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.