could someone explain DFCO and coasting to me? - Fuelly Forums

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Old 04-20-2009, 06:26 AM   #1
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could someone explain DFCO and coasting to me?

sounds dumb, but i have yet to find much detailed info on DFCO mainly and when to use as opposed to neutral coasting.

it was my understanding that the ECU will go into DFCO under higher rpm coasting...i.e. engine braking. any truth to this? what exactly triggers DFCO? when should i try to educe DFCO and when should i just neutral coast? i'm mostly a neutral coaster for now, but the more mpg's the better

btw i'm driving a '04 hyundai accent 5 spd. manual.
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Old 04-20-2009, 06:43 AM   #2
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In short:
You should DFCO when you were already planning on braking or slowing. If you intend to maintain your speed or slow just a little bit, you should neutral coast for best FE.

Longer explanation:
DFCO uses zero fuel, but the engine keeps turning at the same RPM. The energy to pump air through the engine, push past internal engine friction, pump oil and water, etc...that energy all has to come from somewhere. By DFCOing at 2000rpm (or whatever), your engine has to turn more revolutions to get to your destination -- and those revolutions cost energy as described above.

If you neutral coast, you spend a little gas idling at 850rpm (or whatever), but you save a lot of momentum. The total amount of energy spent is less and your MPG is more.

DFCO behavior:
Many people think that DFCO is dependable and happens any time you're over 1000rpm, in gear, and the throttle is closed. In reality, there are many conditions that must be met. Examples:
  • My 2002 GMC 5.3l V8 automatic - After 8 seconds of >1500rpm, DFCO will begin, and will hold down to 1000rpm. I don't have more detail because DFCO is so useless on it.
  • My 2008 VW 2.5l I5 manual - DFCO is immediate after steady-state cruising above 1000rpm. If I've just shifted gears, or was accelerating heavily, or just bump-started, it won't DFCO. If I'm below 15mph it won't DFCO no matter how many RPM.
  • Others' Hondas - DFCO lower limit raises to 1300rpm with accessories (A/C, wipers, etc) on.

I highly recommend using something like the fuel rate meter linked in my sig for learning your car's DFCO behavior. It's the only sure and accurate way I've found so far. You can train your butt to detect it by feel but it's imperfect. A ScanGauge can tell but there's a delay before it's displayed, and that lag can make it difficult to truly understand.
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Old 04-20-2009, 07:11 AM   #3
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MY 1998 GMC pickup with a 5.7 L V-8 will go into DFCO after 3 seconds of coasting in gear with the RPM staying over 1,500. DFCO will cancel at 1,000 RPM.

Here's my recommendation... If you want to slow down/stop then DFCO. I like to downshift and DFCO up to red lights. Moderate to steep hills, I'll DFCO all the way down. If you don't want to lose speed, like on a slight downward slope, neutral coast.

Also, on a somewhat related subject, I do not recommend EOC (Engine Off Coasting) unless the vehicle is rated as being "Flat Towable" otherwise transmission damage can occur.
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Old 04-20-2009, 07:18 AM   #4
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for your average person, DFCO is the way to go. it does slow you down somewhat but they are gas free miles and you have to do nothing to enduce it other than take your foot off of the gas pedal.

I have an auto so it is much better for me. I don't have the option of EOCing unless I want an injector kill switch and then I am not sure how I would start the engine back up.

anyway, until you get better info on your neutral coasting, I would say use DFCO. you could run your same daily run using DFCO and a second time using neutral coast and see what the scangauge says your average fuel economy is on each run. that would give you a better idea of which is better.
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Old 04-20-2009, 07:33 AM   #5
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hm looks like i'm going to be buying a scangauge when i have the cash. thanks guys!
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Old 04-20-2009, 07:46 AM   #6
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I just assumed you had one. most of us do.
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Old 04-20-2009, 07:59 AM   #7
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Quote:
Originally Posted by BEEF View Post
I have an auto so it is much better for me. I don't have the option of EOCing unless I want an injector kill switch and then I am not sure how I would start the engine back up.
I think we've had this discussion before, but...how would an injector kill switch help? Isn't the problem with the automatic transmission, which may get destroyed by EOC? You can kill/restart the engine with the key, just like you do when it's parked.
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Old 04-20-2009, 08:27 AM   #8
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I thought that depended on if it could be flat towed or not.

I didn't think you could start it back up while going down the road. it's monday and my mind isn't completely here yet.
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Old 04-20-2009, 08:31 AM   #9
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Quote:
Originally Posted by BEEF View Post
I just assumed you had one. most of us do.
been wanting to get one for a while now and i just don't have the cash at the moment. just got a house and did a big reno job so yeah....kinda broke.
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Old 04-20-2009, 08:50 AM   #10
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I hardly ever DFCO in the Durango, it waits too long. The Tracker almost always did it, if you were on the freeway and let off completely it would DFCO the instant the torque converter unlocked. The Tracker would DFCO down Sandia like crazy and I'd even lose heat because of it. The Durango would coast down that mountain beautifully if it didn't DFCO, there is a noticeable difference in engine braking when it activates and then it goes too slow and I have to change gears or hit the gas. 2nd gear is too little engine braking(and not enough rpm for it to activate the cutoff) and 1st is too much with DFCO.

I don't have a Scanguage either. I have thought of getting one of those KIWI gauges tho.
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