this might not be grassroots, but I guess it fits here as well as anywhere
how much harm, if any, do you do to a modern FWD automatic if you coast in neutral ? (motor on)
this might not be grassroots, but I guess it fits here as well as anywhere
how much harm, if any, do you do to a modern FWD automatic if you coast in neutral ? (motor on)
I really don't know, but the owners manual for my 2010 Hyundai very clearly says do not do it. Very dangerous. Huh?
If the engine's running, it should be oiling, I would think. I can't imagine it would do much harm. I assume the "DO NOT DO" is basically an accident warning, not a mechanical one.
That being said, why?
The whole you will not have control of your car thing is my bet not a mechanical damage issue. Guess they never heard of depressing the clutch in a manual and coasting . .
The harm is putting it back in drive. Getting it wrong can bust up all kinds of stuff in there, if you get it wrong enough. Rev matching accurately puts less stress on things but meh. When driving automagics I'll keep it in gear.
^^^ what he said. sometimes it may engage smoothly enough. sometimes.....not.
slightly off topic....while visiting my rather mechanically adept gearhead uncle I noticed he put his auto transmission cars in neutral at every stoplight, and would put it back into gear when it was time to move. No idea if that is supposed to ease strain on the transmission or what.
How do you rev-match an auto? How do you know which gear it will choose?
In reply to failboat: I do it in my Honda fit sport. It's easier on the torque converter. Otherwise the converter says "let's go!" And the brakes say "no way!" And heat builds and kills thing. Also, if you have a new enough car, it should have DFCO. Decelerative fuel cut off. It means when the throttle is completely lifted, the injectors stop pouring fuel. In neutral it won't do this. Save money and trans by leaving it in gear while moving!
I have nothing of substance to add. But I thought with a thread titled as such, this was needed:
Trackmouse wrote: In reply to failboat: I do it in my Honda fit sport. It's easier on the torque converter. Otherwise the converter says "let's go!" And the brakes say "no way!" And heat builds and kills thing. Also, if you have a new enough car, it should have DFCO. Decelerative fuel cut off. It means when the throttle is completely lifted, the injectors stop pouring fuel. In neutral it won't do this. Save money and trans by leaving it in gear while moving!
I've heard others say this … is there actually ZERO fuel flow ? as in the engine is cut off ?
hard to wrap my feeble mind around that …
and in my case it's just to try to undo the gas milage damage my Mom does on the rare times she drives … LOL ( to her the throttle is an on/off switch) … this is a '13 Fit base model .. I can (if I do all the driving) get it to, and pretty much keep it at 33 - 35 mpg … if she drives it won't get above 26 - 28
and I never put it back in D until I've stopped at the bottom of the hills … in other words, this is just around the neighborhood, with stop signs at the end of each street
In reply to wbjones: Yup, actually zero. If you have an 'instant mileage' gauge, you can see it go to zero...or max. Depending on how or what it reads. Sometimes you can feel it kick back on.
Lots of cars do this, cut the injectors to decelerate...he'll my 1988 automatic Volvo does it as does my manual 93 miata. Just watch a wideband and you'll see. It uses more fuel to coast to a stop in neutral in an automatic than it does leaving it in gear and letting it coast. Not the case with a big hill, but more so coasting to a stop
In reply to wbjones:
In reply to wbjones:
Most engines, the engine will turn the fuel off until just around 1500rpm during decelerations. Some even downshift to keep it with the fuel off.
Co-incidentally, I'm doing some fuel economy projects- if your mom drives with a lead foot- that's a massive killer anymore on fuel economy- especially from a stop. That's the worst acceleration- as the converter is unlocked for the first 3 gears of acceleration. Which just throws energy straight to the fluid heat. Be gentle to third, and then most can be more agressive.
BUT, even then, one has to be gentle on the gas- many cars will see a fast gas pedal as a major demand, and the trans will quickly unlock and downshift. Again, just throwing energy out in heat.
Still, for your manuver, if you want to keep the fuel off, instead of shifting to neutral- go the other way, and manually downshift to keep the engine speed above 1500 rpm as you come to a stop.
(Another acronym- DFSO- decel fuel shut off)
bigev007 wrote: In reply to wbjones: Yup, actually zero. If you have an 'instant mileage' gauge, you can see it go to zero...or max. Depending on how or what it reads. Sometimes you can feel it kick back on.
That makes no sense.
If here is zero fuel consumption, the engine is off.
Your instant fuel gauge is simply having trouble identifying a math problem (like dividing by zero), and is programmed to give you a zero readout when the DFCO kicks in.
It's not a true zero. Can't be.
In reply to failboat: AAmco says to do it to ease strain from the transmission, but personally, if I'm supposed to do that at all stoplights, I should have a manual trans, anyway.
SVreX wrote:bigev007 wrote: In reply to wbjones: Yup, actually zero. If you have an 'instant mileage' gauge, you can see it go to zero...or max. Depending on how or what it reads. Sometimes you can feel it kick back on.That makes no sense. If here is zero fuel consumption, the engine is off. Your instant fuel gauge is simply having trouble identifying a math problem (like dividing by zero), and is programmed to give you a zero readout when the DFCO kicks in. It's not a true zero. Can't be.
No, it's zero. Fuel is totally and utterly off. Lots and lots of fresh air passes through the engine.
zero, nada, nothing, zip, zilch.
The engine is spinning because the trans is attached to it, which is being spun by the momentum of the car.
In reply to SVreX: My understanding is that the injectors are fully shut off. I'm trying to find a source. It's definitely noticeable on my e36 (It runs rough, so when they kick back on you can notice it more than most cars) It's also obvious on my Civic, but that's a hybrid, so it operates a little different from most (it can actually cut 3 cyls or all 4).
failboat wrote: ^^^ what he said. sometimes it may engage smoothly enough. sometimes.....not. slightly off topic....while visiting my rather mechanically adept gearhead uncle I noticed he put his auto transmission cars in neutral at every stoplight, and would put it back into gear when it was time to move. No idea if that is supposed to ease strain on the transmission or what.
Neutral or Park idle is less load than drive idle, so it would use less fuel.
If it were about trans strain for real- I'd be more worried about using the trans what so ever- the idle load is tiny compared to dirving. Let alone agressive or wide open driving.
bigev007 wrote: In reply to SVreX: My understanding is that the injectors are fully shut off. I'm trying to find a source. It's definitely noticeable on my e36 (It runs rough, so when they kick back on you can notice it more than most cars) It's also obvious on my Civic, but that's a hybrid, so it operates a little different from most (it can actually cut 3 cyls or all 4).
Theoretically, I can be a source. Staring at a test I ran this morning, injector pulse width is zero. And O2 in the exhaust goes from .8% to 20%. Very off.
(but I can't post the data, so don't ask)
alfadriver wrote:SVreX wrote:No, it's zero. Fuel is totally and utterly off. Lots and lots of fresh air passes through the engine. zero, nada, nothing, zip, zilch. The engine is spinning because the trans is attached to it, which is being spun by the momentum of the car.bigev007 wrote: In reply to wbjones: Yup, actually zero. If you have an 'instant mileage' gauge, you can see it go to zero...or max. Depending on how or what it reads. Sometimes you can feel it kick back on.That makes no sense. If here is zero fuel consumption, the engine is off. Your instant fuel gauge is simply having trouble identifying a math problem (like dividing by zero), and is programmed to give you a zero readout when the DFCO kicks in. It's not a true zero. Can't be.
Yep. If the throttle position sensor reads closed throttle, in gear, there will be no fuel flow until it drops down to somewhere between 1000 and 1500 rpm.
The first time I encountered that was on D-jetronic pre-1973 Volvos. If the distributor advance springs wore out, the idle would increase. When it got to 2000 rpm, the fuel would shut off, and then kick in again when it dropped to 900 or so. Back to 2000, etc.
Ever hear an early injected Honda idle up/down/up/down? Usually either due to low coolant or hamfisted race car modification, so the cold idle pellet doesn't warm up and close the passage...same story.
failboat wrote: slightly off topic....while visiting my rather mechanically adept gearhead uncle I noticed he put his auto transmission cars in neutral at every stoplight, and would put it back into gear when it was time to move. No idea if that is supposed to ease strain on the transmission or what.
Trackmouse wrote: In reply to failboat: I do it in my Honda fit sport. It's easier on the torque converter. Otherwise the converter says "let's go!" And the brakes say "no way!" And heat builds and kills thing.
i have to say that shifting into neutral at stops seems like a waste of time. The amount of heat generated from sitting at idle in gear will probably not be enough to "kill" anything in the transmission, once a trans is up to operation temp the only thing that will make it get hotter is actually operation through the gears. sitting at a stop, either in neutral or park, it will still stay at operating temp. This is what the torque converter is made to do, to not truely "grab" until a certain engine rpm which is when it sees heat build up.
the proof is in the pudding, millions of cars have gone hundreds of thousands of miles while keeping the car in drive at every stop light. Would putting it in neutral at every stop make it last longer? maybe but i wouldnt sweat it
bigev007 wrote: In reply to wbjones: Yup, actually zero. If you have an 'instant mileage' gauge, you can see it go to zero...or max. Depending on how or what it reads. Sometimes you can feel it kick back on.
if no fuel flow, wouldn't the engine stop turning over ? I'm in neutral, so no pull from the drive train to make the engine turn over … wouldn't being stopped, with no movement at all bring on the same effect ?
chiodos wrote: Lots of cars do this, cut the injectors to decelerate...he'll my 1988 automatic Volvo does it as does my manual 93 miata. Just watch a wideband and you'll see. It uses more fuel to coast to a stop in neutral in an automatic than it does leaving it in gear and letting it coast. Not the case with a big hill, but more so coasting to a stop In reply to wbjones:
not in the Fit … if in neutral the tach shows ~ 600 rpm, and the instant read out shows 80+ mpg … if left in gear, the tack shows whatever rpm you were at when you lifted and started to decelerate … plus it downshifts (you can feel it and see the tach move) as your speed comes down ….
if the injectors totally shut down, wouldn't the engine stall, or act like it was off (i.e. the dash lights would all be on)
this isn't one of those fancy cars that actually do shut down when you come to a stop, and start back up when you mash on the go pedal
now for the rest of the question …
if I've just filled up (this assumption so that the instant milage read out will change more quickly)
in my new Sonic the milage will go up more quickly if I shift to neutral (6sp manual) than if I lift throttle and just roll up to the stop sign … in the latter, the tach stays up (which is where y'all are saying there's no fuel flow) .. if I shift to neutral the tach shows ~ 600 rpm all the way to stop …engine obviously running …
alfadriver wrote:SVreX wrote: If here is zero fuel consumption, the engine is off. Your instant fuel gauge is simply having trouble identifying a math problem (like dividing by zero), and is programmed to give you a zero readout when the DFCO kicks in. It's not a true zero. Can't be.No, it's zero. Fuel is totally and utterly off. Lots and lots of fresh air passes through the engine. zero, nada, nothing, zip, zilch. The engine is spinning because the trans is attached to it, which is being spun by the momentum of the car.
The instant fuel economy is infinite -- it's traveling non-zero miles with zero fuel consumption. It's hard to represent infinity on a seven-segment display, however, so the software displays something else. "0" is wrong (but I've seen it), more common seems to be "--".
Now, if the engine is off and the car is stopped, you're getting 0 miles for 0 fuel, or 0/0. That's not infinite, it's undefined, and is represented in IEEE floating point as "NaN" (not a number). I haven't seen that in car's fuel milage display yet, though. :)
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