Hello all,
I have a question; When driving my 3Litre BMW Diesel lump around, I often drop the tiptronic automatic box into neutral when I can see the traffic well ahead is stopped. As I can sometimes see 1-2 miles ahead, I do it from 70mph sometimes on the dual carriageway and arrive at the slow traffic at a leisurely 30mph...
My mecahnic has said " if you run an auto in neutral, the pump works extra hard above 30mph, or 55mph in some cars, and the whole gearbox can suddenly go".
Does this make sense or is freewheeling / coasting an auto box possibly stressing the car? It's a 110,000 Mile E39 3L Touring.
And yes, I have heard about coasting "not being in control of the vehicle" but when I'm spening £80/week on diesel to get to work and back (15miles direct, 25 by road each way), I need to save every drop!
Thanks
p.s. I have not tried the "Acetone in Diesel" trick yet - that's next in line. (I was a proponent of this a while back)
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ISTBC, but I believed modern engine management systems effectively shut-off any fuel supply when engines were without throttle input so 'idling' in neutral actually uses more fuel , i.e enough to idle as opposed to none at all.
So it's more economical to leave in gear & coast (slight engine braking though) than to put 'box into neutral.
Edited by woodbines on 16/01/2008 at 15:10
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as towing an auto any great distance is a sure-fire way to destroy the gearbox isn't this just the same thing? I don't see how putting it into neutral is going to save any more fuel than just lifting off the throttle.
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Not an auto box specialist, so its not gospel advice, but....
I also agree it is not good for the box to be in neutral with the driven wheels rolling. Gear boxes are not cheap either, so certainly not worht the risk.
And £80 a week in diesel for 50 mile round trip daily? Something is using too much fuel I suspect? For that mileage/vehicle I would be expecting £ 30/£35 tops ?
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You may be correct that leaving in gear means that the engine uses no fuel, but the important bit is that the engine braking means you will not coast for anywhere near as far.
My real world experience is that costing in nuetral can make a big difference to the amount of fuel used. The OP seems to understand how this works and is reading the road ahead well in advance.
As for coasting in neutral in an auto, I'm not sure but I wouldn't advise it.
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If you are so much worried about saving every drop of fuel, why not drive a smaller engine??
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It's using far too much fuel. I used to do 50 miles a day in my 530. It would cost me about £65 every 8 days and its a 3 litre petrol not diesel.
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>>" if you run an auto in neutral, the pump works extra hard above 30mph, or 55mph in some cars, and the whole gearbox can suddenly go".
Sounds dubious to me - for a bit of sport, it might be worth asking the mechanic to expound upon his theory a little bit more. See how creeative he is!
I would be amazed if the oil pump were not engine driven, so would still be turning, and still lubricating the gearbox. The pump doesn't need to do anything more work when the gearbox is in neutral, because there's nowt happening.
Number_Cruncher
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Doesn't make any odds. Oil pump runs off input side so line pressure is up if engine is running. Nothing to worry about. Shifting it into neutral is an odd thing to do though.
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I can't see a mechanical issue, but some newer cars won't let you go back into D without pressing the brake.
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£80 is about 75 litres 16.5 gallons. 250 miles a week commute is 15 mpg!
You either do a lot more miles in addition to your commute, or your car has a serious problem.
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Thanks for all the replies.
My daily commute is 50/day x 5 = 250miles.
Extra mileage (shops / dump runs / gym / evening and weekend stuff) brings the miles up.
The daily commute is also through 12 miles of country B Roads in order to avoid Hindhead (A3 Surrey) and saves me around 25 mins each way but is more heavy on fuel as it's more accel/decel.
Typically I am seeing587 mailes to 69 litres.
Computer reports 47 mpg but actual is around 38mpg.
I have the 530 estate as I am renovating a house and need to (v. occasionally) hitch up a 2 ton trailer to take rubble/scrap/soil away and buy cement/sand/plasterboard/ fence panels/ etc etc. Also, the ability to fit a sofa/concrete mixer etc in the back helps.
The fuel figures represented above do not include any towing/heavy shifting.
When I got the car the first thing I did was get the transmission fluid changed as it's "sealed for life" but the mechanic said it was "black and stinking" so = cooked. Changing it was the best chance for the box, but am I endangering the life by coasting?
If I decelerate in gear, I don't travel anywhere near as much as freewheeling.
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Ahhh, that amkes sense. Explains why towing an auto is a problem - the engine isn't running, therefore oil pump not running, but 'box still being driven by wheels.
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>>the engine isn't running, therefore oil pump not running, but 'box still being driven by wheels.
Yes, that's the vital info. Some, better, automatic gearboxes do have a second pump, driven at the output end of the gearbox. These gearboxes will provide some lubrication during towing with the engne off, and some will even allow the gearbox to select a gear, thus allowing the engine to be bump started.
Number_Cruncher
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I think it has to be said that if things are so tight that coasting makes a noticeable and worthwhile difference to you then you shouldn't be running an automatic BMW 5 Series.
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Coasting can make a big difference, especially if you are doing 250 miles a week.
regardless of wether things are tight or not, then whats wrong with trying to save a bit cash and reduce your CO2 emissions at the same time?
To those who say that you save more fuel by coasting in gear, don't forget that MPG is calculated by distance/fuel consumed - a car coasting in gear may use less fuel but it doesn;t travel anywhere near as far - there are two parts to the equation!
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Coasting can make a big difference especially if you are doing 250 miles a week.
Unless you spend your entire trip driving down a hill I really can't see it!
reduce your CO2 emissions at the same time?
Everything is wrong with that, the CO2 reduction principle is the most flawed thing I have ever heard.
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You don't always need a hill for coasting. For example, approach to traffic lights and slight inclines and even flat roads can also work for coasting.
Please expand on the flaw in the CO2 reduction? Surely by getting more MPG you are reducing your CO2 emnissions?
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The Mercedes W203 C220CDI automatic I used to have as a company car would go automatically into a freewheel mode when I took the foot off the accelerator below around 80km/h when the lever was in "D" and the mode switch in "S". It was possible to tell because the revs would immediately drop without any effect on the car's speed.
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thats just stupid freewheeling an automatic gear box. besides which to drive more economically you must stay in gear to the last moment then dip the clutch on a manual as for the auto it does this as the name suggests automatically. the key is not to use your brakes as much. just ease off the throttle and if you are saying you can see traffic building 1 - 2 miles ahead of you then just ease off the throttle. as for your fuel consumption i suggest you clean the fuel filter just off the tank, as on the diesel engine wax builds up in this filter so less fuel gets through to the engine and thus works harder. try regular oil changes using the proper oils. air filter changes and to have such a heavy right foot.
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Output pumps disappeared from auto g/boxes in the 60's;you could never bump start them in the normal manner-needed to be towed(or pushed) upto 40 mph+ to start them.
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>>Output pumps disappeared from auto g/boxes in the 60's
There were some fitted into the 80's on some makes.
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We got an auto too ,but based on my experience in the manual Galaxy with the trip/fuel computer. when on the overrun or going downhill.
It reads 99.99mpg
ergo not using fuel
DESPITE still being in gear
and if I am in fifth, i use the small amount of engine braking to slow up/ coast
strangely I have never "freewheeled" downhill or otherwise
i manage about 50 mpg summer, 52.5 mpg over 100 hrs driving recorded on camera
45 mpg winter
though in fairness mostly running solo
cheers
M
Ps
I find the drag induced by my anorak hood is significent
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