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Old 03-01-2008, 11:11 PM   #11
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In the 1990-1999 Eclipse/Talon/Laser the fuel cutoff threshold is at 1188rpm, with 3/4 seconds delay from when the throttle is closed to when the ecu cuts off fuel. This is done to smooth the transition and to 'enhance a pleasureable driving experience.' The fuel cutoff rpm is raised when engine coolant temps are low, which is also when idle speed is higher.
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Old 07-02-2008, 04:50 PM   #12
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anyone know when does a 97 civic coupe dx cuts the fuel?
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Old 07-02-2008, 05:08 PM   #13
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Best way to tell is to shift into 3rd gear at 30 MPH and let your foot off the gas. Wait until you feel the engine start to produce power. The transition is very noticeable for me.

860 RPM with no accessories on, I mean nothing that would kick up idle speed. If I turn the AC on it goes uo to close to 1300 RPM.

Manual transmissions are easy.

Automatics are a lot harder to determine unless you have a scan guage.

When you close the throttle and the manifold vacuum goes very high, there is always some significant reduction in fuel delivery, or you would have high unburned hydrocarbons in the exhaust.

Exceptions would be automatics that dont have a lot of engine braking force when you let completely off the gas. If your foot is touching the gas pedal there is no DFCO.



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Old 07-02-2008, 05:38 PM   #14
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Originally Posted by 8307c4 View Post
No, most engines do not cut off fuel to the engine, ever.
That's 99.999% of them, do NOT.
Thank you.
WRONG!

These days MANY (the majority?) of normal gas (I am NOT talking about "hybrid", but rather regular fuel injected gas engines) will in fact cut fuel under some very specific circumstances.

Specifically, if you are in gear, and your engine is warmed up, and your foot is totally off the gas pedal, and your RPMs (from the wheels turning against the pavement) are "fast enough" (with many cars "fast enough" seems to be somewhere in the 900 to 1300 RPM range, depending upon the vehicle), THEN many (most?) engines will in fact cut the fuel completely (i.e. they use zero fuel in that case, unlike "idle" which will still use a minimal amount of fuel). They do this because the engine computer realizes that you don't currently need any power (caused by burning fuel) in such cases, and therefore cuts the (currently not needed for the car) fuel to the fuel injector(s). And as soon as you go out of this very specific situation, the engine computer automatically restores fuel to the car, without you having to take any actions to make this occur!

NOTE: You will get at least some slowdown ("engine braking") when this occurs (although factors such as well lubricating your engine, transmission, and drive train, can help to lower the slowdown when doing so), unless you are sufficiently going down hill to let gravity be your friend. However, there are still times when this is a good idea FE wise, such as "coasting" up to a red light you are approaching (or "coasting" up to the stop sign at the end of an off ramp). In such cases, you might not care that you are gradually slowing down (due to the natural "engine braking"), especially if/when it means you are getting the distance "free" (i.e. without fuel use)!

NOTE: DFCO (Deceleration Fuel Cut-Off, which is what they call this), was apparently originally designed to help the auto makers meet emission standards, not for fuel economy (the fuel economy savings, is just a nice "bonus", that those on this site like to take advantage of). It turns out that when you really don't need extra power in a car, dumping fuel in the engine (i.e. "wasting fuel", as old style carburetors used to do) will greatly increase the emissions (requiring extra emission control devices to compensate). But when you are going with computer controlled fuel injection (what most cars these days use), it's easy/trivial for the engine computer to detect such a situation and simply cut the fuel when it occurs (thereby not only saving fuel, which people on this site like, but also lowering the emissions that would otherwise occur in this situation).
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Old 07-02-2008, 05:41 PM   #15
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No, most engines do not cut off fuel to the engine, ever.
That's 99.999% of them, do NOT.
Thank you.
1994 Honda Accord EX does, and so does my 2002 Honda Civic Si. That is a pretty good bet most, if not all Honda's since at least 1994 do. I also believe that Honda sells more than .001% of the autos in the US, so your math may be a bit off...
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Old 07-02-2008, 06:30 PM   #16
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...so look at the LP gauge to see if it goes into open-loop while decelerating.
Is this true? If a car is in DFCO mode, it is in open-loop? I have software that can monitor this status, but haven't had much time to play with it. I'm still not sure if my 98 Corolla has DFCO.
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Old 07-02-2008, 06:32 PM   #17
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Originally Posted by zimagold View Post
Is this true? If a car is in DFCO mode, it is in open-loop? I have software that can monitor this status, but haven't had much time to play with it. I'm still not sure if my 98 Corolla has DFCO.
My VW goes open loop during DFCO.
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Old 07-02-2008, 06:39 PM   #18
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How could it not go into open loop. There is no exhaust to measure the oxygen content relative to the atmosphere, since the atmosphere is both inside and outside, it can only be in open loop.

Sensor reading is 0.

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Old 07-03-2008, 06:49 AM   #19
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My 96 Civic DX does, so the 97 probably does, too. In gear, above 1200 rpm.

Look for Open Loop on the scangauge. That's a dead giveaway.
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Old 07-06-2008, 11:26 AM   #20
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Confirmed my 98 Corolla does go into open loop if I shift down into 2 and let off the gas. It won't seem to do it if I'm coasting down in D though. This begs the question whether fuel cutoff is worth doing for engine braking to a stop at a light, where you can achieve DFCO for maybe 5 seconds. That must be a minuscule amount of fuel saved compared to just coasting in D or neutral. Must be weighed against transmission wear and a slight increased risk of hitting reverse!

On a long downhill where you have to stop at the bottom, I'll definitely try to get into DFCO now. Thanks.
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