Neutral to drive coast, is it bad for newer auto trans - Fuel Economy, Hypermiling, EcoModding News and Forum

hendgeofdeath · 03-26-2012, 04:21 AM

I would like to start dropping my automatic transmission into neutral to coast to stop signs. Problem is I'm not sure if its safe if I have to go back to drive while still in motion. I have an 07 Malibu, and its not worth it to me if I'm doing any sort of extra wear and tear on the trans. Your expertise is appreciated.

gone-ot · 03-26-2012, 01:18 PM

...which transmission do you have in your '07 Malibu? The current 6T40 (RPO: MH8) didn't come out until 2008 in the Malibu.

...quickest answer is ask your dealership if the transmission in your Malibu has "input pump" only...which means the lubricating trans fluid stops moving when the input (from torque converter) shaft stops...meaning no-fluid flow = no lubrication = things can fail.

...as a "general" statement only--if the car has to be dolly-towed, because the FWD wheels shouldn't be rotating while towed, the transmission should not be coasted-in-neutral for either long distances or at high speeds.

Krayzie · 03-26-2012, 01:42 PM

Quote:

Originally Posted by Old Tele man

...as a "general" statement only--if the car has to be dolly-towed, because the FWD wheels shouldn't be rotating while towed, the transmission should not be coasted-in-neutral for either long distances or at high speeds.

I thought this was only for engine off coasting? Or is it a problem with the engine on as well...

UFO · 03-26-2012, 02:12 PM

Quote:

Originally Posted by Krayzie

I thought this was only for engine off coasting? Or is it a problem with the engine on as well...

Yes, just engine off. I've been engine-ON coasting with my automatic for a while, and it has the pump on the input shaft.

In any case, it takes practice, but if you can do it smoothly it shouldn't be a problem. I've been doing it everyday for a year or so with my notoriously unreliable O1M VW automatic and it has not developed any issues (yet).

gone-ot · 03-26-2012, 03:42 PM

Quote:

Originally Posted by Krayzie

I thought this was only for engine off coasting? Or is it a problem with the engine on as well...

...

Yes, you are correct, I didn't describe it (...when the input (from torque converter) shaft stops...) well enough, sorry. Engine OFF is problematic.

mcrews · 03-26-2012, 04:52 PM

I hae 255,000 miles on my Infiniti Q45 and have coasted (engine on) for 210,000.
Still have the same tranny.

I will say that a drove a ford tarus 2009 and it didnt like neutral coasting.

gone-ot · 03-26-2012, 06:28 PM

...most of the "older" automatic transmissions had both front & rear hydraulic "pumps" so the car could be push/bump-started. Many of the "newer" AT's now only have a "front" pump, so when the input isn't turning, there's no fluid pressure or flow.

...which type transmission do you think the Infiniti Q45 has?

Whoops · 03-26-2012, 07:59 PM

My experience, I did this with my wife's van. It cost me $2000, to put a new transmission in. The amount of money you can save, just isn't worth the risk, IMO.

roosterk0031 · 03-26-2012, 08:30 PM

And with both of my GM's N coasting over D coasting has no advantage, or at least so small not worth the effort.

Thymeclock · 03-26-2012, 10:28 PM

Quote:

Originally Posted by hendgeofdeath

I would like to start dropping my automatic transmission into neutral to coast to stop signs. Problem is I'm not sure if its safe if I have to go back to drive while still in motion. I have an 07 Malibu, and its not worth it to me if I'm doing any sort of extra wear and tear on the trans. Your expertise is appreciated.

What you describe should cause no harm, but here's what you need to know. The potential problem comes not with putting the transmission into neutral and coasting, but rather when you re-engage it into drive. If you coast to a stop (or an almost stop, meaning below 10 MPH) there will be no harm incurred.

Most automatics are designed with the intent of being shifted from neutral or park into drive, as starting from a standstill. The potential problem arises when you are moving at speeds much greater than that 10 MPH threshold of first gear. I once asked the same question of a transmission technician and he told me the torque converter cannot determine how to handle the sudden transition from neutral into a much higher gear. In other words you not only risk damaging the transmission but the torque converter as well. However if you coast to a stop (or an almost stop) before re-engaging back into drive, there should be no problem regardless of the transmission design.

I've been doing this for a long while now on my four cars (all are automatics) and have found no problem from doing it. However, bear in mind that if the stop sign that is your target point is 1/4 mile off and someone pulls out at a slower speed in front of you while you are going 40 MPH, you probably will need to coast back down to 10 MPH before re-engaging into drive. Thus I never coast to traffic lights - because if the red light turns green while you are approaching it at 30 in neutral, your options are few because you can neither go back into drive nor accelerate.

In my experience neutral coasting an automatic does help improve FE somewhat - but the situations where you can implement it are limited, as traffic often makes it impossible to do.