Coasting with an Auto

[kickflipjr]

Today was the first time i tried drifting down a hill (engine not on). I just put into N, killed the engine, coasted down this one hill for about a mile, and then restarted when it was needed. I have heard that this can be bad for certain auto trannies. I have also heard that it isn't bad on saturns (i don't plan on doing this too often because it is hard on the starter).

[MetroMPG]

yay - another convert!

a mile is a long coast (by local standards). one of my longest around here is maybe half a mile - IF there are no trains coming AND the light after that is green.

[Compaq888]

this is the reason I stopped coasting with engine off. It can kill the tranny easy, and starter. I hate auto.

[kickflipjr]

I used to do it with my old car (5 speed). It just seems dumb with an auto.

Yeah, that hill is nice, long, and not too steep. I have bee coasting it in nutral for years.

[Compaq888]

I don't think coasting with engine on in neutral will damage anything, there will be less resistence but the problem is you can't put it into the gear you want

[rh77]

So, today I tried the coast/engine off in a few places. With the driving I do, I ended up doing it approx. 15 times. If I don't let the transmission kick-down to first gear before a light, then the N-to-D shift is jolting once things get going again. I coasted down a hill, engine off for half a mile and started back up. I've been using the N-idle feature at long lights if I know it isn't planned to change within 20 seconds. Otherwise, I've been trying the shut-down. I have to pay close attention to the traffic pattern, or else I get stuck dead in the water until things get back up and running, and irritated people behind me. Cutting the power deactivates the turn signal for a second or two -- I may look into the engine cut if I get decent evidence that I'm not trashing the transmission. So yes, MetroMPG, I have seen the light, and I'm now converted.

RH77

[Compaq888]

rh77,

so let me get this straight, you coast in N then put it into D? Wouldn't that N bomb the transmission???

Can you make it work like this... See a red light ahead, flip to N, shut down engine. Stop at red light, green hits restart engine, put in D and keep going??

Can you put the shifter from D to N without causing any damage?

[Compaq888]

Here is what I did this morning. When I drive and see a yellow or red light I put in N and let it coast to the light. If I know it's going to be a long light when I put it into N I shut the car down too. When I get to the light I stay there till it turns green and restart the car and put back in D. If the light will be soon I just coast up to the light in N and leave it in N and wait for the green. When the green hits I put in D and drive off.

[rh77]

rh77,

so let me get this straight, you coast in N then put it into D? Wouldn't that N bomb the transmission???

Can you make it work like this... See a red light ahead, flip to N, shut down engine. Stop at red light, green hits restart engine, put in D and keep going??

Can you put the shifter from D to N without causing any damage?

As far as I know, you can't damage the transmission from shifting to D-N or N-D without throttle input. N-bombing the tranny is putting the car in neutral when you're moving, say 65 mph, then in N go full throttle and shift into D. Since it's not a standing start bomb, it probably wouldn't tear things-up immediately, but a few times and it's shot.

RH77

[rh77]

So, I took a VAC up to Calgary and their National Parks in the Rockies over a long weekend. Down a long hill on the Trans-Canada Highway 1, I put the rental Chrysler 300 into N and shut off the car. It coasted at the speed limit down the mountain. When it was time to start it back up, it wouldn't! Dead in the water, we coasted to a conveniently located turn-out on the side of the road. I had to turn the key to the complete OFF position and then start the car! If I did that moving, it would've locked the steering, potentially screwing us even further. I have some Canadian car stories to tell in a separate thread, but MetroMPG, I saw my first Pontiac Firefly!

RH77

[philmcneal]

So, I took a VAC up to Calgary and their National Parks in the Rockies over a long weekend. Down a long hill on the Trans-Canada Highway 1, I put the rental Chrysler 300 into N and shut off the car. It coasted at the speed limit down the mountain. When it was time to start it back up, it wouldn't! Dead in the water, we coasted to a conveniently located turn-out on the side of the road. I had to turn the key to the complete OFF position and then start the car! If I did that moving, it would've locked the steering, potentially screwing us even further. I have some Canadian car stories to tell in a separate thread, but MetroMPG, I saw my first Pontiac Firefly!

RH77

Hahaha Chrysler is a brand I will nevrer call my own. But glad your safe ;) so TEST N coast with engine off in a clear area folks! Do it in the wrong place and boy you'll never want to drive EVER again.

[DaX]

As far as I know, you can't damage the transmission from shifting to D-N or N-D without throttle input. N-bombing the tranny is putting the car in neutral when you're moving, say 65 mph, then in N go full throttle and shift into D. Since it's not a standing start bomb, it probably wouldn't tear things-up immediately, but a few times and it's shot.

RH77

Haha, now I'm thinking of N-bombing my auto tranny right before I do the 5-speed conversion, just for the hell of it and to see how many times it takes to kill it. Just talking about N-bombs makes me LoL.

[Matt Timion]

Haha, now I'm thinking of N-bombing my auto tranny right before I do the 5-speed conversion, just for the hell of it and to see how many times it takes to kill it. Just talking about N-bombs makes me LoL.

LOL... "N-Bombs" almost sound like a cuss word. :D

[rh77]

So, I've tried that a few times in a Rental just to see what would happen (never from a standing start), and it just kind of slips into whatever gear is appropriate and off you go! It's kinda like slipping the clutch in a manual. Evidently, it tears it up. (Didn't know that at the time -- I feel guilty about ripping up Rentals sometimes, because some poor sucker is going to buy it off of a used lot in a year or two. The roughest I've been is with that Chevy Trailblazer on the Rally stages -- particularly the last one where I beat the last 6 car's times.

So, if someone refers to the car as the "N-bomb", I'd be cautious ;-)

RH77

[rh77]

LOL... "N-Bombs" almost sound like a cuss word. :D

OMG, Bob dropped the N-bomb in front of the boss the other day. I thought he'd get fired on the spot!

RH77

[Compaq888]

I feel guilty about ripping up Rentals sometimes, because some poor sucker is going to buy it off of a used lot in a year or two.

RH77


I hate you!!!!!!!!!!!!!!!
Me=owner of a previously rented car.

[rh77]

I hate you!!!!!!!!!!!!!!!
Me=owner of a previously rented car.

Great now you hate me *tear*. What about all the good times we had together here on Gassavers: the O/D discussion, your quarter-mile times. I've ruined it.

OK, now go back and re-read my post. I rent, on the average, 1-2 cars per week. 99% of the time I wouldn't ever do anything that would compromise the longevity of the vehicle. If you polled most people our age out there, they would tear the living **** out of rental cars: throwing it in reverse on the highway, jumping it over hills, driving at redline all day. I'm one of the good guys out there that rents cars. I may have compromised 1 or 2 cars out of almost 500 I've rented in my life. 1 was the Rally trailblazer -- the paint was scuffed. The other was the accidental N-Bomb before I knew the consequences. Go ahead and hate me, but be warned -- there are much worse people out there in the rental world.

My parents bought a used Jeep Grand Cherokee for their Farm, which was a rental. The transmission went out at 160,000 miles and they had miscellaneous problems -- they got a good deal on it, but it was a rental. Anymore, I picture someone like my folks buying the vehicle. Does that make you feel any better ????????

RH77

[Compaq888]

I hate you!!!!!!!!!!!!!!!
Me=owner of a previously rented car.

Great now you hate me *tear*. What about all the good times we had together here on Gassavers: the O/D discussion, your quarter-mile times. I've ruined it.

OK, now go back and re-read my post. I rent, on the average, 1-2 cars per week. 99% of the time I wouldn't ever do anything that would compromise the longevity of the vehicle. If you polled most people our age out there, they would tear the living **** out of rental cars: throwing it in reverse on the highway, jumping it over hills, driving at redline all day. I'm one of the good guys out there that rents cars. I may have compromised 1 or 2 cars out of almost 500 I've rented in my life. 1 was the Rally trailblazer -- the paint was scuffed. The other was the accidental N-Bomb before I knew the consequences. Go ahead and hate me, but be warned -- there are much worse people out there in the rental world.

My parents bought a used Jeep Grand Cherokee for their Farm, which was a rental. The transmission went out at 160,000 miles and they had miscellaneous problems -- they got a good deal on it, but it was a rental. Anymore, I picture someone like my folks buying the vehicle. Does that make you feel any better ????????

RH77


It was a joke, we bought our car from a nissan dealer that bought it at an auction from a rental place. The transmission shifts fine. My cousin messed it up, I fixed it by resetting both the ecu and the transmission module and taking out the extra liquid out of the tranny.

Now my moms car is messed up, I feel sorry whoever buys that.
Engine broke down twice, ecu went crappy and had to be reproggramed, sometimes looses power on the freeway, and other little stuff. Our lease will be over in Nov. this year.

Sorry for the confusion, I don't hate you.

[rh77]

I know -- I'm just playing along. But at the same time, I don't want people to think I'm going around tearing the crap out of rental cars. I go back to the same places, so I might end up with the same car, and I don't want to be stranded. Also, to encourage people not to tear-up rentals, because it's not the rental agency that's going to get screwed, it's someone who buys a "well-maintained, late-model" vehicle. I live about 5 miles from a huge car auction facility. I see hundreds of car model-types that I've rented. Many dealers from all over buy them and sell them as "One-Owner" cars. Who knows what people have done to them, that can't be found by a good mechanic. But really, that goes for any used car anymore. Buyer beware.

RH77

[DaX]

I can see it now...revving the engine to 6500 RPM's...burning oil smoke blowing out of the exhaust pipe [the car burns oil horribly at high rpm's]...slamming it into drive...the squeal of tires and the smell of burnt clutch packs. I just may have to indulge myself.

[Compaq888]

be sure to post the results of how many N bombs it takes to kill the tranny.

[DaX]

be sure to post the results of how many N bombs it takes to kill the tranny.

"How many licks does it take to get to the center of an auto tranny?"

I've N-bombed twice before from what I believe to be about 3500 RPM's (didn't have a tach back then). If I decide to do it I'll be bouncing off the rev limiter. Go all out.

[philmcneal]

so with an auto tranny coasting with engine off in N ... say your going 10-15 mph and you decided to put it back into D. Would the engine magically start up again (roughly hense the name bombing) that would be cool! Or you have to key on?

I noticed some Volkswagaons have a DSG (its a manual transmission controlled by the "best manual driver in the world" i wonder if the computer is smart enough to bump start the engine smoothly...

too good to be true stop dreaming~~

[Compaq888]

The vehicle has to be in N or P in order to restart.

[zpiloto]

DaX:

I've N-bombed twice before from what I believe to be about 3500 RPM's (didn't have a tach back then). If I decide to do it I'll be bouncing off the rev limiter. Go all out

It would be cool if you could record the audio and post it so we could hear the full effect.

[rh77]

I did it with a Pontiac Grand Am a few years ago. Basically you're traveling along at any speed and shift to N. Then Rev it up bumping off of the rev limiter -- that's the loudest sound. Then shift to D. You'd think it was a big production, but it isn't, really. Then oddly enough, the transmission makes little noise. The engine just bogs down while the tranny catches and figures out the right gear like if you were to rev up a car with a clutch and let it out slowly. Then the car acts normal with a downshift for power acceleration unless it's been fried.

RH77

[zpiloto]

Can some one help me with why it destroys your auto transmission if you coast in neutral with the engine off. I've never seen the inside of a transmission and looking around on-line it doesn't really address the issue it always get side tracked into legality, brakes and PS. The torque converter is a fluid coupling and with it in neutral the axel is still hooked up to the transaxel so the gears are still spinning around no harm there. But when you shut the engine off it shut down the pump. Now where is the pump pumping fluid to? Even when the pump is off the gears will still have fluid from when it was on and there is no load. So does a coast of 30-60 second at speeds under 50 MPH cause that quick of a heat build up? and what parts are geting damaged? I can see where if you flat towed it cold for miles it would cook everything.

[n0rt0npr0]

With no fluid *pressure* the auto transmission disengages completely. There is absolutely no way for this engine off coasting in gear or out to destroy a tranny.

However IF you have an electronic transmission, IT WILL most likely confuse the **** out of the tranny. Thats why Compaq888 was having a hard time with his in the first place. It's all because of the way they designed the "fuzzy logic" ....either the engineers factor in that the driver won't use engine off coasting (ie, compaq888) or they will factor in engine off coasting (ie, 2001 olds intrigue)

[psyshack]

I coast our 05 Accord 5AT all the time. ICE off,, ICE on. restart engine and dump it in drive.

[Compaq888]

I coast our 05 Accord 5AT all the time. ICE off,, ICE on. restart engine and dump it in drive.

your tranny must be designed differently than mine. I'm still getting good mpg with coasting in D. Whenever you let your foot off the gas the tranny goes automatically into neutral, the reason the car slows down so much is it's connected to the engine. When you put the lever in N you actually disengage the tranny from the engine and you get a longer coast because your engine is not slowing down the tranny.

It's actually pointless to just stick it in N and coast because the engine still injects fuel. If you're going to stick it to N you better shut off the engine to save fuel. On my car if it's in D and I coast the fuel injection shuts off till like 1000-1100rpm then comes on to prevent the engine from stalling.

You can find out a lot by reading a FSM for your car. If you have one look in the Electronic control section.

[SVOboy]

your tranny must be designed differently than mine. I'm still getting good mpg with coasting in D. Whenever you let your foot off the gas the tranny goes automatically into neutral, the reason the car slows down so much is it's connected to the engine. When you put the lever in N you actually disengage the tranny from the engine and you get a longer coast because your engine is not slowing down the tranny.

It's actually pointless to just stick it in N and coast because the engine still injects fuel. If you're going to stick it to N you better shut off the engine to save fuel. On my car if it's in D and I coast the fuel injection shuts off till like 1000-1100rpm then comes on to prevent the engine from stalling.

You can find out a lot by reading a FSM for your car. If you have one look in the Electronic control section.

From the sound of it your car is a freak. My car does not do this. Anyway, in an automatic, from my understanding, the engine is never actually really connected to the tranny unless isn't fully locked up. My tranny does shift automatically into neutral, but it'll drop revs as the torque converter does it's thing to act independently of the engine.

Rick has said neutral coasting is much better than in gear on his auto, I think it's up to everyone to figure that up for themselves.

[Compaq888]

From the sound of it your car is a freak. My car does not do this. Anyway, in an automatic, from my understanding, the engine is never actually really connected to the tranny unless isn't fully locked up. My tranny does shift automatically into neutral, but it'll drop revs as the torque converter does it's thing to act independently of the engine.

Rick has said neutral coasting is much better than in gear on his auto, I think it's up to everyone to figure that up for themselves.

neutral coasting prolongs the coast but where I drive even If I did put it in N I would run out of space. Even when I leave it in D I have to brake because there is just too many cars. When I coast like 1/4 mile from the light then by the time I make it there it's green and I just continue. I think having an auto is stupid unless it has the manumatic feature on it.

[psyshack]

In the Accord the best coast is N ice off. Then its a draw between N coast ice on. And D coast ice on.

With ice on N coast you can bet Im ripping off a coast as if ice was off. EXCEPT I may be running the A/C. Yes I do run the A/C alot in the summer months.

With ice on in D Im light timing and have a very short coast. A/C on or off.

If Im not going to have a good light timing or something else comes up then Im ice off and stay off at the light.

I love to bust really long ice off N coast. The Accord is a coasting fool!

[mtbiker278]

I've been trying both coasting in D and coasting in N witht he engine on. So far I'm starting to believe that coasting in D saves some gas, but not much compared to N. I have the same problem as Compaq where there's really not enough room to coast effectively. I've tried engine off coasting a couple times and didn't like the feeling. The number one issue I have is the power steering cuts out. The first time I did it I freaked myself out cause I though my steering locked up or something like when you take the key out of the ignition. Thank god it was only a very small arching stop.

Until I get a scangauge or a service manual to figure out whether or not my injectors shut off I think I'm going to stick to engine off coasting.

[ketel0ne]

I read this whole posts and like some other posts, there is not really an answer to the question as to whether coasting in auto in N with the engine on and no throttle then shifting to D is bad.

I have seen where it has been stated that people do not see an increase in an auto between coasting in D or N, well I know for sure my Yaris will coast faster with it in N, on a steep downhill in D the car holds to about 60 or 65 for a long time. In N it goes upwards of 70 almost immediatly. In D I don
t have to slow down the coast at all, in N I would be doing 90 or better by the bottom of the hill (mountain).

So is the N to D no throttle safe?

[Compaq888]

I read this whole posts and like some other posts, there is not really an answer to the question as to whether coasting in auto in N with the engine on and no throttle then shifting to D is bad.

I have seen where it has been stated that people do not see an increase in an auto between coasting in D or N, well I know for sure my Yaris will coast faster with it in N, on a steep downhill in D the car holds to about 60 or 65 for a long time. In N it goes upwards of 70 almost immediatly. In D I don
t have to slow down the coast at all, in N I would be doing 90 or better by the bottom of the hill (mountain).

So is the N to D no throttle safe?

Depends on a lot of things. Your transmission being the primary. You can try it once. if you don't like how it acts don't do it again.

[SVOboy]

Liking how it acts has nothing to do with how safe it is.

Anyway, it's fine. Giving in throttle is what causes issue because you're putting stress on the transmission. At idle it's just slipping around like auto transmissions slip slip slip.

[Gary Palmer]

I read this whole posts and like some other posts, there is not really an answer to the question as to whether coasting in auto in N with the engine on and no throttle then shifting to D is bad.

I have seen where it has been stated that people do not see an increase in an auto between coasting in D or N, well I know for sure my Yaris will coast faster with it in N, on a steep downhill in D the car holds to about 60 or 65 for a long time. In N it goes upwards of 70 almost immediatly. In D I don
t have to slow down the coast at all, in N I would be doing 90 or better by the bottom of the hill (mountain).

So is the N to D no throttle safe?

On an automatic, I would strongly recommend not shifting into neutral, then shifting back to drive. Every time you shift, it requires a number of clutch's, which are small disc's really, to released and recompressed. Each time they get compressed, they get a small amount of wear. With the number in their, the wear isn't worth any gain in fuel efficiency, in my perspective.

Addititonally, in most automatic's, if something happened to cause the engine to turn off, then the automatic doesn't have any way of re-engaging, because the solenoids work off of oil pressure, which is generated by a turning pump, which runs off of the engine. If the automatic is in gear and the engine dies, then the transmission will keep the engine and the pump turning, so that you still have the load of the engine on the car, to help slow it down. If you have it in neutral and that happens, then you have no load, which is bad, bad!

[omgwtfbyobbq]

I was thinking, if the auto really does cut fuel when coasting, then breaking up driving into small P&G intervals should really help out with FE. The driver could pick a target speed, and window, so depending on how much of the time they pulse and how much they glide, pumping losses could be reduced by that ratio. I'm thinking something like a 20% increase in FE could be attained with this technique in an auto, but it really depends on whether the injectors shut off. Does anyone know how I can test this, for cheap? ;)
I've seen the fancy induction tach the smog places uses, but is there a low tech solution that can tell me if there is a signal or not?

[SVOboy]

Hmm, just hook a light up to your injector's switched ground and if it's not flashing then the injector ain't turnin' on.

[omgwtfbyobbq]

So there's enough current to light an LED? I'm hoping the current draw won't screw with the pulse...

[SVOboy]

My injector powering lit up my LED very well when I was messing around the other day. I doubt it'll mess with the pulse, I have my superMID hooked it to my #2 injector without issue.

[omgwtfbyobbq]

Sweet! Probably try this out on friday. :D

[cfg83]

Compaq888 -

this is the reason I stopped coasting with engine off. It can kill the tranny easy, and starter. I hate auto.

I am not sure. Did you stop coasting with the engine off on your auto or your stick?

CarloSW2

[Traugbot]

i had a tranny go out in my old car (not from n-bombing) and it is not fun. I got stuck in an intersection, so if n-bombing is bad for your tranny i wouldnt advise doing it

[Erdrick]

Alright, so let's say that I am driving along at 40mph. I am coasting down a hill, and decide to put the car into neutral. I end up going about 50mph by the end of the hill, still in neutral. Without touching the gas, if I were to then, at 50mph, shift the car back into drive, would that hurt the transmission?

[kickflipjr]

Alright, so let's say that I am driving along at 40mph. I am coasting down a hill, and decide to put the car into neutral. I end up going about 50mph by the end of the hill, still in neutral. Without touching the gas, if I were to then, at 50mph, shift the car back into drive, would that hurt the transmission?

well, you should rev up the RPM (while in neutral before you put it in drive) so they match what you would be at at 50mph (say 2,000rpm) and then put it in drive. Doing that will hurt your car less.

I really think costing with an auto may be more trouble then it is worth (unless you find a long gradual hill).

[Snax]

The effectiveness of rev matching is probably going to vary by vehicle. Not only that, but torque converters absorb the bulk of it and likely mitigate much of the wear associated with shifting in and out of gear.

Play around with it. Whatever works the most smoothly is the way to do it.

If I were to try to reve-match in our truck, it will clunk aggressively into gear every time at highway speeds. It produces a virtually unnoticeable engagement by simply leaving it alone until the gears are engaged again.

[JanGeo]

May be that the higher speed of the engine produces a quicker shift from the increase in automatic hydralic pressure. I think unless there is a freewheeling action in the torque converter than rev matching is a good idea. The clutch in top gear would wear much less if the RPM of the engine is closer to the normal RPM for the car speed. I would think that there would not be that many times when you would have to coast at that high a speed - more likely at lower speed when you would be shifting back into drive at less than 30mph. The last thing you want to do going down a mountain is to rely on brakes to slow you down if the drop is big enough leaving it in gear may be a lot safer.

[brucepick]

... Whenever you let your foot off the gas the tranny goes automatically into neutral...

As written elsewhere in this thread, I also think it varies with different cars. I put a tach into my car so I could see better what was going on with engine rpms.

My car has auto trans without lockup converter, so ymmv, as always.

Leavng it in D on a nice long downhill coast will see the rpms drop to 1500 and stay there.

Putting it in N for a similar nice long coast will see the rpms drop to about 800. Idling in N when not moving, rpms = 700-800.

My take on it is, coasting in D has the car's momentum spinning the engine via the tranny. It's not fully "engaged" as when your foot is on the gas, but those rpms have to be coming from somewhere. My foot is off the gas so I think it must be the car that's driving the engine. Those coasts seem to lose speed faster than the ones where tranny is in N. That always gives me the best coast. Oh yeah - electric rad fan, so the airflow is NOT driving the engine!

I drive Rte 9 in CT daily, it has loads of long up and down grades where I pulse to about 70-75 going down, and then glide the max possible till I need to help it get up the next hill.

[atomicradish]

I've done D to N and N to D coasting in my automatic for over a year now with no adverse effects. I didn't rev match either, mainly because I misunderstood what it was. I had thought that autos rev matched on their own since it always finds the apropriate gear when I shift back to D. I assumed rev matching was for driving a manual and then simply finding the apropriate gear to put it after gaining speed.

Does anyone really know whether or not it is bad to coast off of a long sloping hill in N in an automatic? Some people say yes, others say no.

Also, what are the adverse effects of engine-off coasting in N for autos? From my limited knowledge on transmissions, how would it be any different than in a manual (which I gather is fine to do)? When in N aren't the engine and transmission disengaged in both an auto and a manual? If so, what makes it 'wrong' to do so in an auto but not a manual?

Thanks to anyone who can answer this.

.. unfortunately the conflicting opinions really don't help someone who is confused to begin with.

[Snax]

The primary issue on many automatic transmissions with engine-off coasting is the functioning of the lubrication pump and shift pressure system. Without the engine turning, the transmission pump is not. This can result in odd or catastrophic wear and engagement of gears in the transmission. (The specifics of which I don't fully understand, but having felt the effect of it once while coasting, it does not sound or feel good for the transmission.)

Some cars by contrast, typically compacts, don't operate this way and are perfectly happy engine-off coasting in neutral. You just have to figure out which camp your car is in.

The best acid test is to check the manufacturers recommendations on towing the vehicle. If they give the ok for flat towing without disconnecting the driveline, coasting in neutral is fine, engine off or on. If they say don't flat tow, then don't coast with the engine off. Engine on coasting is likely no problem for any vehicle, but that's just an educated guess.

[atomicradish]

The primary issue on many automatic transmissions with engine-off coasting is the functioning of the lubrication pump and shift pressure system. Without the engine turning, the transmission pump is not. This can result in odd or catastrophic wear and engagement of gears in the transmission. (The specifics of which I don't fully understand, but having felt the effect of it once while coasting, it does not sound or feel good for the transmission.)

Some cars by contrast, typically compacts, don't operate this way and are perfectly happy engine-off coasting in neutral. You just have to figure out which camp your car is in.

The best acid test is to check the manufacturers recommendations on towing the vehicle. If they give the ok for flat towing without disconnecting the driveline, coasting in neutral is fine, engine off or on. If they say don't flat tow, then don't coast with the engine off. Engine on coasting is likely no problem for any vehicle, but that's just an educated guess.

Thanks for the advice Snax. Appreciated.

The Metro is definetly a compact. I'll check for that right now.

[bbgobie]

Got my scanguage a couple days ago. The scangauge reports fuel consumption is definately lower while coasting in N. I would expect the scanguage to report 0 fuel consumption if the engine on my car was being turned over by the tranny. More testing of coasting from 100-0 will be needed, but I'll need to find space to do this.

[bbgobie]

As written elsewhere in this thread, I also think it varies with different cars. I put a tach into my car so I could see better what was going on with engine rpms.

My car has auto trans without lockup converter, so ymmv, as always.

Leavng it in D on a nice long downhill coast will see the rpms drop to 1500 and stay there.

Putting it in N for a similar nice long coast will see the rpms drop to about 800. Idling in N when not moving, rpms = 700-800.

My take on it is, coasting in D has the car's momentum spinning the engine via the tranny. It's not fully "engaged" as when your foot is on the gas, but those rpms have to be coming from somewhere. My foot is off the gas so I think it must be the car that's driving the engine. Those coasts seem to lose speed faster than the ones where tranny is in N. That always gives me the best coast. Oh yeah - electric rad fan, so the airflow is NOT driving the engine!

I drive Rte 9 in CT daily, it has loads of long up and down grades where I pulse to about 70-75 going down, and then glide the max possible till I need to help it get up the next hill.

Your describing engine braking. Yes your car is driving the engine, but that doesnt mean your using fuel.

I'm no expert, but I've never heard that coasting in D your engine will cut off fuel. Your engine is forced into an rpm to match that of the tranny or else it wouldnt be a gd thing for either, to keep that rpm fuel must be injected. If coasting in D meant no fuel going to the engine hybrid cars wouldn't have to shut off the engine when coasting.

Another thing to consider, if you think your car shuts off fuel while coasting in D, try this and this will be easy to measure. Drive up to a decent speed, downshift to like 2nd and coast. You should be able to quickly figure out if you are using fuel. It'd be the equivalent to a few hard accelerations.

[ma4t]

You know, I tried coasting in neutral a few times. I noticed that the RPMs stayed the same. There was no shock when I shifted back into drive.

From all that I'm reading about damaging the transmission, I am not going to keep doing it.

M

[Raccoonjoe]

The best acid test is to check the manufacturers recommendations on towing the vehicle. If they give the ok for flat towing without disconnecting the driveline, coasting in neutral is fine, engine off or on. If they say don't flat tow, then don't coast with the engine off. Engine on coasting is likely no problem for any vehicle, but that's just an educated guess.


If you're worried about damaging your transmission, this is the advice I would follow. However, if you are like most folks out there, you don't *want* to kill the tranny, but are aware of the risks and such associated.

In the Jeep, I coast in N with the engine on. Not only because of the fact that she's hard to start sometimes (and the wear on the starter), but because of the lack of lubrication in the transmission with the engine off. I've torn apart a few trannies in my time, and can tell you I would *not* recommend it, unless the manufacturer says OK to straight flat tow. If they require driveline dis-assembly, then leave the motor on. (personally?? I would never flat-tow anything w/o disconnecting the driveshaft.....possible exception being a Jeep (or other vehicle with manual t-case), where you can effectively disconnect the driveline by shifting the t-case (and tranny) into Neutral)

I have never rev-matched my engine when shifting from N to D. The tranny is smart enough to find the correct gear...and if you keep your foot out of the throttle, the car will shift into "freewheel mode", similar to coasting in D. Autos are notoriously difficult to drive in the mountains, as they do *not* have any real capacity for engine braking. You remove throttle input = transmission slips and free-wheels. You take it out of gear = motor idles like it was sitting in your driveway in N.

This is what I would do if you have questions. Find a friend with a manual (or drive one yourself). Take it out on the freeway, wind it up to ~70. Push in the clutch and coast along with the clutch in for a few seconds.....then dump the clutch out. You should feel the car throw you forward, as the motor/tranny mate, and try to match RPM. (also a good reason for hard shifts, 'specially with inexperienced drivers). Your auto tranny does a lot of things for you.....auto shifting + auto clutching. If your auto tranny slams into gear (acting like the manual in the above example) when shifting from N -> D at highway speeds (and *absolutely no* throttle input), your tranny is most likely on it's way out.

If all else fails.....stick a few of the HDD magnets (those extremely powerful "rare earth magnets" to the bottom of the tranny pan. Drive for a while, then change your tranny fluid. If your causing damage, you will see the metal shavings/crap in the fluid or stuck to the magnets.....


*edit*

Sorry, this post got waaayy longer than I thought. Stepping off my soapbox now....

[ffvben]

the automatic trans fluid is the link from engine torque to wheel motion. this process heats up the fluid from all that friction. this fluid must be cooled down, so shutting your eng. off in a auto while coasting, would stop the pump from pushing fluid to the radiator/cooler. this might fry your fluid which in-turns cooks your clutches/plates inside trans. it could warp/crack clutches/plates/bearings/servos if the temp is to high. now coasting in neutral with engine running seems fine, the fluid is being pumped through the cooler.
as SNAX states , check your owners manual for towing your veh. on 4 wheels some you can tow others you have to disconnect drive line.( mainly awd or 4x4)
just a note : I have seen a heavy duty manual trans (f350-f450 trucks) with cooler lines also. they might need them for the heavy work load to keep gears cooler.

[omgwtfbyobbq]

If all else fails.....stick a few of the HDD magnets (those extremely powerful "rare earth magnets" to the bottom of the tranny pan. Drive for a while, then change your tranny fluid. If your causing damage, you will see the metal shavings/crap in the fluid or stuck to the magnets.....


*edit*

Sorry, this post got waaayy longer than I thought. Stepping off my soapbox now....
Good idea!

[ma4t]

We have a couple of hills that allow that. If the light at the bottom is red, I'll EOC if I'm sure I'm going to be sitting for a while.

I don't plan on keeping the vehicle for more than another year, so I won't know for sure if it causes any long-term issues.

m

[brucepick]

I suspect it's already been said: coasting in neutral will usually coast longer than coasting in D. Also if you have a tach you can see that idling Neutral coasting gives lower rpms than idling in Drive coasting. So for most cars without fuel cutoff, the lower rpms will give better fuel economy.

For neutral coast with engine off, auto tranny, I'd check the owners manual re. flat towing - that is, with the drive wheels on the ground in neutral.

Some manuals say it's OK, some may say never do it. Mine says up to 40 mph and 40 miles distance. So I only coast with engine off if I'm below 40 mph. Otherwise the engine is idling, coasting in neutral.

If the tranny fluid pump is driven off the tranny's input end and the engine is off while coasting, there is no fluid pressure in the system and some parts won't have the proper lube. Just my guess.

[s2man]

I put my ScanGauge in this week, and I've learned one thing already. With the TC locked, I get better mileage coasting in N. In D, the rpm stay up around 1700, but in N they drop to about 800. And the SG reports about 25% better milage coasting in N. At low speeds, with TC unlocked, there is no difference in rpm or reported FE (though I can coast a little further in N)

So I've stopped slipping it into N every time I come up to a stop sign, only using N coast over about 40mph. I don't like the way my car handles N to D at highway speeds with the engine idling, so I blip the throttle before putting it back in D. This makes it much smoother.

And that's my $.02

[veloman]

It seems like EOC with an auto is kind of dumb. Why risk screwing your tranny for a few extra PENNIES worth of gas a week?

I can coast for a half mile or so from 55mph to 25mph with just letting my foot off the gas on my auto civic. Below 29mph, the deceleration is quite slow, so it works fine.

Save pennies, spend hundreds? lol

[bfg9000d]

yeah bumping the transmission like such is not worth it. I can see coming up to a stop light and throwing it in N and then back to D when you get ready to go. But going 40 so mph and throwing it back in D. You might not hurt your transmission in the short term but overtime its gonna rip it up. Plus paying for a new transmission and having it installed will waste your savings in gas. Just let off the gas and let it coast. If you have your tires psi up high enough you should achieve a pretty good glide.

[popimp]

I don't think the coasts are being done with the engine off, just in Neutral.

[bfg9000d]

Yeah but throwing it in gear. From N to D when going 40 50 70 mph is not such a hot idea. Its still hard on the transmission.

[popimp]

When I do it at higher speeds I don't feel anything. In lower speeds between 30-35mph I feel a little thump. I should probably stop doing it at lower speeds. Please explain how it messes up the tranny. All I hear is that it is not good, but no one has posted why.

[bfg9000d]

found it on another website.

It is not advisable to shift to N while on the move as this may damage the internal parts of the device. When engaged at N, deceleration will allow the output shaft to overrun the hydraulic clutches and the gear trains of the gearbox. All hydraulic clutches at this gear will be at a disengaged position. The driving and driven plates will be separated slightly as they rub against one another, causing premature wear and tear. At D position, the governor or speeds sensor in the Electronic Control Transmission is still in operation. There will then be an automatic downshift, also known as coast downshift, to give better control of the vehicle and engine braking.

[omgwtfbyobbq]

That seems like it's for a specific transmission, especially the last bit about an automatic downshift. Any idea what for?

[bfg9000d]

It was talking about trans in general not a specific one. or atleast thats what I got from it. could be wrong.

[omgwtfbyobbq]

I think it's likely meant for a specific trans or family of transmissions since no automatic transmission I've driven downshifts after putting it into D from N or coast downshifts as the article puts it.

[bfg9000d]

I dont know. Bumping the transmission back and forth like that from N to D. Doesnt sound real good. After all you are playing with the transmission. You can be penny rich and dollar poor.

[omgwtfbyobbq]

I can do it all day long in my manual and all it'll do is wear the clutch. I don't see how it would do anything but the same thing (except w/ clutch packs) in an automatic. And, if it does blow, I'll have an excuse to drop a manual in. ;) If you search the forum, you'll find there have been posters who have asked the manufacturer if doing it is ok, and they said it was np. I suppose that would be the best bet...

[bfg9000d]

For now what I have been doing is just letting off the gas. It seems to coast for quite a long time this way and my rpms are way below one when doing such. I have tried it a few times in my older car. I dont notice much difference when in gear or not. I guess it varies from car to car. I tried it once in my newer car it gave me a slightly longer coast but not enough to warrant me bumping the transmission all the time.

[popimp]

I'll be damned I looked in my owners manual and saw this statement:

CAUTION!
Damage to the transmission may occur if the following
precautions are not observed:
• Shift into PARK only after the vehicle has come to
a complete stop.
• Shift into or out of REVERSE only after the
vehicle has come to a complete stop and the
engine is at idle speed.
• Do not shift from REVERSE, PARK, or NEUTRAL
into any forward gear when the engine is above
idle speed.
• Before shifting into any gear, make sure your foot
is firmly on the brake pedal.

Should I stop doing my coasts in Neutral? I guess there goes my MPG.

[skewbe]

FWIW, here is my interpretations, taking into account a fair degree of manufacturer paranoia:

• Shift into PARK only after the vehicle has come to
a complete stop.
N/A, stay out of park when moving forward.

• Shift into or out of REVERSE only after the
vehicle has come to a complete stop and the
engine is at idle speed.
N/A, stay out of reverse when moving forward

• Do not shift from REVERSE, PARK, or NEUTRAL
into any forward gear when the engine is above
idle speed.
If I may paraphrase: "Don't do neutral drops and smoke the tires for a block!!"

• Before shifting into any gear, make sure your foot
is firmly on the brake pedal.
Because there is an interlock, or they don't want the car moving accidently.

[popimp]

So should I stop?

[2TonJellyBean]

skewbe's not worried... any chance of overshooting your neutral shift? if not, i wouldn't be either

[popimp]

Someone enlighten me. What is actually going on when I put the van in Neutral at 65mph then back to Drive at 55mph? Why would this or wouldn't this mess up the tranny? The owners manual says not to. So I'm curious. The FE gains are obviously there.

[skewbe]

I suspect that the "small bump" is the transmission going into gear, and that the bump is attenuated at higher speeds as the tranny picks a higher gear to put the van in.

I don't see that the owners says not to, it says to shift into drive with the engine idling (or do not shift into drive with it not idling). I assume the brake comment was to keep the car from moving accidently, just an assumption though.

Does the manual say anything about towing the van? Does it need a dolly or anything? Does oil circulate in the trans when in neutral w/engine idling?

Best bet would be to ask the manufacturer or someone with model specific info, tell them you are conducting countless FE experiments and need to switch from neutral to drive and vice versa at various speeds. Tell them you are on a closed course too so they don't get too sidetracked.

[Telco]

I wouldn't worry about it, as was posted earlier this is a little manufacturer's butt covering. This is put in so when Little Johnny floors it in neutral, then shifts into drive, and drops the trans, they can deny the warranty claim as abuse. Just shift into drive, then apply the gas. Don't apply gas then shift in.

Now you don't want to coast with an auto with the engine off, almost all conventional transmissions have their fluid pumps driven by the input shaft. If the fluid isn't being pumped then bearings go dry and the transmission will burn itself up. If towed, the trans won't survive a 20 mile ride, so it might take 40-50 miles cumulative of engine off coasting to cause damage. If the trans pump is driven off the output shaft then there is no problem with engine off coasting because the rear end will be driving the pump. Sometime in the 1950s (I don't know exactly when) the makers started going to an input shaft pump so anything newer than that, with a couple of exceptions, are not going to be good for engine off coasting.

[Snax]

Popimp, and anybody else who is wondering:

An automatic transmission is essentially no different when it comes to the physical connection of gears to other gears and reductions and such in the gearbox. Aside from the obvious automatic nature of it, the only difference is that the gears always remain in constant mesh with either the output or input shaft depending on design, and each gear has it's own clutch and is selected by engaging the hydraulic pump that tightens it's clutch band or plate. No two of the gear clutches are engaged at any one time, and lockup bands or converters can only engage when one of the gears is actually engaged.

So essentially an automatic transmission, just a like a manual, shifts in and out of neutral numerous times between each gear selection as you drive. Selecting neutral on the shifter just tells the computer not to select a gear. Placing it back in drive, due to the pumping and valving action will in virtually every case, prompt the system to reselect the appropriate gear to re-engage.

The bottom line is that unless your transmission has an obvious issue with reselecting the proper gear, engine on coasting in neutral is no different to it in terms of wear except as it relates to the number of shifts required to regain your target speed. In fact, leaving it in gear while coasting down to a stop can actually cause MORE wear as the system selects each successively lower gear.

[bfg9000d]

well coasting in N to a stop will without a doubt not hurt the trans. Still I would believe the more shifting that occurs the more wear on the trans. If you feel a jolt when shifting to N to D I wonder how much stress that puts on the trans?

[popimp]

The manual says not to tow on the ground.

[Snax]

That just means the hydraulic pump is input shaft driven and that towing with the engine off could do serious damage to the internals.

If on the other had you are feeling significant jolts when shifting from N to D, then it's probably not a good thing to be doing. The stress on the gearbox, if the control system is operating correctly, should produce less shock on the driveline than the average manual shift however, as the torque converter should absorb much of the difference in rpm.

[popimp]

The Jolts are only felt at lower speeds around 30mph. At highway speeds, there is no jolt, just a smooth shift.

[Hockey4mnhs]

so dont rev match becase its alot smoother when i rev match at to 2k at 55mph?

[popimp]

Hockey4mnhs:

Rev match? At 55mph my shifting is smooth. It's a lower speeds where I feel a small jerk.

[2TonJellyBean]

Hockey4mnhs:
Rev match? At 55mph my shifting is smooth. It's a lower speeds where I feel a small jerk.

Hrmmm... perhaps the actual engagement force is the same regardless of speed but the momentum at higher speeds makes it hard for you to notice?

[omgwtfbyobbq]

Hockey4mnhs:

Rev match? At 55mph my shifting is smooth. It's a lower speeds where I feel a small jerk.I think that's a feature my auto has too. It'll shift into a higher gear then a lower gear at speeds below say, 45-50mph, in order to minimize shock and wear compared to just dropping it into 1st-3rd. It goes away at highway speeds since it just drops it into 4th and that's that.

[popimp]

So I'll keep coasting then.

[skewbe]

:)

[bfg9000d]

Just let me know when you go in for trans work.

[omgwtfbyobbq]

I think we all will... I mean, if someone does have trans trouble, it'd be great if they pulled it apart and figgered out what died. Then we could compare it to our trans models and see if those could fail in the same way. The best way to determine if it's worthwhile is to use the difference in mileage it results in to figure out how much the driver will save over some distance, find out when it fails, then compare it to the lifetime of other transmissions of that type. If it fails much sooner, and the cost is more than the gas savings, then it's a no brainer not to do. Otherwise, it's a wash up to a point, or definitely worthwhile.

[popimp]

I'll keep you up to date on tranny work. I'm not real worried right now. It's still under the manufacturer warranty and I bought a service contract from warranty direct that covers up to 100,000 miles. We'll just wait and see I guess. Thanks for all the input concerning coasting.

[landspeed]

Hi all!

Just to say that some cars, like my 'new' 1989 Nissan Bluebird 2.0iZX auto, come with a gearbox that can be switched in / out of neutral without the box getting confused at all - it very gently selects the right gear!.

I will let you know if the gearbox fails (noting that the gearbox is now nearly at 100,000 miles, which is when these gearboxes have been known to fail anyway!)

[ma4t]

I've done it. It didn't seem like it really did much because it was a short distance. I just drive conservatively, and get good FE for the vehicle I have.

There is a lot of talk about trading in your existing vehicle for something that gets better FE. Years ago I compared how much I was spending on gas with a guy who had just got a new Civic. He was saving about 5 to 10 dollars a month and started heckling me. So I asked him what his payment was. My car is paid off. He never brought it up again.

MA4T