Coasting downhill in my car in neutral: how bad?

[Carnac the Magnificent!]

To amuse myself, sometimes I slip my car's automatic transmission into neutral when I'm driving down mountains, letting gravity do its thing.

Admitting that this isn't exactly "helping" my car, how much is it hurting it? My two concerns center on: (a) does doing this on a hot day, when the radiator needs to really circulate those fluids, yet the car is basically idling and (b) does it "hurt" (whatever that means) the transmission when I pop it back into drive at about, say, 50 mph? :rolleyes:

Thanks.

[MonkeyMensch]

I can only speak for the heating question. Idling the engine creates considerably less heat than running it up at speed. The reason temps tend to skyrocket while idling is generally because you're stuck in traffic with virtually no airflow through the radiator. On the other hand if you're rolling downhill at 40 or 50 you're cool.

[Bippy the Beardless]

On a manual car (and I assume auto as well) don't switch off the engine to save fuel when doing this, as the breaks will not work with the engine off (on a Ford Fiesta any way this is true). Luckily I experimented on a very shallow hill, and restarted the engine easily enough.

[x-ray vision]

It shouldn't hurt anything besides wearing out the brakes faster and heating up and possibly warping the drums and rotors. The radiator will circulate the coolant the same as with the trans in gear. Popping it back in gear at 50mph also shouldn't hurt anything as long as everything is working as it should. In some states it'e illegal to coast in neutral because you might need to accelerate quickly to avoid an accident.

[bernse]

On a manual car (and I assume auto as well) don't switch off the engine to save fuel when doing this, as the breaks will not work with the engine off (on a Ford Fiesta any way this is true).
Your brakes will work but you wont have any power boost from the vacuum created by the engine which may make you think they don't work at first.

Either way, it is extremely unwise to kill your car when it is moving, period. Especially at highway speeds.

[MaryEFoo]

Carl said it is a matter of control. You've given away your control.

[MaryEFoo]

My friend Carl would say it is a matter of control. You give away your control.

[MaryEFoo]

Drat.

[Squink]

Or rather, My friend Carl said Drat. ;)

[minor7flat5]

Originally posted by bernse
Either way, it is extremely unwise to kill your car when it is moving, period. Especially at highway speeds. Imagine what would happen if you turned the key one click too far and, all of a sudden, your steering wheel locked :eek:

Please do keep this possibility in mind if you ever need to turn off the ignition while driving (e.g. stuck accelerator).

[Mikahw]

Imagine what would happen if you turned the key one click too far and, all of a sudden, your steering wheel locked

In my car at least, it's impossible to turn the key to the "lock" position if the shifter is anywhere but "park".

[Xema]

Originally posted by minor7flat5
Imagine what would happen if you turned the key one click too far and, all of a sudden, your steering wheel locked

I imagine I'd turn the key one click in the other direction, thereby fixing the problem.

[RufusLeaking]

If you turn the engine off with the key the steering wheel doesn't lock until you pull the key out. But then it is such a habit to pull the key out after switching the engine off you might do this without thinking.

[herman_and_bill]

I have heard that in an auto tranny you are running the gears and not the pump so you might be wearing out the gears and burning the oil.
Also some places have laws against coasting in neutral, California maybe. Google shows that Maine had one but it was repealed.

[ronbo]

Originally posted by Country Squire
To amuse myself, sometimes I slip my car's automatic transmission into neutral when I'm driving down mountains, letting gravity do its thing.

The answer to the actual question is:

You are probably not helping, but not hurting.

Modern automatic transmissions generally allow the drive shaft to freewheel faster than the engine speed. You can experiment to see if you really travel faster and further with the transmission in neutral instead of in gear. If you have a tachometer, see if the engine speed drops to idle speed when you release the gas. If it does, shifting to neutral has no effect.

If you have an auto that does not allow freewheeling, and choose to drive in neutral, brakes are made for stopping cars. Any additional wear on brakes saves tremendous wear on the engine. Brakes are easy to replace.

If the engine is on, your tranny is getting oil.

In many states, it is illegal to turn off the engine to coast.

Hope this helps.

[racer72]

You are doing a disservice to your transmission. If you are coasting at 40 mph, the internal parts of your transmission are turning at a 40 mph clip too. But with the engine idling, the transmission is not get the hydraulic pressure to keep the clutch packs in contact. This will cause premature wear inside the transmission. It is not as severe as towing a car on the drive wheels with the engine off but close to it. You could get away with coasting in GM cars with 2 speed automatics produced from the early 50's to the mid 60's. The hydraulic pump was driven off the output shaft of the transmission, not the input shaft like virtually all other automatic transmissions. Cars with these transmissions suffered during periods of idling in place. The transmissions would overheat and quit working. The nice thing about these transmissions is that they cars that had them could be push started.

[samarm]

bernse said
Either way, it is extremely unwise to kill your car when it is moving, period. Especially at highway speeds.

It has alread been said, but I think it's worth repeating: DON'T SWITCH OFF THE ENGINE WHEN YOUR CAR IS MOVING!

It is very dangerous, as your steering may lock. Also, you lose power assistance to the brakes, meaning you have to pump the pedal much hard than you would normally.

[Carnac the Magnificent!]

Originally posted by samarm
It has alread been said, but I think it's worth repeating: DON'T SWITCH OFF THE ENGINE WHEN YOUR CAR IS MOVING!


I didn't say a thing about turning off the engine.

So far, we have some Dopers who says idling has no ill effect, while others say it does.

Any others?

[Beer Penguin]

So is racer right or is ronbo?

[LouisB]

My guess is that little to no damage would be done to the transmission. IF any damage IS done, it would be almost theoretical.

What you do lose is the engine braking effect. If the hill in question is long, or particularly steep, you will increase the wear on your brakes and in severe cases you might end up with brake fade due to heat. Plus, if the car is out of gear, you lose the ability to quickly accelerate, should that be required.

Coasting down a hill is never a good idea unless the engine dies and stopping would be dangerous. Switching the engine off while coasting is an even worse idea, unless the accelerator is stuck.

[Osip]

I have a manual transmission Honda Insight.

I push in the clutch every time I go down a hill. This really does boost the overall gas milage of the vehicle. Traveling a 1/2 at idle speed = less gas than the same at 3K rpms.

Going down the hill with speed does not cause the engine to heat any higher.. at the idle speed water is still moving thru the engine, but the air rushing thru helps cool things off.

Of course, if I gain to much speed I have the option of releasing the clutch and having the gears slow me down. So I do not wear the breaks down to much.

The ins and outs of all this for an automatic transmission I do not know.

[lucwarm]

With an automatic transmission, the engine disengages when you take your foot off the gas. So, in effect you are in neutral even the shifter set to "D" if you are coasting down a hill. So if you slip the shifter to "N," it will have no effect on your car's drive train.

The only effect will be that if you want to speed up, you'll have to put the car back into gear.

To test this, keep your eyes on the tach when you drive. Every time you take your foot off the throttle, the RPMs will drop to idle the same as if you throw the clutch out in a manual car. No matter how fast you're going.

Just MHO.

[Fritz The Cat]

One thing I did not see mentioned about shutting off the engine when coasting is the loss of power steering.

[handy]

Country Squire, what kind of car is this? Make, model & year.

[Carnac the Magnificent!]

Originally posted by Fritz The Cat
One thing I did not see mentioned about shutting off the engine when coasting is the loss of power steering.

LET ME REPEAT... I am not talking about shutting off the engine. I have no intention of shutting off the engine. I promise not to shut off the engine.

Okay? Thanks!

[Myrr21]

With an automatic transmission, the engine disengages when you take your foot off the gas. So, in effect you are in neutral even the shifter set to "D" if you are coasting down a hill. So if you slip the shifter to "N," it will have no effect on your car's drive train
I find this hard to believe, given that automatics will drive forward if you aren't actively pressing the brakes, but maybe I'm missing something...

[bernse]

Originally posted by Country Squire
LET ME REPEAT... I am not talking about shutting off the engine. I have no intention of shutting off the engine. I promise not to shut off the engine.

Okay? Thanks!
You are not the only one posting in this thread, Squire. We're not just talking to you. OK?

[Thin Ice]

It seems to me that whatever small gain in fuel mileage is not worth the added danger you've put yourself in by coasting down grades. The engine's compression is a major factor in keeping your speed down, along with the brakes. That's why you downshift to lower gears going downgrade, so the compression can keep the speed down enough so you don't have to use the brakes continuously. Relying on brakes alone is an easy way to overheat them and boil the brake fluid. If that happens then you have no brakes at all until they cool down, which will probably be after you've plowed headon into that ditch or the truck coming up the grade.

[DarrenS]

The Car Talk brothers on coasting downhill (http://cartalk.cars.com/Columns/Archive/1996/September/05.html).

[Annie-Xmas]

Two words: ASK GARP.

[lucwarm]

Originally posted by Myrr21
I find this hard to believe, given that automatics will drive forward if you aren't actively pressing the brakes, but maybe I'm missing something...

Now that I think about it, you're right. Nevertheless, if I am driving my wife's car (an automatic) at 60 mph on the highway and I see traffic slowing up ahead, so I take my foot off the gas, the tach goes right down to 800 rpm -- idle. The only way this is possible is if the engine is disengaged.

So perhaps the automatic transmission disengages if the car's motion would cause the engine to speed up, but not vice-versa.

[Thin Ice]

Lucwarm, the reason the engine speed drops to idle when you take your foot off the gas is because the engine output and transmission input are not connected mechanically; there is a fluid-filled torque converter on the front of the tranny. The TC is connected to the engine's crankshaft via the flywheel and spins with it. Inside the TC and fixed to it are fins that act as a pump, flinging fluid outward. This fluid enters another set of fins, a turbine, and causes it to spin. The shaft of the turbine is connected to the tranny.

[GusNSpot]

There are lots of variations in transmissions folks.

Some will not disengage ( free wheel) if you take your foot off the gas.

Some will free wheel in D but not in 3'rd and is usually a slight down shift in gearing.

Some automatics always (free wheel) and they are harder on brakes because they have no engine braking.

Some transmissions have overdrive as an selection and those are usually a (free wheel) condition. In overdrive that is. But some were not Free wheel even then.

There is no difference nor as much strain on a tranny by shifting from neutral to drive while coasting at 50 MPH than there is downshifting from drive to second to accelerate out of a dangerous situation ala James Bond. In a properly set up tranny, you will drop two gears by just flooring it from cruising to full on the floor. trannys get plenty of fluid movement at idle to take care of a neutral to drive shift at 50.

The largest damage of trannys is people who drive around in overdrive while in city traffic where the tranny is constantly shifting up and down.

One reason for more Free Wheeling transmissions these days is the fact that cars are so small and light. An 1980 Caddy or Buick or Chevy Impala are not as likely to have free wheel in the tranny.

Just because you are young and have not driven all models is not a problem, but there are older and different cars on the road that you all seem to be totally unaware of. But I admit that is hard to say for sure of the last 5 years or so, I need to read the data plate to tell them apart. Now ask me about anything from 1948 to 1965..... Bawahahahaha

About coasting... If the engine is not turned off, and the speed does not build to a point where you are speeding (against the law you know) then putting an older transmission in neutral is no more dangerous than simply driving down the hill in the free wheel equipped cars, or using the average ‘overdrive’ in most cars so equipped.

The boogie man about needing sudden acceleration is just scardykat people wringing their hands. If you can’t remember to leave the engine running, do it in bumper to bumper traffic in CA, can’t shift back into drive, then, you should not do it at all..... You are to stupid to be in a car in the first place. ::: sheesh ::::

[Blown & Injected]

Originally posted by racer72
You are doing a disservice to your transmission. If you are coasting at 40 mph, the internal parts of your transmission are turning at a 40 mph clip too. But with the engine idling, the transmission is not get the hydraulic pressure to keep the clutch packs in contact. This will cause premature wear inside the transmission. It is not as severe as towing a car on the drive wheels with the engine off but close to it. You could get away with coasting in GM cars with 2 speed automatics produced from the early 50's to the mid 60's. The hydraulic pump was driven off the output shaft of the transmission, not the input shaft like virtually all other automatic transmissions. Cars with these transmissions suffered during periods of idling in place. The transmissions would overheat and quit working. The nice thing about these transmissions is that they cars that had them could be push started.

But, if this is for the engine is not off folks, the torque converter is still turning the pump. The converter body, not the input shaft, turns the pump. So as long as the engine is running the pump is moving fluid and since there is no pressure on the parts there is no need for high fluid pressure.

[Blown & Injected]

Originally posted by Osip
I have a manual transmission Honda Insight.

I push in the clutch every time I go down a hill. This really does boost the overall gas milage of the vehicle. Traveling a 1/2 at idle speed = less gas than the same at 3K rpms.

Going down the hill with speed does not cause the engine to heat any higher.. at the idle speed water is still moving thru the engine, but the air rushing thru helps cool things off.

Of course, if I gain to much speed I have the option of releasing the clutch and having the gears slow me down. So I do not wear the breaks down to much.

The ins and outs of all this for an automatic transmission I do not know.

I would believe that this car has a coast program in the computer that lills most of the fuel to the engine during coast conditions. If it does, then you are forcing it to idle fuel when doing what you talk about. I bet you are using more fuel!

[abdul]

For that matter, when you're in a Honda Insight, and you're driving around trying to strategically FURTHER increase the gas mileage, you might just be beating a dead horse... Did you lose a bet? <=

[x-ray vision]

Did anyone mention DON'T SWITCH OFF THE ENGINE WHEN YOUR CAR IS MOVING!

[something clever]

wow, there is so much incorrect information in this thread. but i guess that is normal when it comes to cars. everyone thinks they're an expert. ill sum all the points up in one post, i saw a few posts with good information but they are surrounded by wives tales and just plain ignorance.

Modern cars have lockup torque converters.. when youre going down the highway at 70mph and let off the gas, the rpms will NOT drop to idle. When a torque converter locks up, the car is essentially like a manual transmission, there is no slippage, it is a direct mechanical link. This usually happens above 45mph as long as overdrive is turned on, and you are not at wide open throttle.

With that said, no, it wont hurt. coast down a hill, big whoop dee doo. Some of you are partially right. Towing an automatic RWD car with the rear wheels on the ground is a bad idea if youre going long distances. If that is the case you need to remove the driveshaft.

But since we're not talking about towing.. as long as the engine is on, the front pump in the tranny will be getting fluid. AND SINCE WE ARE IN NEUTRAL AT THIS POINT, there is no need for a high line pressure, because the transmission is in neutral.

having the engine go from idle (probably5-800rpm) to say 2,000 or so is fine for the engine but it could eventually cause problems for the transmission, since you dont have a performance car or anything, you would most likely NEVER NOTICE. if you want to be safe and you have a tach, get the rpms back up to ~2000 or so before putting it back in gear. its similar to what you would do if you had a manual tranny.. 80mph on the freeway, you pop it in neutral going downhill.. you wouldnt justput it back in to gear and pop the clutch, youd try to match the engine rpm with where it should be. same principle.

[GusNSpot]

Originally posted by x-ray vision
Did anyone mention DON'T SWITCH OFF THE ENGINE WHEN YOUR CAR IS MOVING!
Item # 15, 389 that the SDMB is FULL of folks that do not read even short therads before swelling with pride form seeing their post up.. ;j \

::: sheesh ::: :smack:

[x-ray vision]

Get a clue Gus.

[handy]

"One thing I did not see mentioned about shutting off the engine when coasting is the loss of power steering. "

Yeah, but you can still steer.

[Freiheit]

What about manual trannies?

I live on a hill, and my dad criticizes me for going down it in neutral every day.

[Skillet38]

I have a manual transmision in my Toyota Tundra. I often pop it into neutral going downhill. NO i'm not going to burn up my brakes, I know how to use a clutch and usually only do it when there's a hill ahead to provide natural braking.

I also downshift to assist in braking on occasion ( I know some people equate this with stomping on kittens ).

The only real concern I have is the sudden change in oil pressure. I'd like some insight into this from any gear heads out there, as to potential stress to the motor.

Country Squire funny how in your OP you never mentioned turning the motor off lol yet it became a big topic.

[Rick]

Originally posted by something clever
wow, there is so much incorrect information in this thread. but i guess that is normal when it comes to cars. everyone thinks they're an expert. ill sum all the points up in one post, i saw a few posts with good information but they are surrounded by wives tales and just plain ignorance.

Quite true

Modern cars have lockup torque converters.. when youre going down the highway at 70mph and let off the gas, the rpms will NOT drop to idle. When a torque converter locks up, the car is essentially like a manual transmission, there is no slippage, it is a direct mechanical link. This usually happens above 45mph as long as overdrive is turned on, and you are not at wide open throttle.

Well I can't say for sure on all lockup converter auto transmissions, but on the ones I teach on the converter unlocks whenever the brake is applied or the throttle is returned to idle.

With that said, no, it wont hurt. coast down a hill, big whoop dee doo. Some of you are partially right. Towing an automatic RWD car with the rear wheels on the ground is a bad idea if youre going long distances. If that is the case you need to remove the driveshaft.

In some cases a long distance can be defined as just a couple of miles (yes I have seen transmissions get killed by a tow truck driver not dollying or flat bedding a 5 mile tow.)

But since we're not talking about towing.. as long as the engine is on, the front pump in the tranny will be getting fluid. AND SINCE WE ARE IN NEUTRAL AT THIS POINT, there is no need for a high line pressure, because the transmission is in neutral.

I agree with this statement.

having the engine go from idle (probably5-800rpm) to say 2,000 or so is fine for the engine but it could eventually cause problems for the transmission, since you dont have a performance car or anything, you would most likely NEVER NOTICE. if you want to be safe and you have a tach, get the rpms back up to ~2000 or so before putting it back in gear. its similar to what you would do if you had a manual tranny.. 80mph on the freeway, you pop it in neutral going downhill.. you wouldnt justput it back in to gear and pop the clutch, youd try to match the engine rpm with where it should be. same principle.

I have coasted down hills in N before. when it was time to re-engage the trans, there is a shock to the system as the clutches and brakes inside the unit apply. While this shock may not necessarily shorten the life of the trans, for sure it, won't be increasing it.

On the transmissions that I am familar with and teach on, when in drive and the throttle is closed the troque converter will unlock, and if on flat ground the engine will return to idle (or very close to it) If decending a hill the speed of the car will be increasing (gravity) and the rpm will hang at above idle. This is not really engine braking, as it will not occur on flat ground. I have never really given much thought as to why it does this. If somebody REALLY wants to know I suppose I could pull out some hydraulic system charts and apply diagrams and figure it out. In any case, if the driver downshifts one gear, then the transmission is set up in such a way that engine braking is present. Feel free to try slowing down in your own car in D and then repeat in the next lower gear.

While I am on the subject Osip Most cars nowadays (and I will say most, becasue there could be an exception somewhere) the engine management computer turns off the injectors on decell. This means that if you take your foot off the gas the injectors shut off. This is done for both fuel mileage and emissions (hey no injection, no emissions) so by putting your car in neutral you are probably increasing the amount of fuel used.
:smack:
My car has a trip computer. When driving in the hills if you look at the instanious fuel mileage you will see it peg at 99.9 MPG when you go down a hill, as no fuel is being injected.

[Skillet38]

Originally posted by Rick
...
While I am on the subject Osip Most cars nowadays (and I will say most, becasue there could be an exception somewhere) the engine management computer turns off the injectors on decell. This means that if you take your foot off the gas the injectors shut off. This is done for both fuel mileage and emissions (hey no injection, no emissions) so by putting your car in neutral you are probably increasing the amount of fuel used.
:smack:
My car has a trip computer. When driving in the hills if you look at the instanious fuel mileage you will see it peg at 99.9 MPG when you go down a hill, as no fuel is being injected. [/B]

interesting observations. the point I want to make (in my manual tranny), is that I go Faster downhill coasting than if I take my foot off the gas. In fact the instances when I'm tempted to do so are such that I actually accelerate with either the clutch in or the tranny in N . . . . .

[Rick]

Originally posted by Skillet38
interesting observations. the point I want to make (in my manual tranny), is that I go Faster downhill coasting than if I take my foot off the gas. In fact the instances when I'm tempted to do so are such that I actually accelerate with either the clutch in or the tranny in N . . . . .

Manual trannys are a different kettle of fish than a automatic. In a manual there is a direct connection between the engine and the rear wheels at all times when the car is A) In gear and B) the clutch is engaged (No foot on clutch pedal)
So when you take your foot off the gas in manual the engine works to brake the drive wheels, this does not happen in an automatic in Drive. (usually)
So yes pushing in the clutch or shifting into N you will pick up speed on a down hill.

[GusNSpot]

Originally posted by x-ray vision
Get a clue Gus.
It was a joke? It was a joke? It was a joke?


Argahhhhhhhhh :smack: :eek: ;j :wally

Where is the clue store?
.