dynamho

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

zoom44

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

dynamho

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

Raptor2k

bump

Gomez

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??

dynamho

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.

Gomez

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....

Smileynh

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

Help?

Raptor2k

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

two rotors

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.

captain mercury

word up...me too

dynamho

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

zoom44

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

Gomez

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.

Raptor2k

Gasp

Raptor2k

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

Smileynh

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. --

Raptor2k

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.

alcimedes

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.

Astral

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.

Gomez

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.

Raptor2k

With a torque-less car, does it matter?

Astral

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.

Raptor2k

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

bah

Astral

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.