D Or N When Slowing Down ?

[flotsam]

Colin Barber said months ago something along the lines that a car uses more petrol when in neutral than when in gear coasting, that might have been in relation to a manual so not sure of the auto effects like Kremmen mentions above, I just leave my manual box in gear a lot more now than I used to!

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In neutral, the engine management system has to supply enough fuel for the engine to idle.

In gear and coasting, it cuts the fuel.

[simontaylor]

Colinbarber made it clear in this post.

http://www.lexusownersclub.co.uk/forum/ind...c=20509&hl=auto

Colin, can you confirm this summary please?

Basically, coasting in N is not good for 4 simple reasons;

1-Safety, you are not in as much control of the car.

2-Failure, if the engine stalls while coasting, which is more likely while in N, power steering and braking will not work for long, and the LS is a heavy car. In fact, you probably will not hear it stall as it is so quiet any way.

3- In N the oil is not being circulated properly and could make parts of the box hot and/or overheat, which could be compounded by poping a hot box into Drive when it really needs all oil circulating for cooling. This is why you must not tow an auto car with the driven wheels on the ground.

4- Fuel conservation occurs when the box is slowing the car, as the fuel cut off is enabled and zero fuel is used while slowing down. In N, fuel is being used to maintain idle speed.

But when stationary (at the lights etc) being in N is ok besause the wheels are not turning and you will not be poping it back into drive at speed. When stationary, the load on the box is reduced when in N, and less fuel is used.

Colin, is that an accurate summary, and is it the case that all LS400s have a fuel cut while the autobox is braking the car? I specifically have a '95 MkIII with the stepped transmission stick. I actually thought that fuel cut offs were not used on petrol cars because the cat could get damaged by unburnt petrol, but it was ok on diesel cars for some reason, perhaps injection technology has moved on????

Many thanks in advance..... Simon.

[MrADeveci]

Well done that man !!!

That was the exact thread I was talking about ............... ::thumbs up::

[lexicon]

Im with Lexicon on this one. Read him and believe!!

LOL - cheers for that :D

Well done that man !!!

That was the exact thread I was talking about ............... ::thumbs up:: 

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Can't see anything that contradicts what I said - I think we're all saying the same thing here.

[ColinBarber]

Colinbarber made it clear in this post.

http://www.lexusownersclub.co.uk/forum/ind...c=20509&hl=auto

Colin, can you confirm this summary please?

Basically, coasting in N is not good for 4 simple reasons;

1-Safety, you are not in as much control of the car.

2-Failure, if the engine stalls while coasting, which is more likely while in N, power steering and braking will not work for long, and the LS is a heavy car.  In fact, you probably will not hear it stall as it is so quiet any way.

3- In N the oil is not being circulated properly and could make parts of the box hot and/or overheat, which could be compounded by poping a hot box into Drive when it really needs all oil circulating for cooling.  This is why you must not tow an auto car with the driven wheels on the ground.

4- Fuel conservation occurs when the box is slowing the car, as the fuel cut off is enabled and zero fuel is used while slowing down.  In N, fuel is being used to maintain idle speed.

But when stationary (at the lights etc) being in N is ok besause the wheels are not turning and you will not be poping it back into drive at speed.  When stationary, the load on the box is reduced when in N, and less fuel is used.

Colin, is that an accurate summary, and is it the case that all LS400s have a fuel cut while the autobox is braking the car?  I specifically have a '95 MkIII with the stepped transmission stick.  I actually thought that fuel cut offs were not used on petrol cars because the cat could get damaged by unburnt petrol, but it was ok on diesel cars for some reason, perhaps injection technology has moved on????

Many thanks in advance..... Simon.

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Yes that is an accurate summary. Your point about fuel cut on petrol cars not being good for cats isn't accurate though. If you have fuel cut then there isn't any unburnt fuel because it's been cut. Traction control that cuts the spark is not good because then you do get unburnt fuel on the cat which ignites and overheats the cat.

Fuel cut is used on manual and autos. It probably happens more on a manual than an auto because when an auto is coasting the revs are normally lower and get to the point where fuel-cut is stopped quicker.

I can notice when fuel cut occurs on my car and you can also see it on the fuel computer. If I come off the throttle at say 50 mph my mpg reading will go to 99.9mpg. Once the vehicle gets to around 30mph the engine speed gets to around 1000-1200 rpm and I can feel a very slight pick up. The fuel computer will then drop down to a normal reading (about 80mpg). If I then drop the gearbox down to 4th or 3rd the engine speed will increase to 1500-2000rpm, fuel cut comes on and the mpg reading goes back up to 99.9.

[MacRS200]

Bloke I played rugby with, who was a traffic cop, told me "use the brake to slow down not the engine, brakes are cheaper to replace than a clutch". Same theory I guess.

[JellyBean]

But whatever is the point in buying an auto and then pulling & pushing the gearlever about?

It's like buying a dog and then going up to the front gate and barking yourself.

[youngteam]

My previous auto car (Mitsubishi FTO) would automatically change down a gear if it started to runaway with itself down a hill, etc. In fact, as it was a tiptronic "sports" car, it actively encouraged you to drop a gear to lose some speed.

[tonupkid]

LOL. A sports car that drops a gear to help you loose speed.

Methinks, perhaps it was doing so for another reason.

[Straight-Six]

Does anyone know why we have a 3,2,L on an auto box if its not for engine breaking then...apart from towing caravans up steep/very steep..and silly hills

Im sad i know but have just got my manual out to find out the answer..

"3-Position for engine braking(shifting into overdrive not possible) "

"2-Position for more powerful engine braking "

"L-Position for maximum engine braking "

So surely if this is what these gears are what Lexus designed them for then thats what we should be using them for.I think if they were worried about harming the engine they wouldn't recommend the above...no?

[ColinBarber]

There's nothing wrong with using the gears for engine braking although with normal driving there's no point, just use the brakes.

I tend to downshift when I want to overtake so when I press the throttle I just go rather than waiting for the lazy gearbox to downshift automatically.