DFCO: Deceleration Fuel Cut-off
 

I was searching for the algorithm of the DFCO ( Deceleration Fuel Cut-off ) in our RX-8s without much success.

I find the explanations found regarding our DFCO to have some ambiguity, which begs clarification. To name a few:

1. Deceleration detection method: Is it just the engine RPM's rate of change or something else?

2. Does anyone know the threshold values or conditions for the deceleration rate when the fuel cut-off engages?

3. What parameters cause the fuel cut-off to disengage and resume fuel flow (ie, when the RPM hits 1000 on deceleration?, when the clutch is disengaged? or a function of both?).

I can only deduce from the existing information, but some affirmations or guidance from those more knowledgeable would be nice.

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I've looked into the following threads for "DFCO" and iterations of "decel(eration)" and "fuel cut(off)".

a) The engineers at Mazda are bright people, so....

b) Long downhill - "engine braking"

c) Improving Gas Mileage Tips

d) Engine Break

e) Is There Any Harm to Coasting

if you had a scanalyser you could see for your self when it happens and then reference that to the code:)

throttle position is the major input i think

I see. I've done some external hunting and this excerpt from a Toyota document on fuel injectors clarifies a bit, but not entirely. I think documenting via a scantool is the best way to go. I however am curious if our RX-8s are equipped with the Fuel Tau Cut mentioned below.
-------------- Toyota Doc
Deceleration Fuel Cut
During closed throttle deceleration periods from moderate to high engine speeds, fuel delivery is not necessary or desirable. To prevent excessive dec emissions and improve fuel economy, the ECM will not open the injectors under certain decel conditions. The ECM will resume fuel injection at a calculated RPM.

Referring to the graph, fuel cut-off and resumption speeds are variable, depending on coolant temps, A/C clutch status, and the STA signal. Essentially, when extra engine loads are present, the ECM will begin fuel injection earlier.

Fuel Tau Cut is a mode employed on some engines during long deceleration time with the throttle valve closed, During these times, excess oxygen would enter the catalytic converter. To prevent this, the ECM will very briefly pulse the injectors.
---------------- Toyota Doc

bump

I've got all the info on this at home somewhere, but I won't be back for a few days.

Why for you ask, kimosabe??

My feeling is that one of the primary raison d' etre for car owners' forums is to document as much knowledge about the car inside and outside of what is readily available from the manufacturer by leveraging the collective experience of forum members. I hope that's a good enough reason to ask. :-)

Your information would be a welcome addition to the knowledge base should you be willing to share it.

I've shared here 4400 times....relax, I'm one of the good guys.

Will update when the lacquer on my floor drys and I am again able to enter my humble abode.... :)

So this was discussed yesterday. Is it better for mileage to keep the car in gear and not neutral when decelerating?

Help?

I'd like to know to. If so, I'm making a big mistake...

Most of the information you seek is in the Mazda RX-8 2004 Service Highlights Manual,there is a fair description of the fuel cut parameters.
Like Gomez I am on the road and will not have access for a couple of weeks.Maybe someone who has the manual can give you the info.By the way it is similar to the fuel cut scheme of RX7s from 1986 onwards--I just cannot recall the numbers.

word up...me too

Thanks in advance Gomez and two rotors. I appreciate any further info on this.

the 2004 service highlights isnt available now:(. i believe it is being incorporated iwith the 2005 service highlights complete set. i cant find one on its own at all. although i hear one shop i know may have a couple lying around

The quick answer to this is yes. How much fuel will you save over the cycle of one tankful?? Depends on the amount of decelerating you do. I do heaps, I rarely use my brakes in town. I get 300 miles out of a tank with mixed city/country driving.

Idling uses a lot of fuel. Deceleration with the throttle fully closed uses zero fuel....nada, zip. The Renesis cuts fuel to one rotor during decel with the throttle opened....ie, you haven't fully backed off the gas. This single rotor fuel cut is performed to reduce deceleration shock.....allows a more comfortable drive.

According to Mazda, the fuel cut prevents overheating of the catalytic converter due to misfire and improves fuel economy.

Gasp

I guess people already know about the significance of the fuel cut-off.

can someone sticky this and change the title, add it to the FAQ!?!?!?

KEEP THE CAR in gear decelerating, DO NOT IDLE!

Something like that. -- :)

I'll see how my next tank goes. Still can't believe the lack of iinterest in this thread, or it's just a minority of 8 owners who usually coast in neutral like I used to do.

I did until I read a thread a while back about the car cutting off fuel while in gear. Haven't bothered putting it in neutral since then, but never really saw much change in gas milage either way.

Makes sense for me though, for the most part I'm either cruising or in stop and go traffic. Idle or accel. Not a lot of long breaking or decelleration in there anywhere.

I never coasted in neutral anyway, because I have an aversion to not being in gear.. I always want to be in gear, to be in control.

While we're on the topic of fuel cutoff, were you aware that pressing the accelerator pedal to greater than 50% of throttle opening prior to engine cranking cuts off the supply of fuel to the combustion chamber? De-flooding.

No need to go to 100% to cut off the fuel....apparently 50% will do.

With a torque-less car, does it matter?

i keep revs above 3K, most of the time downshit to around 4.5-5K, that's plenty torque to get out of the way or get going for most cases. it matters when you go to fumble with your shifter to get it in gear and revmatch.

But doesn't rev matching counter the point of fuel cut off? ._.

bah

out of 20 seconds to slow down from a highway ramp: 17 seconds in DFCO, 3 seconds revmatching. vs 20 seconds in our hungry idle? i think idle would use more gas.

as i said, i don't care much about mileage (last tank 15mpg), i just like to stay in gear & control.

Wow. Guess I need to change an old habit. I often coast down long hills. Going to be leaving it in gear from now on.

I must say that this fuel cut-off comes as a pretty big surprise! Just as with Raptor, I too have always coasted in neutral...very very rarely in gear! However, it's gonna take some getting used to though.

I'm a lil surprised at the inactivity of this thread as well! This is pretty awesome...definetely gonna have to try it out.

Fuel cut doesn't care if you are in gear - when the PCM sees no throttle input and the wheel sensors are still spinning, it pulls fuel until the RPMs get down to 2200 or so.
You can prolong the agony by leaving it in gear, but the fuel difference is negligible.
The main reason for fuel-cut is anti-buck, not fuel economy.
I suppose over the life of the car the 1/2 to 3 seconds or so that you are not under power while moving might add up to a mililiter or two...

I was taught at an early age, by some damn good drivers, never to coast long distances in neutral, especially down hills. Low speed up to a red light or backward out of my gently-sloping driveway, stuff like that would be OK. Reasons given were lack of control (RWD), delay in accelerating if needed, and brake wear. Downhill at highway speeds, stay in appropriate gear for amount of engine braking desired. 'Never thought about fuel consumption but now that we know about this DFCO, add that to the list of reasons. Also coasting downhill in neutral is illegal, at least in California, presumably for the other reasons listed.

I go through a dozen sets of traffic signals on the way to/from work each day. I decelerate up to a red light rather than coast up to one (because I enjoy using the gearbox....not the brakes). I'm hopeful that in the fullness of time I'll find this practice allows me to one day drive up to the bowser......not push my car to it!!!!

I agree......you're not going to add 20 miles to a tank during normal driving. You might add one or two though, and that may make the difference one day :) .

Yeah, my tank is at the half way line and I have my usual 120 miles. I did play around with a WRX for a little bit, however, so meh. Oh yeah, the synchros sound sweet.

Yeah, I'm at 1/2 tank right now, with 120 miles..


I did 2 highway pulls with a 3000GT, paced my brother in 4th at like 7000RPM for about 10 minutes, raced my brother in his MSProtege, and have been doing a good amount of general buzzing around.


Heh.

I didn't read all the responses to this thread but why would you coast in neutral. If you ever had to make an emergency manuver, it would take more time to stick it in gear. And if you keep it in gear while decelerating it, it's easier on the brakes therfore leading to longer life of the brake pads/rotors. It also prevents the pads from overheating and obviously you slow the car down faster while in gear.

We've had this debate here a number of times in the past. Any minute now you'll get some geezer pipe up about how brake pads are cheaper than gearboxes. Don't use engine braking....use your brakes, dumbass.

Funny thing is though, in over twenty years of driving manual cars, I've never had a clutch or gearbox replaced. Spent plenty on brake pads/rotors though.

'Cuz brakes are cheeper than clutches!

That said, I tend to use whatever forces are available to me - I'll leave it in gear and keep the clutch in while I use my brakes and then gradually slip the clutch back on to apply some engine braking.
Hell, I pull my e-brake on the highway sometimes if I don't want to give away my intention to slip back into a spot in another lane when I've been shut out by a crawler.

Gomez - you've never replaced a clutch? You must not hold onto vehicles for long.

The RX-8 is my second new car. All the used cars I've purchased in the past have been vetted by me for abuse before purchase. I like to buy something that hasn't had the bejeezuz thrashed out of it.

I drive in a spirited fashion....but I'm mechanically sympathetic. I don't burn rubber.

I didn't push my Honda too hard, either. I still used up the clutch by 100k miles.
Most are designed for about 60k miles.
I've never owned a vehicle that didn't require a new clutch at some point and I've mostly bought new since 1987.
I have plenty of mechanical empathy, but friction is friction.

I've always agreed with the old fashioned "pads are cheaper then engine/trannys" slogan.

Its for this very reason that I find this decelleration fuel cut-off to be so interesting...because i've always been one to use brakes, not an engine...to stop.

I haven't read into any past debates about this, but I find this to be pretty intregeing.

And btw, my old daily driver has about 80K miles on the clutch....still no slip whatsoever.

100k, bloody hell.

I've owned 16 cars. I've only had one that I kept for more than 40,000 miles.....and it was an auto. My RX-8 has done 30k miles.

:rolleyes:

Ah, OK. I'm only up to 11 over the last 24 years, but I drive them.http://www.mazdamaniac.com/portal/sm...n_rolleyes.gif

Wow, you all kill clutches. My infiniti g20t had the original one at 120k till I Sold it.

It was alot of hwy miles but I still had a spirited 20 miles of curvy backroads to drive everyday.

I think the wear on the transmisison when decelerating in gear is rather minimal--not much different, surely, than acceleration, and probably even gentler? And the engine is probably not worn all that much either by deceleration with fuel cut off.

So really the only wear point is during downshifting, if you don't revmatch and end up slowing the car down via clutch friction.

It's also illegal in most states to coast... :)

werd...

really? i wonder how they could ticket you for coasting...

As Astral said earlier, I always keep it in gear because then you're always in controll. In spirited driving I dont see any situation where it would be benifitial to be breaking in neutral as apposed to downshifting to engine break or heel/toeing. I always rev down to at least 3rd gear before putting in the clutch and breaking. Trust me, downshifting comes in handy when the light changes in the middle of you decelerating and you wanna get back on it in a hurry.
I had no idea this had an effect on our fuel economy though. Nice to know that my style of driving is conducive to how the car is tuned.

It's unusual, and I haven't heard of a case made in years. It mostly applies to road tractors and other very large vehicles.

Seems to be a safe law to have for vehicles of that size and weight... I wouldnt want any trucks near me coasting in neutral, especially going down hill!