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Old 09-30-2009, 03:38 PM   #51 (permalink)
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I should phrase my thoughts better. I had thought I had said pretty much the same thing as the last two of you did.

Thanks for replying though.

Wait: Old Mechanic- Are you saying that you go down hill in low gears with the ENGINE TURNED OFF SO NO GAS IS USED? I don't think I have tried that yet because I keep thinking the engine would restart, but if my key is turned to OFF, I guess the engine wont start, eh? Oh well I was doing almost the same thing but I had the engine ON, and this lead to fairly high RPMs as the low gears slowed me down.

Sorry if I misunderstood.

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Old 09-30-2009, 04:14 PM   #52 (permalink)
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DFCO - deceleration fuel cut off

The car will automatically shut off the fuel flow in an engine-braking situation. No need to turn the ignition off yourself.
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Old 09-30-2009, 04:59 PM   #53 (permalink)
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In that situation, I would still bump in 5th, but then immediately clutch in and shift to 4th. I do it all the time. 5th gives a smoother bump, and then a second later I'm in the gear I need to tackle the hill.
That's actually what I do, so maybe that's why I haven't stalled out going up a hill. In practice, I almost never bump-start on the uphill, or my "uphills" are very gentle, aka 1% or 2% inclines as on freeways.

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Old 09-30-2009, 05:27 PM   #54 (permalink)
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Quote:
Originally Posted by Jammer View Post
I should phrase my thoughts better. I had thought I had said pretty much the same thing as the last two of you did.

Thanks for replying though.

Wait: Old Mechanic- Are you saying that you go down hill in low gears with the ENGINE TURNED OFF SO NO GAS IS USED? I don't think I have tried that yet because I keep thinking the engine would restart, but if my key is turned to OFF, I guess the engine wont start, eh? Oh well I was doing almost the same thing but I had the engine ON, and this lead to fairly high RPMs as the low gears slowed me down.

Sorry if I misunderstood.
If you are using the engine to slow the car down going down a steep slope. you are using no fuel with the ignition on due to DFCO as Pale stated.

Example:

You crest the hill (top) and start down. The hill is very steep so you downshift with your foot off the gas. No need to turn the engine off. It's not using any fuel anyway. Steering and brakes work normally.

If you can not slow down enough using the gears and your are worried about the engine revs getting too high, apply the brakes momentarily to kill some speed, but keep the car in gear with your foot off the gas pedal. You are getting infinite mileage when your foot is off the gas and the slope is keeping your revs up in any gear.

As you approach the bottom of the hill you have two choices. If you have enough time use EOC (engine off coasting). If you don't have enough time, put it in neutral and let your speed increase some (Maybe 10 MPH). You want to use the speed to get you as far up the next up slope as possible without danger or a ticket.

Its a lot like a roller coaster, but you have to be safe with your maximum speed.

If the speed limit is 50 MPH, your highest speed would be at the bottom of the hill, and your lowest speed would be right before you reached the crest of the hill. This is just like a roller coaster. I use the roller coaster as an example because it can go very far without any power at all, after its first climb under power.

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Old 09-30-2009, 06:35 PM   #55 (permalink)
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Quote:
Originally Posted by Old Mechanic View Post
If you are using the engine to slow the car down going down a steep slope. you are using no fuel with the ignition on due to DFCO as Pale stated.

Example:

You crest the hill (top) and start down. The hill is very steep so you downshift with your foot off the gas. No need to turn the engine off. It's not using any fuel anyway. Steering and brakes work normally.

If you can not slow down enough using the gears and your are worried about the engine revs getting too high, apply the brakes momentarily to kill some speed, but keep the car in gear with your foot off the gas pedal. You are getting infinite mileage when your foot is off the gas and the slope is keeping your revs up in any gear.

As you approach the bottom of the hill you have two choices. If you have enough time use EOC (engine off coasting). If you don't have enough time, put it in neutral and let your speed increase some (Maybe 10 MPH). You want to use the speed to get you as far up the next up slope as possible without danger or a ticket.

Its a lot like a roller coaster, but you have to be safe with your maximum speed.

If the speed limit is 50 MPH, your highest speed would be at the bottom of the hill, and your lowest speed would be right before you reached the crest of the hill. This is just like a roller coaster. I use the roller coaster as an example because it can go very far without any power at all, after its first climb under power.

regards
Mech
Thanks again. I mean no offense, after all your the mechanic, but I can not see where downshifting down a mountain with the engine running results in NO fuel being burned. After all I think the DFCO cut off is around 2,500 RPMs, so I have to assume gasoline is burning to that point. I guess I am curious if it is possible to go EOC down hill by gearing down to slow down with the key in the off position, or will this idea end up burning gas some how? My mind is real fuzzy on this one.

Also there seems to be a limit to braking during EOC in my Cobalt XFE, or is that my imagination? As far as I can tell if I am going a long distance in EOC my brakes work fine for a few pumps then do not work until I start the motor back- which can be extremely dangerous around these curves here. I am still learning how to do all of this safely, but I no longer have the fear I did back when I first tried it and had trouble.

I do plan on getting some home insulation and taping it around the battery in the trunk and see if that helps my battery to charge/keep a charge or not.

My best.
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Old 09-30-2009, 08:28 PM   #56 (permalink)
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Jammer -

Quote:
Originally Posted by Jammer View Post
Thanks again. I mean no offense, after all your the mechanic, but I can not see where downshifting down a mountain with the engine running results in NO fuel being burned. After all I think the DFCO cut off is around 2,500 RPMs, so I have to assume gasoline is burning to that point. I guess I am curious if it is possible to go EOC down hill by gearing down to slow down with the key in the off position, or will this idea end up burning gas some how? My mind is real fuzzy on this one.

Also there seems to be a limit to braking during EOC in my Cobalt XFE, or is that my imagination? As far as I can tell if I am going a long distance in EOC my brakes work fine for a few pumps then do not work until I start the motor back- which can be extremely dangerous around these curves here. I am still learning how to do all of this safely, but I no longer have the fear I did back when I first tried it and had trouble.

I do plan on getting some home insulation and taping it around the battery in the trunk and see if that helps my battery to charge/keep a charge or not.

My best.
I just started doing this on a steep hill I have when I leave for work. If I keep it in 3rd gear and actually accelerate such that I am at about 2800 RPM, I get the DFCO without using EOC. I know this because I have a digital A/F ratio guage, so I see it go lean. The benefit is that as long as the RPMs are above 2500 RPM, I have full brakes and full steering all the way down the hill and I have less need to use my brakes because I have engine braking in my favor, just like a semi. I do use a little fuel to get above 2800 RPM, but I am 100% safe doing it. I learned this recently, so now I can't wait for a great big hill on the grapevine to test it on.

My rule of thumb is that I have one lonngggg power assisted brake. I say this because I gleaned (incorrectly?) on this forum that the vacuum reservoir doesn't dissipate as long as I keep my foot in the brake. I also don't try EOC in curves because I need my (mechanical belt-driven) power steering.

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Old 09-30-2009, 08:39 PM   #57 (permalink)
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When you are using the gears to slow the car down, the vacuum in the intake manifold approaches the level of outer space.

There is no air to combine with the fuel to create combustion.

If fuel was being injected it would go out the exhaust as unburned fuel.

You can check this fairly easily if you doubt what I am saying.

Take a phillips head screwdriver. Place the point on the fuel injector and the other end on your ear. You should be able to hear the injector ticking. Use it like a stethoscope.

Rev the engine and the ticking will become more rapid. Then left off the throttle and the ticking should stop for a short time as the engine slows down to idle speed.

If you hear it stop ticking when you let off the throttle you have just heard DFCO working on your car. No ticking=no fuel delivered. The same thing should happen when you are using the engine to slow down the car, as long as your foot is off the gas completely. If that was not the case when you pressed on the gas the unburned fuel in your exhaust would ignite with a big bang.

I can't provide better proof of that.

The brakes work with vacuum assist, the booster holds enough vacuum for a few applications then the assist doesn't work. You still have brakes but the pedal effort is much higher.

Using DFCO downhill means your brakes and steering work normally.

I was almost killed driving in mountains very close to where you live. I was climbing a steep grade and there was an 800 foot drop just past the guardrail on my side of the road. A big rig came around a blind turn (left for me right for him) coming in the opposite direction. Just behind him a Subaru lost it and came across into my lane, missing the guardrail by 18 inches about 200 feet in front of me. If I had been 200 feet further down the road he would have knocked me over the guardrail down the 800 foot drop.

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Old 09-30-2009, 09:12 PM   #58 (permalink)
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If you have a ScanGauge, your instantaneous MPG display will read 9999 MPG when you're in DFCO. The 4 liter V-6 in my Mustang does it above 1500 RPM. It will do it at even lower RPM with cruise control engaged. I use DFCO as a substitute for my service brakes whenever possible. It saves brake wear and fuel.
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Old 09-30-2009, 09:22 PM   #59 (permalink)
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If you have a ScanGauge, your instantaneous MPG display will read 9999 MPG when you're in DFCO. The 4 liter V-6 in my Mustang does it above 1500 RPM. It will do it at even lower RPM with cruise control engaged. I use DFCO as a substitute for my service brakes whenever possible. It saves brake wear and fuel.
Some do, some don't. Mine doesn't know it's in DFCO, so I read normal MPG. Hrmmmmm, I have an SGI, not an SGII. Maybe my SG1 is just dumber.

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Old 09-30-2009, 09:24 PM   #60 (permalink)
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Cfg, does sg1 have an open loop indicator?

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