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

I just heard of this, so learning that it was implemented fifteen years ago is slightly embarrassing. I read a few times that it has pretty much been on all vehicles since OB II came out in 1996. Some guy wrote that he thought that DFCO was required by OB II, so I looked into it, and found:

Therefore Deceleration fuel cconditions cutoff (DFCO) is sued to utoff control catalyst temperature during vehicle coastdown, when the engine intake manifold pressure is drive too low to a, allow llow complete combustion. To prevent unburned fuel from entering the converter, the fuel injectors are shut off by the engine controller.

So, DFCO was implemented for emissions, not mileage?

I know for a fact is was in the 1981 Datsun Z cars. Might have gone back to 76 but not absolutely sure. DFCO allowed the elimination of air injection into the exhaust system. They may have tried it on some of the last carburetors. Electronically shutting off the idle circuit, but that is speculation.

Easy test is to place a stethoscope on an injector and rev the engine, let the throttle go completely back to idle and the injectors will stop ticking completely until the RPM drops to just above idle speed. That may not work in some of the more modern versions but it worked in a 1981 Z car long before OBD 1 or 2.

regards
Mech

The old carbed VWs (don't remember what year it started- '71?) had a vac diaphragm added to the throttle so that it would gently close on decel instead of snapping shut, to reduce emissions.

Nice find! Thanks for posting this. Care to share your source?

My dad's 84 Nissan Sentra was carborated and would cut fuel flow when coasting. However my 98 Honda Civic is injected and has OBD2 and does not cut off fuel when coasting. I have heard that some cars do not cut fuel because there is an increase in emissions as the fuel come back on.

When I changed the chassis wiring harness in the engine bay of my truck, I used a Dremel tool with a wire wheel to clean all of the chassis ground posts, all of the grounding nut surfaces, and all of the ground loop connectors on the harness. After that, I coated all of the exposed metal with dielectric goop, then put them back together.

The net result was (other than to get rid of a really annoying intermittent condition whereby my transmission would just decide to go into limp-in mode for 5 seconds while at speed - with no codes, of course) that my truck now tends to go into DFCO a lot more easily than before. There are sections of my commute where the truck will actually do DFCO regularly, whereas before it never did.

I'm tending to lean toward "DFCO = fuel savings," myself.

The problem dates back a long time. When you close the throttle butterfly on an engine that is running at high RPM the manifold vacuum can approach the close to perfect vacuum of outer space. Under those conditions combustion is impossible and unburned hydrocarbons go ballistic.

Even my 37 Ford had a small spring loaded valve in the butterfly so when you got that super high vacuum, some air would flow through the engine. It also had the intake manifold heated by exhaust gas through a passageway that was right under the carburetor, which rotted out a lot of the aluminum manifolds long ago, on an engine that still had a hole in the grille and radiator for a manual crank option.

In the late 60s air injection became practically mandatory, where air was pumped into the exhaust to help with burning the fuel more completely. This was the era of air injection lines into the exhaust manifold.

Later when catalytic converters became mandatory, manufacturers eventually went with fuel injection which allowed for more precise control of fuel delivery. When they realised that they could just shut the injectors off when the vehicle was decelerating while still in gear, then they could eliminate the additional complexity of air injection systems.

In the transitional period from carburetors to FI the manufacturers tried a lot of different "solutions" with carburetors. I think Toyota was one of the last to finally go over completely to FI.

By the time 3 way catalysts were universal, FI was essential to preserve the cats.

It's interesting to see what developments are coming with the availabliity of massive memory and super fast computer calculations. Multiple injections directly into the cylinder while combustion is underway allow ultra high compression with regular fuel. Spark knock is impossible. A long road from the DB 601 engine in the German Me 109 of the mid 1930s with direct injection. You have to see the humor in the "new" direct injection that dates back almost 80 years in aircraft, possibly even longer in other applications.

DFCO was one of the first "advancements" that became possible with computer controls. Nissan went with an EFI system, licensed from Bosch in 1975. It was simultaneous injection, not sequential. They did not need any catalytic converter, EGR, or air injection in the 1975 Federal versions of the first 280 Z. I believe this had DFCO but I can't be absolutely sure. My 76 280 had a N47 early EFI head, on a factory new 83 short block with flat top pistons. This raised the compression from 8.5 to 1 to almost 10 to 1. 160 PSI to 190 PSI on a compression gauge.

It required the base timing to be dropped from 10 to 7 degrees and premium fuel only. With the .75 overdrive 5th gear and the stock 3.54 rear axle 3k RPM was good for right at 85 MPH and at 65, she would push close to 30 MPG highway, with a Motorsports aero kit and lexan headlight covers.

regards
Mech

Wow, Old Mechanic, that was some great information! Thanks for sharing it with us!

Wow, I use references when talking to my girlfriend! How did I quote something without providing the reference?! Some college graduate I am!

http://groups.engin.umd.umich.edu/vi...ganesan_w2.pdf

I think that this was part of a college course. Google just pointed me to the .pdf

I read some of it and then just searched for DFCO. That is some heavy information!

So do carbureted engines from 1980's to 1990's waste petrol/gasoline when engine brake/ downshifting? Take the lancer 2nd generation for example, does it have a valve that shutts off fuel from the carb to the engine? Or tldoes the engine forcefully suck in fuel to the engine and waste it. Also does it mean that we should coast to neutral when stopping? Thanks alot.

Yes, carb'd engines should be in neutral (engine on or off) when coasting.

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!

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.

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.

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

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.

I knew it! :D

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

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

By the way, the previous page shared this: http://media.caranddriver.com/images...s-original.jpg 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.