Fuel Injector shut-off while Coasting - Fuelly Forums

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Old 05-04-2006, 01:27 PM   #1
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Fuel Injector shut-off while Coasting

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?
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Old 05-04-2006, 01:51 PM   #2
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My understanding is that

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.
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Old 05-04-2006, 02:28 PM   #3
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Re: My understanding is that

Quote:
Originally Posted by 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.

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.
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Old 05-04-2006, 02:49 PM   #4
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Re: My understanding is that

Quote:
Originally Posted by 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.
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.
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Old 05-04-2006, 02:56 PM   #5
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Re: Fuel Injector shut-off while Coasting

Quote:
Originally Posted by mtbiker278
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?
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Old 05-04-2006, 02:57 PM   #6
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i know that helga does it.

i know that helga does it. just my $.02
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Old 05-04-2006, 03:00 PM   #7
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Re: My understanding is that

Quote:
Originally Posted by philmcneal
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.

Quote:
Originally Posted by 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.
my car is ODBII.
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Old 05-04-2006, 03:10 PM   #8
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I love my OBDI. The

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.
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Old 05-04-2006, 06:10 PM   #9
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Re: My understanding is that

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
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Old 05-04-2006, 06:17 PM   #10
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yes, suddenly driving has

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