Fuel cut-off threshold speeds - Fuelly Forums

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Old 08-11-2007, 11:03 PM   #1
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Fuel cut-off threshold speeds

Anyone else checked to see at which speeds their engine cuts fuel off completely when coasting in gear? My '07 Aveo5 SVM will only switch to full fuel cut-off (indicated by going open-loop on the ScanGauge II) if the coast starts at above 40 mph (any gear), and lasts until the engine gets down to around 1200 RPM or so. If I let off the gas in gear below 40 mph it'll continue to supply some fuel, reducing the amount of engine braking.
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Old 08-11-2007, 11:26 PM   #2
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It depends what gear you are in for my 07 Fit. In 4th or 5th it cuts off right away if you start coasting anywhere above about 1300 RPM, and it will stay cutoff down to about 1000 if you are slowing down. In lower gears it won't do it unless you start the coast about 2000 RPM, and it will start fueling again at about 1500 RPM.
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Old 08-11-2007, 11:31 PM   #3
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i thought it was 38mph on my car but i cant varify that because i dont have a scangauge.
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Old 08-12-2007, 01:25 PM   #4
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Is this only for newer cars?
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Old 08-12-2007, 01:36 PM   #5
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Quote:
Originally Posted by GeekGuyAndy View Post
Is this only for newer cars?
No. For example, my "throttle body injected" (an early style of fuel injection) 1991 CRX has fuel cutoff. However, fuel cutoff is usually only present on computer controlled fuel injected cars.

But since practically all cars these days use ECUs (engine computers) to run fuel injectors, there is no reason why this features shouldn't be in practically all modern cars. Not to say that the feature is in all current cars (it depends upon how the car maker programmed the ECU), just that there is no technical reason why it couldn't be.
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Old 08-12-2007, 02:20 PM   #6
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Originally Posted by GeekGuyAndy View Post
Is this only for newer cars?
Pretty sure that some manufacturers were using it even in the 80's.
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Old 08-12-2007, 04:30 PM   #7
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But I didn't think the Suby has it. How would I know?
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Old 08-12-2007, 06:49 PM   #8
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Look for a 0 gph on the scangauge. If you had a manual trans, there would be other ways...
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Old 08-12-2007, 08:30 PM   #9
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On my car the fuel cutoff is not linked to vehicle speed, just engine speed, so the ecu will cut off fuel in all gears. I had a look in the ecu and found the cutoff point is 1188 rpm when the engine is fully warmed up. This point is raised during warmup to match the higher idle speed when cold. There is also a delay timer, hysteresis, built into the cutoff so the ecu doesn't cut off fuel at 1200rpm then turn it back on at 1188, which would cause some jerkiness in the driveline.
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Old 08-12-2007, 08:41 PM   #10
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Rotareneg -

I don't think I ever see complete fuel cutoff.

(following is from http://www.gassavers.org/showthread.php?p=46638)

What I read on saturnfans.com is that for my drivetrain, fuel does not completely cutoff. It goes down and down, closer to 0, *but*, then the RPM gets to the "idle RPM", the fuel is restored, because the ECU/PCM never wants to go below the idle RPM. Here is where I read it :

Negative Fuel Trim...Constant
http://www.saturnfans.com/forums/showthread.php?t=89959
Quote:
Bigdaddy94sc2 says :
For STFT to drop when slowing down is normal, due to decell fuel cutoff. 25% is the max it can go to, then it goes into decel fuel cutoff when it sees brakes applied, speed decreasing, and no ABS activation. It doesnt totally cut fuel, but darn close.
Other drivetrains are much better at this and can be very thrifty about cutting fuel when the right combination of driving events appear to the ECU/PCM, i.e. a certain combination of "0 throttle + in gear + brakes applied".

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