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Old 08-27-2010, 01:06 PM   #1 (permalink)
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Please explain DFCO (Deceleration Fuel Cut Off)

Hey guys,

I know that it stands for Deceleration Fuel Cut Off and I also know that most modern vehicles including my Scion employs it because I see it on my scangauge. However what I would like is a definition of how it works.

Meaning, I still here the engine running while its doing it, despite the fact that it is not getting any fuel (Supposedly), how does that work? Why would it do it while decelerating in gear and not while neutral coasting?

And most importantly how does it restart the engine while rolling and without any bump start feeling?

Thank you ..

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Old 08-27-2010, 01:12 PM   #2 (permalink)
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DFCO can only happen while your transmission is in gear. So, the wheels are spinning the engine. To restart, it just injects fuel since its already spinning. If you were in neutral and DFCO happened (which it can't), the engine rpms would drop to 0 and you would have to bump start or key start.
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Old 08-27-2010, 02:02 PM   #3 (permalink)
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Some cars have what is referred to as DFCO but is actually Deceleration Enleanment. My car with the Buick 3800 is one of those cars but I'm not sure how common that is.
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Old 08-27-2010, 02:36 PM   #4 (permalink)
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You're not using any fuel while you DFCO, but it saps the vehicle's momentum to keep the engine turning. You're going to have to burn fuel to get that momentum back. DFCO will give you better mileage than using the brake pedal, but less mileage than driving without brakes and engine-off (or maybe even engine-on) coasting to a stop.
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Old 08-27-2010, 02:51 PM   #5 (permalink)
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Momentum isn't a problem going down a hill... My question is how come the DFCO turns off while still engine braking? I'll be watching the ScanGuage and it will just change from 9999 to ~60mpg after a minute or two. I have to alternate between engine-on coasting and no-fuel engine braking while on long descents. It's like the Ford engineers in Michigan couldn't conceive of the hills we have here in Colorado. Is there a reason for this? Is there a way to change it?
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Old 08-27-2010, 04:37 PM   #6 (permalink)
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DFCO is employed because when you are engine braking the effective compression is very low, and any fuel injected into the engine has a very good chance of not igniting and being exhausted with very high amounts of unburned hydrocarbons, as well as damage to the converter when the unburned fuel passes through the converter.

With manifold vacuum of higher than 22 inches, the amount of air in the cylinders is only a small percentage of the amount that would create the designed compression ratio. This is a recipe for misfires.

Determining if your car has DFCO is easy, just let the car slow down in gear until you feel the fuel injection engage and the car will either stop slowing down altogether or slow down at a much lower rate. The point is easy to feel.

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Old 08-27-2010, 07:42 PM   #7 (permalink)
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Quote:
Originally Posted by RobertSmalls View Post
DFCO will give you better mileage than using the brake pedal, but less mileage than driving without brakes and engine-off (or maybe even engine-on) coasting to a stop.
Not maybe.

Engine-on coasting is what got me above 47mpg (or below 5L/100km) .
I was using DFCO and braking as little as possible before I started coasting, so the combination of both is inferior to engine-on coasting.


My engine uses 0.5L/H while idling.
So it takes 10H to use up 5L.
As long as I do 100km in those 10H of idling, or 10 km/h, I'm still only using 5L/100km and getting 47mpg.
As long as I'm coasting faster than 10 km/h (6 mph), I'm doing (far) better than 47mpg even with the engine on.
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Old 08-27-2010, 07:52 PM   #8 (permalink)
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euromodder -

Quote:
Originally Posted by euromodder View Post
Not maybe.

Engine-on coasting is what got me above 47mpg (or below 5L/100km) .
I was using DFCO and braking as little as possible before I started coasting, so the combination of both is inferior to engine-on coasting.

...
Yes, but I think DFCO has a strategic purpose for us. Going down a steep grade while coasting is not a great idea from a brake-overheating POV. However, if I can get engine-braking + DFCO on the downhill, I can avoid using my brakes and risking dangerous speeds.

If I had a fuel-injector cutoff switch, I could use it for manual DFCO.

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Last edited by cfg83; 08-27-2010 at 08:54 PM..
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Old 08-27-2010, 07:55 PM   #9 (permalink)
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round.boater -

Quote:
Originally Posted by round.boater View Post
Momentum isn't a problem going down a hill... My question is how come the DFCO turns off while still engine braking? I'll be watching the ScanGuage and it will just change from 9999 to ~60mpg after a minute or two. I have to alternate between engine-on coasting and no-fuel engine braking while on long descents. It's like the Ford engineers in Michigan couldn't conceive of the hills we have here in Colorado. Is there a reason for this? Is there a way to change it?
For me that usually happens at a specific MPH+RPM. Have you noted the point where this happens to you?

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Old 08-27-2010, 08:41 PM   #10 (permalink)
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Basically the car knows it doesn't need to dump fuel into the engine to keep it going so it cuts it off. If you took your car out of gear it would respond by turning the injectors back on. Also, if your RPMs fall below some pre-set number it does the same.

DCFO used with EOC can indeed give you better FE than EOC alone in certain circumstances. I can give you an example: On my drive home from work there are so many chances to EOC for long distances that the headlights can drain the battery so low I may not be able to restart the engine if I mis-time a light and have to stop. In order to prevent this from happening, I frequently would bump-start the engine at times just to get the alternator to charge the battery slightly. I would do this at lights to make sure I wouldn't be stranded. Thus, burning gas that was getting me nowhere.

Now I have an injector cut off switch that allows me to DCFO at any RPM and will not restart the engine when I take it out of gear. I use it when approaching lights that are red or I know will be red before I can get to them. I'm not wasting momentum, since I use it when I know I'm going to be stopping anyway. DCFO also keeps all the underhood pumps working, including A/C if it's on.

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