Fuel Injector shut-off while Coasting

[mtbiker278]

I saw on a couple of threads that the fuel injectors on some autos shut down while caosting in gear. Does this apply to most newer cars? I've been coasting to stop lights and such in neutral cause my rpms stay at idle, but if the injectors shut-off while coasting in gear this would be pointless.

Anyone know more on this subject?

[iburnh2o]

My understanding is that some fuel injected cars at one time did shut off fuel on decel, BUT when OBDII came to be they discontinued this trick in order to keep fuel burning and the cat converter up to temp.

[philmcneal]

My understanding is that some fuel injected cars at one time did shut off fuel on decel, BUT when OBDII came to be they discontinued this trick in order to keep fuel burning and the cat converter up to temp.



what no way! maybe that's why the scangauge doesn't report 0 L/100km when your foot is off the accelerator and revs are above at least 1000 (should say in your manual). But some reported that it is a scangauge bug and the answer is still not clear...

all i know is, at least the instanteous gauge says im' getting kick *** mileage than if my foot is on the accelerator, so if you do have to slow down, do it in gear, neutral just forces you to use more brakes.

[n0rt0npr0]

My understanding is that some fuel injected cars at one time did shut off fuel on decel, BUT when OBDII came to be they discontinued this trick in order to keep fuel burning and the cat converter up to temp.


I think you are right on. I think only those TBI OBDI vehicles did this due to the fuel vapors remaining in the intake manifold(harder to control those loose vapors(and of course they did it for emissions). And maybe even the DPFI, MPFI and SFI vehicles too...but on those it would amount to such a small cutoff that it might be immeasurable. I'll have access to a scanner to find out whether my engine does this upon decel in june.

[MetroMPG]

I've been coasting to stop lights and such in neutral cause my rpms stay at idle, but if the injectors shut-off while coasting in gear this would be pointless.

your garage shows your car as a "5AT" - is it an automatic?

[Diemaster]

i know that helga does it. just my $.02 :D

[MetroMPG]

maybe that's why the scangauge doesn't report 0 L/100km when your foot is off the accelerator and revs are above at least 1000 (should say in your manual). But some reported that it is a scangauge bug and the answer is still not clear...

it's clear to me: my car's factory service manual says injection shuts off under deceleration. the scangauge never showed this, so i'd call that a bug in the scangauge.

My understanding is that some fuel injected cars at one time did shut off fuel on decel, BUT when OBDII came to be they discontinued this trick in order to keep fuel burning and the cat converter up to temp.

my car is ODBII.

[krousdb]

I love my OBDI. The SuperMID shows 0.0 pulse width and pags out at 99.9 km/L (235 MPG) in any gear over 1200 rpm with the throttle closed.

[Silveredwings]

Both my VW and BMW do it. My experience is that if the catalyst is hot and the engine speed is high enough, the ECU decides to shut down unnecesary idle fuel. Atleast in the BMW with an analog mpg gauge, I can see exactly when the shutoff happens. Assuming the cat is hot, if the RPMs are above about 1500 while coasting in gear, it usually takes about 2-3 seconds of normal idle flow but then shuts off. It keeps it off until the RPMs fall below about 1000 (idle is about 650). If I'm on a long hill and I have to keep the speed from increasing (either for traffic or to avoid speeding), I can sometimes do this for a long way. It really pushes up the average mpg.

It's an interesting game to play because it's often better to 'spend' the momentum I've already invested in, and coast farther with the clutch in letting the engine idle. In these cases I also consider how far I am likely to coast. If it's pretty far, sometimes it's better to speed up slightly (as in a pulse) and then glide at a higher speed. Since the idle fuel flow is constant, my higher speed in these cases turns into mpg numbers.

And then there are times when I need to slow down and surrender my precious momentum, such as for traffic, stop signs, lights, sharp corners, etc. Rather than just squander it on merely heating and wearing my brakes, I try to take advantage of the shutoff function. In these cases, vehicle speed is irrelevant and RPM is all important. I have to choose the right gear and then brake only when I finally have to stop. :)

Now if only there was a way I could derive the defining algorithim. Of course then I'd want to add automatic shutdown/bump starts, then automatic FAS and restarts when stopped, regenerative braking, not to mention aero, A/C and P/S mods, and... :P

[MetroMPG]

yes, suddenly driving has become complex enough that even the radio is a distraction! never mind cell phones, sightseeing, daydreaming...

[Silveredwings]

yes, suddenly driving has become complex enough that even the radio is a distraction! never mind cell phones, sightseeing, daydreaming...

yeah, that's all I'm sayin'... ;)

Well said.

[mtbiker278]

your garage shows your car as a "5AT" - is it an automatic?


my car is an auto with five gears. A first on the civic. I do notice that it likes to hold the gear while deccelerating. It'll get down to about 800 rpms before down shifting when coasting.

So are you guys saying that it is more likely that OBD1 cars do this and not so much newer ones? I know metro said his was OBDII. Is there a way to find this out without having to call a honda engineer?

[MetroMPG]

my car is an auto with five gears.

i think it's likely that most, if not all OBDII cars do this once they've reached operating temperature.

but... i believe only cars with *manual* transmissions do it.

if the injectors shut down in an automatic, there's nothing to keep the engine turning (no direct connection to the wheels). the engine will stop when the fuel stops.

[mtbiker278]

if the injectors shut down in an automatic, there's nothing to keep the engine turning (no direct connection to the wheels). the engine will stop when the fuel stops.

I was actually wondering about that. I do experience engine braking when deccelerating, but probably not to the point where if the fuel cut off the car would keep going. I keep thinking I should have bought a manual, but the 5th gear ratio at 0.525 is the only thing saving the automatic. I can cruise at 50mph turning over at 1600 rpms.

[DaX]

if the injectors shut down in an automatic, there's nothing to keep the engine turning (no direct connection to the wheels). the engine will stop when the fuel stops.

Unless the lockup solenoid is activated... I wonder if this is the case?

[mtbiker278]

No clue. The inner workings of an auto tranny are still a mystery to me. I could tell you all about genes, DNA, and complex molecular reactions, but auto trannies .... it's like pandora's box

I doubt there is a service manual I could get my hands on at this point in time since the car is so new. I suppose I could bombard Honda with e-mails, but I have a feeling they'd be reluctant to share the specifics.

So if the injectors don't shut off completely, do you think that the fuel input would be reduced since there is no load (however I'm not sure if engine braking would be considered a load by the ECU) on the engine as opposed to being at idle?

[DaX]

So if the injectors don't shut off completely, do you think that the fuel input would be reduced since there is no load (however I'm not sure if engine braking would be considered a load by the ECU) on the engine as opposed to being at idle?

My intuition would tell me that if any fuel at all is injected, it is very little. My knowledge is solely based off of OBD-1 technology, but I do know that MAP, TP, and RPM all play key roles in the amount of fuel injected, and deceleration is 0% throttle, so that has to count for something.

[rh77]

Via the ScanGauge, I noticed something. I figured that coasting in Drive would be more efficient because of the FI cutoff, but that doesn't seem to be the case. When forced to idle in Neutral and coast, the FE goes up considerably, even at slower speeds. So anymore, I just pop it into N and coast just about anywhere possible, and the results have yielded at least a 1-2 mpg increase, not including more FE with engine-off coasting.

Something to consider...

[SVOboy]

Via the ScanGauge, I noticed something. I figured that coasting in Drive would be more efficient because of the FI cutoff, but that doesn't seem to be the case. When forced to idle in Neutral and coast, the FE goes up considerably, even at slower speeds. So anymore, I just pop it into N and coast just about anywhere possible, and the results have yielded at least a 1-2 mpg increase, not including more FE with engine-off coasting.

Something to consider...

When I noticed that my cel was coming on as a result of decelerating in D (WTF?) I went back to neutral and so far this tank has netted many more miles halfway through, :p

[MetroMPG]

Via the ScanGauge, I noticed something. I figured that coasting in Drive would be more efficient because of the FI cutoff

are you sure your automatic has a cutoff? i'd be surprised.

When forced to idle in Neutral and coast, the FE goes up considerably, even at slower speeds.

i noticed teh same thing in my mom's camry automatic when i borrowed it to find its EPA highway speed. while driving through town, the coasting in N beat coasting in D.

[philmcneal]

Via the ScanGauge, I noticed something. I figured that coasting in Drive would be more efficient because of the FI cutoff

are you sure your automatic has a cutoff? i'd be surprised.

When forced to idle in Neutral and coast, the FE goes up considerably, even at slower speeds.

i noticed teh same thing in my mom's camry automatic when i borrowed it to find its EPA highway speed. while driving through town, the coasting in N beat coasting in D.


i keep telling people, sure you won't have the power of a manual transmission but you can still save gas. Just abuse N and don't use your brakes.... but NOOOOOOOOOOOOOOOOOOOOOOOO people think I'm rubbish....


enjoy paying for the gas sucker. If you haven't noticed, in D there's a potenial truth that somehow the gasoline engine is still feeding power to the torque converter which in turns go to the wheels. With it in N all power to the torque converter is now CUT and enough gas is pumped into the engine to prevent stalling.

By putting it in N you elminate the middle man, doesn't matter if D coasts as well as N, there's still resistance from the torque converter!.

[rh77]

Via the ScanGauge, I noticed something. I figured that coasting in Drive would be more efficient because of the FI cutoff

are you sure your automatic has a cutoff? i'd be surprised.

When forced to idle in Neutral and coast, the FE goes up considerably, even at slower speeds.

i noticed teh same thing in my mom's camry automatic when i borrowed it to find its EPA highway speed. while driving through town, the coasting in N beat coasting in D.

From what I've read, I heard that the fuel injectors give slight pulses of fuel on decel instead of complete cutoff, but I didn't consider it to be a significant amount of fuel. As far as know, it pulses until around 800-900 rpm, then it kicks-into full injection mode to maintain idle, even in gear. Neutral coasting has yielded huge benefits, believe you me.

[mtbiker278]

Looks like I'm going to have to do some more testing on my car. Without the immediate feedback of a scangauge I have to wait through about half a tank to get any kind of estimation. Even then the trfiic around here is so fickle that I could very well see nothing.

[mtbiker278]

Did anyone ever figure out if there was a limit to the speed you can coast an automatic in neutral? There's a hill on the highway that I could coast down but my speed would be around 55-60mph

[SVOboy]

What do you mean by limit to the speed?

[mtbiker278]

is there a limit to the point where it wouldn't be good for the tranny to shift from N to D. I remember seeing a thread from Compaq about this but I think they only resolved the "DO not N-bomb" for an auto.

[SVOboy]

That's only for revving high in neutral. You're transmission should be seamless anywhere if you're not revving it up. My friend's 02 civic will slip in and out of gear at any speed. Lucky him, mine does the whole grinding, jerking, N to D ****, :(

[mtbiker278]

that's what I thought and hoped for. I've tried it a couple times and it seemed to work ok. I'm just paranoid cause it's still new. I'm starting to think that buying a new car wasn't such a good idea. My 01 civic coupr was awesome cause it was too old to void the warranty but still new enough that you didn't have to worry about it. Still pissed at that bastard that rear-ended me and drove off.