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Old 07-12-2007, 08:58 AM   #1
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Driving Technique: Slightly Tap the Brakes Once

On my girlfriend's Volvo there is an instant MPG display. When coasting, I noticed that slightly tapping the brakes once causes the MPG to go way up. As opposed to just lifting the foot from the accelerator in order to coast. (Different fuel map?)

Has anyone else tried this? If someone has a scangauge or other instant MPG display, please try it and let us know.

Tom.
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Old 07-12-2007, 09:34 AM   #2
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Does this happen every time in a chain of P&Gs, or only the first time after you've used the brakes? I'm wondering if maybe your brakes are dragging slightly and the quick tap allows them to release fully.

On the other hand, if they were dragging enough to noticeably affect FE I expect you'd perceive the change when you tap, so maybe it is a mapping thing.

I've been staying away from neutral coasting to see how close I could get to the same mileage in gear, but if I can remember I'll try it on the way in to work tomorrow morning to see if I get anything similar. There's a lovely long descent at the entrance to my neighborhood, just steep enough to keep the VW rolling at a roughly constant speed. It'd be perfect for this test.

Rick
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Old 07-12-2007, 10:02 AM   #3
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When I press the brakes, my mixture can go up to 19:1, due to the extra air entering the manifold (this is when at idle, and not moving). It might be something to do with this?
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Old 07-12-2007, 10:53 AM   #4
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It may be a throttle hold to aid in smooth driving and when the brake is press the fuel is cut off for engine braking to start. As soon as the fuel is cut off the MPG would go up.
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Old 07-12-2007, 11:42 AM   #5
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I'll have to try this with my wife's taurus. I think there is some kind of throttle hold for smoothness on it.
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Old 07-12-2007, 11:58 AM   #6
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I think it might have something to do with vacuum assist brakes, and the air from the brake booster getting sucked into the manifold... if it stays up while you're on the brakes it would indicate a vacuum leak in the booster diaphragm... if it's just a spike it's normal for vacuum assist brakes...
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Old 07-12-2007, 12:37 PM   #7
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Not sure if this might apply, but on my wife's Odyssey, if you are going downhill and tap the brake, the car will downshift. They call it something like "grade logic" and it is supposed to help you to not pick up too much speed on downhills and use the brakes less.

Maybe the Volvo has something similar.

And, here's where I start making stuff up. Maybe when the car downshifts, it also more aggressively cuts fuel, and maybe that is why you get the increase in MPG.
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Old 07-12-2007, 01:09 PM   #8
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Quote:
Originally Posted by Bill in Houston View Post
And, here's where I start making stuff up. Maybe when the car downshifts, it also more aggressively cuts fuel, and maybe that is why you get the increase in MPG.
It doesn't downshift as far as I can tell. When I slightly tap the brake once the car does not slow down noticeably at all, maybe .5 or 1 mph. If it downshifted, then the RPM's would have to go up to maintain speed which would increase fuel consumption.

> It may be a throttle hold to aid in smooth driving and when the
> brake is press the fuel is cut off for engine braking to start. As
> soon as the fuel is cut off the MPG would go up.
I think that's what's going on...

I'll try to get actual numbers next time I drive her car.

Looking forward to Rick's post after his commute tomorrow.

Tom.
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Old 07-12-2007, 01:50 PM   #9
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Quote:
Quote:
It may be a throttle hold to aid in smooth driving and when the
brake is press the fuel is cut off for engine braking to start.
I think that's what's going on...
But if that were the case, surely you'd feel it? When I coast in neutral it's like gliding on air; if I instead just take my foot off the gas and engine braking kicks in, it's (as someone else described it) "like driving in wet sand." I got the impression from your first post that whatever's happening is pretty transparent... you're not feeling/seeing much of a change other than a sudden jump in reported instantaneous FE. Is that correct?

I'd think that would also rule out a downshift; again, that would be something you'd sense, either by feel or by the sound of the engine. (No tach, I take it?)

Rick
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Old 07-12-2007, 02:29 PM   #10
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could just be a glitch in the electronics system...
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