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  #1  
Old 06-07-09
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Default Rolling in an auto

Hi guys,
Not sure if its been covered before, but the other night i was driving down brookton highway (basicalls a 5 - 10 kilometer down hill run) and had the auto in neutral and rolling (to save petrol) just wanted to know if this is ok to do with an auto, or if i should not do that at all??

Thanks

Marc
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  #2  
Old 06-07-09
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Hi, this is not a fantasic idea, as the auto tranny needs oil pressure to lubricate the internal parts of the tranny, the pressure is supplied from the torque converter driving the oil pump in the front of the tranny, at idle speed,(as you would be coasting in neutral), this would not spin the torque converter quick enough to pump the required oil to all parts of the transmission, the short term effects would be minimal, long term effect $3,500,00 for a reco tranny........................that is a shit load of fuel...........dont you think??...........so i would say it is not a good idea at all, ...........a manaul on the other hand in neutral.......no problem!!
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  #3  
Old 06-07-09
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hhmmmm 3500 is alot of money... and alot of fuel!! seems like im not saving anything! On my ford falcon I have a trip computer telling me the current fuel usage, when im rolling down a hill in neutral in that (auto too) the usage keeps changing, but the rpms stay the same, and idea why this happens and if it would happen on the paj. Also on the falcon, if i slow down using the motor (semi-auto) by selecting a lower gear, it uses roughly 0.2 liters per hour (much less than idling!) anyone know if the pajero does this too?? I know its like comparing apples and oranges but im curious to know as both are semi-auto petrol motors of roughly the same size.

Thanks

Marc
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To-do:
brake upgrade, oil seals (again!!)
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  #4  
Old 06-07-09
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We used to call that "angel gear" LOL. Problem used to be that you'd get to the bottom of the hill and find that the engine had quietly died and you suddenly had no vacuum to power the brakes..

Not worth it IMO. Better to use the engine braking to control your decent.

Fuel usage in this scenario is governed by the ECU. I most modern cars the fuel supply is cut right back on overrun to help the vehicle come within emission guidelines. So you won't actually save any fuel at all by using "angel gear"..
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Old 15-08-09
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Quote:
Originally Posted by marcthelegend View Post
........anyone know if the pajero does this too?? I know its like comparing apples and oranges but im curious to know as both are semi-auto petrol motors of roughly the same size.

Thanks

Marc

Yes, the Pajero runs a fuel cut on 'over run', basically the engine just becomes an air pump and the engine will run cooler as there is no fuel being added and no ignition taking place..

On one of my other cars I have an additional water temp and oil temp guage and when coasting down a long hill in 'over run', I have seen the water and oil temps temp drop about 10 degrees or more...
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Old 15-08-09
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Quote:
Originally Posted by MSF View Post
Yes, the Pajero runs a fuel cut on 'over run', basically the engine just becomes an air pump and the engine will run cooler as there is no fuel being added and no ignition taking place..

On one of my other cars I have an additional water temp and oil temp guage and when coasting down a long hill in 'over run', I have seen the water and oil temps temp drop about 10 degrees or more...
This is great news! will definitely start selecting lower gears when going down a hill??? or does it do that just normally rolling??

Regards

Marc
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05 NP GLX 3.8 Auto. 2" Lovell/Bilstein Lift, ARB Deluxe Winch Bar, Granke mk3 12,000lbs winch, Uniden UH015sx, HID spotties, Roof mounted light bar, Work lights, Upgraded stereo, Tinting, 2.5t tow, dual battery setup (homemade), Radar Renegade tyres, wired up dummy lights, Bushskinz Sump/Intercooler plates, home-made diff breathers (front and back) and a cheap ebay snorkel.

To-do:
brake upgrade, oil seals (again!!)
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  #7  
Old 15-08-09
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Quote:
Originally Posted by Hawk-eye View Post
Hi, this is not a fantasic idea, as the auto tranny needs oil pressure to lubricate the internal parts of the tranny, the pressure is supplied from the torque converter driving the oil pump in the front of the tranny, at idle speed,(as you would be coasting in neutral), this would not spin the torque converter quick enough to pump the required oil to all parts of the transmission, the short term effects would be minimal, long term effect $3,500,00 for a reco tranny........................that is a shit load of fuel...........dont you think??...........so i would say it is not a good idea at all, ...........a manaul on the other hand in neutral.......no problem!!
Same goes for flat towing an Auto car. Always remove the tailshaft if you have no option other than to flat tow.

As for 'no problem' with a manual. There is a warning on the back of the sunvisor in my Hilux 4WD about removing the tailshaft for flat towing, something to do with damaging the transfer case. Anyone know anything about this?
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  #8  
Old 15-08-09
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Several comments:

I believe it is illegal to have a moving vehicle in neutral. Could be wrong.

Many EFI vehicles will "idle rich", others will enter "closed loop" control - cycle from rich to lean and back again. This could explain why the Falcon fuel consumption changes while coasting in neutral?

Flat towing: years ago, on a weekend away with friends, a diesel Hilux blew an oil line (separate hose somewhere near the alternator?) and dumped it's oil - at highway speed. Unfortunately, the oil light wasn't working, so the first the owner knew about the problem was when the engine stalled - at highway speed. Long story short, engine became a boat anchor, and had to be towed home. I questioned pulling the tailshaft for the flat tow, and the owner checked the manual - said it wasn't necessary.
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  #9  
Old 16-08-09
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Quote:
Originally Posted by marcthelegend View Post
This is great news! will definitely start selecting lower gears when going down a hill??? or does it do that just normally rolling??

Regards

Marc

It will do it in any situation where you are on a trailing throttle or engine braking is occuring. Ie, not accellerating, not coasting on slight throttle..

So, you may not have to select a lower gear to be on a trailing trottle, the roadspeed just has to be higher than idle coasting...
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  #10  
Old 18-08-09
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I've asked this before but can't remember the answer. Using engine braking on an auto I believe some Mitsis have a lock up converter. I have an NJ and I don't believe I have one. Is it possible to get one fitted after market? I am specifically talking about working in 4llc down a steep hill and having the engine braking negated by run on.
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