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Old 06-26-2016, 04:54 PM   #1 (permalink)
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Fuel injector shutoff while coasting on an automatic?

This question applies to cars with automatic transmissions only. I've read in multiple places that "modern" cars will shut off the fuel injectors when you lift off the gas, allowing you to coast/engine brake to a stop while in gear without using any fuel. So if you were braking to a stop in front of a red it would be better to stay in Drive instead of idling in neutral like you might in a manual.

Of course for most of you that's common knowledge. My question is, what do they mean by MODERN cars? As in, anything past 1985 modern? Or like, 2013+ model year BMWs and high-end cars that have engine stop/start and that kind of stuff? How would you know if a given car had the injector shutoff feature? Would it say in the manual? Could you listen for it? For the record I'm trying to teach someone how to drive a late-90's Nissan Maxima (A32) for better MPGs and I need to know which technique would be better.

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Old 06-26-2016, 06:24 PM   #2 (permalink)
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My 1991 Honda does it. I think most fuel-injected cars should do it. An easy test that works with a manual might work with an auto too. Manually shift into 1st gear and accelerate from a stop up to 3000 RPM. Take your foot off the gas and you should feel the injectors come back on around 1000-1500 RPM. My car will buck a little when this happens.

And idling in neutral will be more efficient if you're not trying to slow down. But yes, keep it in gear if you want to slow down.
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Old 07-03-2016, 12:31 PM   #3 (permalink)
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Search "DFCO" or deceleration fuel cutoff. I was really happy when someone told me about this. Knowing about how it works gets me that extra MPG I really need sometimes!
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Old 07-03-2016, 12:44 PM   #4 (permalink)
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It was legally mandated at the same time as the OBD-II port, for model year 1996 cars, although some had it earlier, and the requirements for the computer to turn off fuel vary.
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Old 07-03-2016, 02:12 PM   #5 (permalink)
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I downshift while coming to a stop when driving a manual tranny for the purpose of achieving DFCO and reducing wear on the brakes. I'll select the highest gear that maintains DFCO to extend the distance I can do this, and also arrive at the stop quickly enough that it doesn't annoy other drivers.
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Old 07-03-2016, 04:23 PM   #6 (permalink)
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Quote:
Originally Posted by Gasoline Fumes View Post
... An easy test that works with a manual might work with an auto too. Manually shift into 1st gear and accelerate from a stop up to 3000 RPM. Take your foot off the gas and you should feel the injectors come back on around 1000-1500 RPM. My car will buck a little when this happens. ...
Another way to test for DFCO capability would be with a ScanGauge or UltraGauge (or a mechanics' scan tool). With any of these you car read the O2 sensors output (and often the short term fuel trim too) That data will show you when/if the injectors get cut off.

EDIT: My point partly is that if you don't "feel" DFCO it does not mean it does not exist. Sometimes I don't feel the shift, and I have felt it hundreds of times, yet my scan gauge will show the shift to DFCO happening nonetheless.
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Old 07-04-2016, 12:34 AM   #7 (permalink)
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Quote:
Originally Posted by redpoint5 View Post
I downshift while coming to a stop when driving a manual tranny for the purpose of achieving DFCO and reducing wear on the brakes. I'll select the highest gear that maintains DFCO to extend the distance I can do this, and also arrive at the stop quickly enough that it doesn't annoy other drivers.
I sometimes do this too on my commuter bike. Only problem is I have to make two or three throttle blips for multiple downshifts to drop speed in a timely manner... I wonder if the fuel used for blips (infinitesimal, probably) and more importantly clutch wear for those little RPM adjustments on engagement is worth the reduced wear on the brakes?
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Old 07-04-2016, 01:34 AM   #8 (permalink)
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My little brothers 1991 Jeep Wrangler does it as well. It is Fuel injected.
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Old 07-09-2016, 07:49 AM   #9 (permalink)
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Quote:
Originally Posted by parablooper View Post
Of course for most of you that's common knowledge. My question is, what do they mean by MODERN cars? As in, anything past 1985 modern? Or like, 2013+ model year BMWs and high-end cars that have engine stop/start and that kind of stuff? How would you know if a given car had the injector shutoff feature? Would it say in the manual? Could you listen for it? For the record I'm trying to teach someone how to drive a late-90's Nissan Maxima (A32) for better MPGs and I need to know which technique would be better.
You'll probably find that anything EFI does it, it was a fairly early trick to aid economy. The question then is how useful that feature is for that particular vehicle. Most older cars will only DFCO if RPM is over 2000, so in my Jeep Wrangler, that's next to useless as I'd rarely be drivng at that kind of RPM, (the downshift seems hardly worth it either).

On my Renault I also need 2000rpm to enter DFCO but with it's low gearing it's far more useful. DFCO generally cuts off again at 1500rpm. Given the fairly narrow DFCO engagement envelope of older cars, I wouldn't confuse someone I'm trying to teach too much with it. Those parameters can also be dependent on road speed and RPM so it can be quite confusing if you don't have instruments.

On my Fiat I'm able to enter DFCO at just about any speed over idle in all but first and second.
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Old 10-27-2016, 12:50 AM   #10 (permalink)
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In your situation, yes it is more efficient to leave it in gear assuming you have DFCO. You need to learn your car, and learn to predict situations of where to use DFCO and where to coast. Some cars you can feel it, some you can't. I can feel it with mine. If you have a car with ridiculously noisy fuel injectors, like my dad's Merc, then yes, you can also hear it. Otherwise, don't expect to.

Kind of backwards, but high performance and high revving vehicles such as my dad's VFR 800 and S2000 have DFCO that is available RIGHT AWAY, and I think down to 1500 RPM(VFR). Don't have more information about this since it's been a while since I have ridden the VFR, I prefer my CBR.

My car doesn't get DFCO until it hits about 140-150 degrees Fahrenheit, I have to be around 1300 rpm or above for it to initiate. My car maintains DFCO down to 950 RPM when warm, and down to I think around 1100-1200 RPM when warming up. It usually lurches out of DFCO abruptly because I don't expect it to back out that early.

Accord(HA!)ing to my service manual, the auto version of my car gets another 100 RPM of DFCO, cutting out at 850 instead of 950.

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