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Discussion Starter #1
Hey friends,

I'm looking to find out every thing I can about the DFSO system in my new Focus. I have done some searching on this forum and have found bits and pieces, but perhaps a single thread on the topic can combine all the knowledge this forum holds into one place.

I am trying to create some hypermiling strategies and would like to know the speeds at which DFSO turns on and when it turns off. Also, I like manually turning on the AC button when I am coasting to the light and turning it off when the light turns green. I want to know if having the AC on as I am coasting down to the light will keep the DFSO from activating.

Thanks in advance!
 

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Best way to do this is with a scangauge and look at the closed/open loop status. On mine the DFSO would work with or without A/C and it would stay activated upon coast down until ~25MPH.
 

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Discussion Starter #3
Best way to do this is with a scangauge and look at the closed/open loop status. On mine the DFSO would work with or without A/C and it would stay activated upon coast down until ~25MPH.
Thanks for the data point. And I realize that owning a scangauge is the way to go but not all of us have one of those. I was about to purchase an ultra-gauge until I learned of the battery drain problem.

Perhaps those with scanguages and technical knowledge on the DFSO system could share their knowledge in here so we all benefit. Can a person "feel" when the DFSO is activated?
 

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..........Also, I like manually turning on the AC button when I am coasting to the light and turning it off when the light turns green. I want to know if having the AC on as I am coasting down to the light will keep the DFSO from activating.

Thanks in advance!
Just a heads up, doing this very frequently is actually quite hard on the actuator for the A/C. Whatever money you're trying to save on brakes by doing this, you may end up paying 10x for new A/C parts. [poke] Rather than using cheap brake pads, you're using an expensive climate control component to slow the car down.
 

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Just a heads up, doing this very frequently is actually quite hard on the actuator for the A/C. Whatever money you're trying to save on brakes by doing this, you may end up paying 10x for new A/C parts. [poke] Rather than using cheap brake pads, you're using an expensive climate control component to slow the car down.
I think he's trying to save on fuel, not brakes. But this is a very good point. You could never in a million years save enough fuel by activating/deactivating the A/C on decel to make up for the $1500 it will cost to replace the compressor and/or EM clutch.
 

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The way I understand this system is the fuel system shuts down any time you're coasting in gear with no accelerator use. Since it's in gear the rolling of the tires keeps the engine cranking.
 

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The way I understand this system is the fuel system shuts down any time you're coasting in gear with no accelerator use. Since it's in gear the rolling of the tires keeps the engine cranking.
The thing I can't wrap my head around with this is that if there's no fuel being injected then there's no combustion in the cylinders while coasting - wouldn't the engine sound be much different while coasting? Even no engine noise at all or just some kind of mechanical whirring sound?
 

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Discussion Starter #9
Just a heads up, doing this very frequently is actually quite hard on the actuator for the A/C. Whatever money you're trying to save on brakes by doing this, you may end up paying 10x for new A/C parts. [poke] Rather than using cheap brake pads, you're using an expensive climate control component to slow the car down.
I always heard that turning the AC button off and on was the same as the compressor cycling which it does continually when in use. Did I receive incorrect information? I did it a million times in my 2001 Focus and the engine died (thrown rod at 150k) before the air conditioner ever did.
 

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Discussion Starter #10
I have a scangauge, if you tell me what to look for I can get the data for you.
Thanks!

Do you notice the DFSO activating only at particular speeds; for example, only after decelerating from 50mph but not 40mph? Or does it turn on as long as your foot is off the gas and you are over 25mph? Also, have you ever seen it on under 25mph? And have you found the 3 second delay after lifting off the gas to be true in your experience?
 

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Discussion Starter #11
The thing I can't wrap my head around with this is that if there's no fuel being injected then there's no combustion in the cylinders while coasting - wouldn't the engine sound be much different while coasting? Even no engine noise at all or just some kind of mechanical whirring sound?
Yeah, I keep trying to listen for something different. The engine is pretty quiet so it is hard. I think I notice a feeling like engine breaking when the DFSO is activated -- kind of like the car is not coasting as effortlessly.
 

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I always heard that turning the AC button off and on was the same as the compressor cycling which it does continually when in use. Did I receive incorrect information? I did it a million times in my 2001 Focus and the engine died (thrown rod at 150k) before the air conditioner ever did.
The compressor itself doesn't cycle on and off when the AC is set to ON (or front defrost is enabled)... the on/off that you can hear, usually while the car is at idle, is the fans for the AC radiator itself... those go on and off, but the AC compressor is on the entire time. It's basically a clutch that connects the compressor to the belt of the motor.

Having that clutch enable and disable every time you come to slow down is not good for the compressor. Like anything that isn't "good for the car", per say, it won't necessarily fail, but it does take away from its life.

And turning the AC on while slowing down doesn't save gas in any way.. it just adds to the compression braking, assuming you have manual and keep the gears engaged as you slow down.

Unless of course, you want AC on because it's hot out, and are just turning it off while accelerating... that will save a bit of fuel each time, but I don't think that's what is being referred to.
 

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Discussion Starter #13
The compressor itself doesn't cycle on and off when the AC is set to ON (or front defrost is enabled)... the on/off that you can hear, usually while the car is at idle, is the fans for the AC radiator itself... those go on and off, but the AC compressor is on the entire time. It's basically a clutch that connects the compressor to the belt of the motor.

Having that clutch enable and disable every time you come to slow down is not good for the compressor. Like anything that isn't "good for the car", per say, it won't necessarily fail, but it does take away from its life.

And turning the AC on while slowing down doesn't save gas in any way.. it just adds to the compression braking, assuming you have manual and keep the gears engaged as you slow down.

Unless of course, you want AC on because it's hot out, and are just turning it off while accelerating... that will save a bit of fuel each time, but I don't think that's what is being referred to.
Yes, I have the AC on because it is hot out. I thought up the "genius" idea of turning the AC on as I coast to a light essentially getting "free air conditioning" as I posited the engine was off due to DFSO. Then when the light turns green I turn the AC button off and allow the cool air to recycle in the car with the fan only. Then I repeat this process at the next opportunity to coast. But if what you said is true, and I am causing extra wear and tear on my AC system, I will stop. Are you a mechanic?
 

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Yes, I have the AC on because it is hot out. I thought up the "genius" idea of turning the AC on as I coast to a light essentially getting "free air conditioning" as I posited the engine was off due to DFSO. Then when the light turns green I turn the AC button off and allow the cool air to recycle in the car with the fan only. Then I repeat this process at the next opportunity to coast. But if what you said is true, and I am causing extra wear and tear on my AC system, I will stop. Are you a mechanic?
A mechanic? No. Electromechanical Engineer. You're right that it will use less gas that way, because it's off whenever you have it set to off. But that isn't the intended use of the AC unit, and if you do that 20 times each time you drive, then yes, it will add significant wear. Will it fail while you own the vehicle as a result? That's anyones guess. If you don't plan to own the car much past the end of warranty, then do as you wish. It shouldn't kill it that quickly anyways.

Even the auto climate won't really bother to turn off the AC much (which is why I always have it on manual override). It pretty much just leaves it on all the time.
 

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The thing I can't wrap my head around with this is that if there's no fuel being injected then there's no combustion in the cylinders while coasting - wouldn't the engine sound be much different while coasting? Even no engine noise at all or just some kind of mechanical whirring sound?
even with no fuel, the engine is still pumping air from 20" of vacuum upto atmospheric. it still goes through the compression cycle there will still be the slight "pop" of noise from each cylinder as the "high" pressure cylinder gets released into the atmospheric exhaust system when the valve opens
Also i think it is seamless when it goes into DFSO, as the fuel injectors ramp down to zero flow.
 

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Thanks!

Do you notice the DFSO activating only at particular speeds; for example, only after decelerating from 50mph but not 40mph? Or does it turn on as long as your foot is off the gas and you are over 25mph? Also, have you ever seen it on under 25mph? And have you found the 3 second delay after lifting off the gas to be true in your experience?
You have to explain what I'm looking for. Are you talking about when the MPG gauge sometimes reads 9999 instead of 100-200 when no gas is applied?
 

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Discussion Starter #17
You have to explain what I'm looking for. Are you talking about when the MPG gauge sometimes reads 9999 instead of 100-200 when no gas is applied?
Yes, when it reads 9999 it is in open loop or DFSO mode. If you could make a few notes when your cars goes into DFSO and what speeds you were at and how long it lasted.. that would be fantastic!
 

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Discussion Starter #18
A mechanic? No. Electromechanical Engineer. You're right that it will use less gas that way, because it's off whenever you have it set to off. But that isn't the intended use of the AC unit, and if you do that 20 times each time you drive, then yes, it will add significant wear. Will it fail while you own the vehicle as a result? That's anyones guess. If you don't plan to own the car much past the end of warranty, then do as you wish. It shouldn't kill it that quickly anyways.

Even the auto climate won't really bother to turn off the AC much (which is why I always have it on manual override). It pretty much just leaves it on all the time.
So if I have it on regular AC (not max ac), and occasionally press the button when the desired temp is reached, that would be the same as turning the entire unit off?
 

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The thing I can't wrap my head around with this is that if there's no fuel being injected then there's no combustion in the cylinders while coasting - wouldn't the engine sound be much different while coasting?
The short answer to your question is no.

If you had an exhaust system with a deep sound, you would hear a deep exhaust note even when coasting with no fuel (I've tried it many times).

Think of the engine as an air pump.
 

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The compressor itself doesn't cycle on and off when the AC is set to ON (or front defrost is enabled)...
What???

A/C compressor cycling is perfectly normal and will happen quite frequently depending upon a number of factors.

Imagine what would happen if the compressor didn't cycle on and off.
 
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