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Old 08-28-2013, 11:13 PM   #1 (permalink)
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Saturn experts? DFCO parameters?

I know DFCO (deceleration fuel cut off) vs EOC vs Idling/neutral coasting has been beaten to death. (I know the issues with braking, safety, etc on hills)

but..

Can anyone point me to the DFCO parameters for a sohc 1997 stick shift saturn?
Or if you happen to know what it takes exactly to be in fuel cut off.
Do you need to be in closed loop?
Certain coolant temp, exhaust temp?
Over 1200-1500 rpm?
Over 15 mph?
I've not found any difinative source that spells out exact parameters, or if they even exist beyond the basics.

I'm ready to start getting a little more serious about this saturn. Shouldn't be too hard to see 50mpg on a tank I wouldn't think. Saw 45 already.

I don't yet know how long of a coast it takes to bother with EOC if DFCO will do the job. A really long eoc coast (freewheelin) will obviously be better than DFCO and then having to burn gas the rest of the way. I have one such spot I drive often and will eoc there but I'm leaning towards more DFCO and not bothering with so much eoc.

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Old 08-29-2013, 02:38 AM   #2 (permalink)
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The DFCO info should be in your factory service manual. Your instrumentation will let you know, as it will display 9999 MPG on the instant MPG gauge.

DFCO is very different than EOC. With DFCO, you are using no fuel, but you aren't coasting efficiently. The mass of the drivetrain, as well as the force required to compress intake air, is creating a brake that slows the car down quickly.

I can't think of a situation where DFCO is better than EOC. For example, if you see a red light ahead you should immediately cut the engine, coast to a stop, and not idle. With DFCO you would not be able to coast as far to the stop, and you would still be idling once you got there.

For P&G DFCO would drastically reduce the distance you could coast compared to EOC or neutral coasting.

If you know that you need to decelerate ahead, say going from 60 mph to a 45 mph zone, DFCO could be used instead of your brakes. The better method would be to EOC sooner so that you have a long coast and you get down to the desired speed.

DFCO is useful when you don't want to use brakes, like going down large hills. But that is pretty much the thing, DFCO is a brake.
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Old 08-29-2013, 08:12 AM   #3 (permalink)
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thanks cbaber.
I don't have a factory service manual or scangage to show me the lovely 9999mpg.
I already understand what you are saying about dcfo vs eoc. I love eoc to the point of replacing gearbox oil in my last car with 0-20 synthetic and running 55 psi in the tires for really long coasts.
But, what I like about dcfo is the lack of effort/simpleness of it. Ease off the petal and fuel injectors are off! No key off needed, no kill switch to add, no bump start, etc. My non-eco passengers don't even know I'm doing it.

Just trying to find the rules my cars ecm uses to be in fuel cut off to understand it better, to be sure of when I'm coasting with injectors off.
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Old 08-29-2013, 03:04 PM   #4 (permalink)
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I can feel when my injectors turn back on in my prizm (from around 1000 rpm with no accessory load, to 1500 rpm with the AC running,) what I would do is coast down in gear and turn the engine off like you would for an EOC, but leave it in gear. If there is no change in brakeing force, it was in DFCO.
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Old 08-30-2013, 12:04 PM   #5 (permalink)
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assuming it is setup like the rest of GM's lineup of the era:

minimum coolant temp
maximum TPS
maximum MAP
timer delay
minimum RPM
minimum MPH

those are the requirements that you would run into. exact numbers, no idea.
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Old 08-30-2013, 07:48 PM   #6 (permalink)
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Quote:
Originally Posted by davelobi View Post
thanks cbaber.
I don't have a factory service manual or scangage to show me the lovely 9999mpg.
I already understand what you are saying about dcfo vs eoc. I love eoc to the point of replacing gearbox oil in my last car with 0-20 synthetic and running 55 psi in the tires for really long coasts.
But, what I like about dcfo is the lack of effort/simpleness of it. Ease off the petal and fuel injectors are off! No key off needed, no kill switch to add, no bump start, etc. My non-eco passengers don't even know I'm doing it.

Just trying to find the rules my cars ecm uses to be in fuel cut off to understand it better, to be sure of when I'm coasting with injectors off.
Learn to EoC and bump start more discreetly and your passengers won't know it
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Old 08-31-2013, 10:06 AM   #7 (permalink)
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there is no fuel cutoff on a saturn.
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Old 09-01-2013, 01:47 PM   #8 (permalink)
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OP you should be able to feel DFCO come on. If you have it, which I would think you would.
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Old 09-01-2013, 03:36 PM   #9 (permalink)
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I have a ScanGauge II in my '01 Saturn. And I can tell you that invoking DFCO takes some work. I don't know the exact parameters. But it takes more than 1500 RPM to trigger it. Plus there is a time delay, several seconds long. Like I said, I don't know the specifics. But I've only seen "9999" on the instant mpg display a few times, all of which were on long down-dill coasts.
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Old 09-27-2013, 03:22 AM   #10 (permalink)
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davelobi -

I think my 1999 SW2 has "mild" DFCO. I say this because I have a digital AFR. When the car is in 3rd Gear and *over* 2500 RPM I can see DFCO. Instead of oscillating around 14.7 I see "L" for lean. Once the car decelerates down to 2500 RPM, normal closed loop operation kicks in.

On today's cars I think the DFCO is much more aggressive, aka in the 1500 RPM range.

I have never seen DFCO below 3rd gear and I have rarely been in 4th gear + above 2500 RPM.

Because of the high RPM I only use DFCO to "engine break" down steep hills. On gentle slopes in 3rd gear the car slows down too fast before falling out of DFCO.

CarloSW2

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