neutral, this has an increased drag

Hypermiler | Project Gutenberg Self-Publishing - eBooks | Read eBooks online

According to Paul Williamsen, the product education manager at Toyota, “All contemporary Toyota and Lexus vehicles (and every other car built since the 1990s that I’ve looked at) can detect the condition when engine revs are higher than idle with a fully closed throttle: Under these conditions, all current to the fuel injectors is stopped, and no fuel is injected.” That means if your foot is off the gas while the car’s in gear, you’re not using any fuel.

Tom Read, GM’s powertrain spokesman, agrees: “Shifting into neutral in an automatic will cancel fuel cutoff. Thus, it is better to remain in gear and let the drive wheels pull the engine airflow down to where fuel cutoff can be enabled or where fuel flow is minimized.”

Gas Pains: Mileage Myths and Misconceptions - Car Comparison - Feature Article - Page 3

By the way, the previous page shared this: Turn Off Your Air Conditioner

Several people have insisted that using the air conditioning does not increase fuel economy, I tell them to stop watching Mythbusters, and the conversation gets stuck between a rock and a hard place.

This page describes it for the 1990 GM Prizm: | Repair Guides | Emission Controls | Deceleration Fuel Cut-off System | AutoZone.com

I found this statement I cannot confirm: "According to Mazda, the fuel cut prevents overheating of the catalytic converter due to misfire and improves fuel economy." DFCO: Deceleration Fuel Cut-off - RX8Club.com

Also, this: Green Car Congress: GM says deceleration fuel cutoff in Cruze increases fuel economy by up to 2%

United States Code Title 42 - THE PUBLIC HEALTH AND WELFARE, CHAPTER 85 - AIR POLLUTION PREVENTION AND CONTROL, SUBCHAPTER II - EMISSION STANDARDS FOR MOVING SOURCES, Part A - Motor Vehicle Emission and Fuel Standards, Sec. 7521 - Emission standards for new motor vehicles or new motor vehicle engines, (m)(1):
“Within 18 months after November 15, 1990, the Administrator shall promulgate regulations under subsection (a) of this section requiring manufacturers to install on all new light duty vehicles and light duty trucks diagnostics systems capable of—
(A) accurately identifying for the vehicle's useful life as established under this section, emission-related systems deterioration or malfunction, including, at a minimum, the catalytic converter and oxygen sensor, which could cause or result in failure of the vehicles to comply with emission standards established under this section,
(B) alerting the vehicle's owner or operator to the likely need for emission-related components or systems maintenance or repair,
(C) storing and retrieving fault codes specified by the Administrator, and
(D) providing access to stored information in a manner specified by the Administrator.”
https://www.gpo.gov/fdsys/pkg/USCODE...tA-sec7521.htm

Clearly, the EPA established the standard, which was developed and named afterward, and I imagine the same applies to DFCO. I did not find that acronym or “deceleration” in any of the regulations or elsewhere.