Auto 'coasting'

[redz]

i have been advised that shifting into neutral to 'coast' in an auto can cause problems?

true or not?

[Turks]

I'm sure you would be fine for very short periods... why would you want to?


Turks.

[pcdee]

I'm sure I read somewhere it's not advised.

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[Cockneyjanner]

In standard driving lessons and in advanced driver training it is always advised against doing this, as you are less in control of the vehicle whilst in neutral.

Why would you want to do it?

[AW8]

I am guessing the some may want to do this to try to improve mpg.

I agree re the car control issue to some extent albeit descending a steep hill in (wrong) too higher selected gear in manual or steptronic auto will still result in a compromise in control and/or additional wear to brakes.

The main reason why coating is discouraged in an auto is because it can allegedly damage the gearbox. Arguably it's not ideal for differentials either.

I am sure the question may be asked as to why the OP would want to own an X5 and have an agenda to save on running costs......We could ask same of everyone who upgrades to another 4x4 or other car with improved mpg as a motivating factor.


Oh and I don't coast in mine btw.

[X5Sport]

If you don't turn the engine off then no damage will be caused as the circulation pump is still running.  Putting it into 'D' before coming to a complete stop can burn the gear set clutch and brake bands though. It's always been a no no with autos ever since I learned to drive four decades ago.

One other  thing to note is that most modern engines actually cut fuel off while on the overrun so you would not save any fuel.  Even autos now have clutch lock ups to keep the gearbox turning the engine and generate engine braking.

I know my Dad used to do it in our old Morris Oxford traveller but that had a carb, not fuel injection.  Can't see any point to doing it nowadays.

Richard

[AW8]

I am not a transmission/drivetrain engineer but I understand the thoughts behind comments in this link

http://forums.anandtech.com/showthread.php?t=121267

[shadrack]

i know i once knocked my x5 into N, then when i put it back to D doing around 70 it brought up 'trans fail safe'

on a different point the re lack of control and coasting, newish volvo trucks have a mode called eco+ and this drops them out of drive to free wheel on hills etc automatically so you get 44t free wheeling to save a few co2s from the 13-16ltr engine

[andym3]

[quote="redz"]
i have been advised that shifting into neutral to 'coast' in an auto can cause problems?

true or not?
[/quote]
Don't do this in a auto, the gearbox will go bang in no time!!!

[allforus]

On the F15 in Eco pro mode the gearbox goes in to free wheel if the down hill section is long enough  and no battery top is required

[Finglonga]

[quote="AW8"]
I am guessing the some may want to do this to try to improve mpg.
[/quote]

Good theory they have, but in gear the engine it being turned by the wheels and not using any fuel, it is cut off completely. When coasting the engine will be ticking over and using more fuel.

I would be more concerned in getting the box back into Drive after coasting in Neutral if the vehicle is still moving.

[X5Sport]

[quote="allforus"]
On the F15 in Eco pro mode the gearbox goes in to free wheel if the down hill section is long enough  and no battery top is required
[/quote]

But the gearbox components are all in the correct position to take up drive again which won't be the case in Neutral. 

There must be some connection as the fuel will cut off on the overrun whereas in true free wheel it must be using fuel at idle so not saving anything at all - which seems pointless (to me).

[redz]

i have always assumed, maybe incorrectly, that it saves fuel. also i notice that quite often when going down hill the revs are really high and i get engine braking. have never had this in an auto before but suppose its just as easy to switch to manual if it bothers me that much!

[X5Sport]

Current generation autos all have engine braking to prevent run aways due to brake overheating/failure.  Some BMWs have a power regen system that kicks the alternator in on the overrun too as a way of keeping the battery topped up.

Richard