Automatic transmission - coasting damage?

[BenScoobert]

I have a 2003 GMC Yukon XL 5.3 vortec.

Here in Britain petrol is pricey, so I run my yukon on LPG at less than half the price. I get about 18MPG(UK Gallon) but we can always do better

However, there is a long hill on my drive to work which I can coast down.
I was wondering does anyone know if it does damage if you still have the engine running but put the auto gearbox in neutral.

I have done it a couple of times and instead of reading 3Litres/100Km, it reads 1Litre/100Km.
Someone told me you shouldn't coast automatics, so until I find out more info I'm back to leaving it in drive.

[Angmaar]

No, there is no damage done by coasting.

[UFO]

When I coast my automatic, it's when I intend to stop a ways ahead and engine braking is too much deceleration. I leave it in neutral until the light turns green, there seems to be a significant reduction in fuel consumption idling in neutral vs. idling in drive.

I've also pushed it back into drive while still coasting occasionally, and it doesn't seem to do anything bad, but I hesitate to do it regularly.

[BenScoobert]

Originally Posted by Angmaar

No, there is no damage done by coasting.

Any idea why some people think it does? Maybe they are thinkingthe engine is being stopped...

Originally Posted by UFO

When I coast my automatic, it's when I intend to stop a ways ahead and engine braking is too much deceleration. I leave it in neutral until the light turns green, there seems to be a significant reduction in fuel consumption idling in neutral vs. idling in drive.

I've also pushed it back into drive while still coasting occasionally, and it doesn't seem to do anything bad, but I hesitate to do it regularly.

I also drop it back into drive whilst moving, seems smooth to me even at 50-60MPH

[Angmaar]

Originally Posted by BenScoobert

Any idea why some people think it does? Maybe they are thinking the engine is being stopped...

That's probably it. Most automatic transmissions can't be driven when the car is off.

[saand]

the age old auto transmission coasting debate,
There are plenty of threads on this one but here is my understanding of auto transmission coasting.

The general check people seem to do is to check if their car can be flat towed (towed by a truck with all 4 wheels on the ground compared to just the back 2 wheels). If the car can be flat towed then there is little risk in coasting in neutral since flat towing is basically coasting in neutral.
People are generally concerned about a lubrication issue, with the transmission unpowered some people believe that the oil pump isn't pumping much or any oil in the transmission which will cause gradual damage to the transmission.

As for my own experience, i have been neutral coasting for maybe 6 months now and i have had 0 issues, i pulse and glide (glide in neutral) then pop it back into gear after reving the engine to rev match.
I also have recently installed override switches for the torque converter lock up and the gear selection solenoids. With the torque converter lock up control i have little worries about shifting back into gear when coasting because i just unlock the torque converter shift into drive (the slush box takes up the slack i believe without any stress or damage) the engine naturally matches RPM when i put the gas pedal down a bit then i lock up the torque converter and keep driving. Its only an issue when i forget to unlock the torque converter when i coast down to lights, i pop it back into gear and the car has stalled out once or twice.
Regarding going back into drive while still moving at high speed, when i didn't have override control of my transmission i did have some issues if i was traveling at around 60 mph, if i went back into drive without rev matching well the transmission would shift down a gear and rev really high.

[Thymeclock]

Regarding going back into drive while still moving at high speed, when i didn't have override control of my transmission i did have some issues if i was traveling at around 60 mph, if i went back into drive without rev matching well the transmission would shift down a gear and rev really high.

Yes, that is the problem and it can be detrimental.

Automatic transmission design can vary, and whether you can shift back into drive while in motion without incurring damage is questionable. One of the most expensive repairs imaginable on any vehicle is an A/T rebuild - so be careful. Saving a few $$ in fuel is not worth causing well over a thousand $$$ in repair.

I'd suggest you consult with a transmission shop and ask them the question directly. They can give you good reasons why you shouldn't do it, if it can't be done without doing harm.

[BenScoobert]

I know mine can be "flat towed" but it involves stopping and holding 2wd hi and 4wd low for 10 seconds. Obviously this is something I cant do at 50-60mph.

Due to the large engine of the yukon it cruises at 50mph at about 1500rpm, idles at about 700, I'll take note today whether the rpm spikes when I go from neutral to drive at 50mph.

I don't suppose anyone on here knows anything about the specific transmission this has? It was built in USA in 2003 and privately shipped to UK, but when I check gallery pictures on edmunds website it seems mine is identical to the 2004. It has the 285bhp 5.3 vortec.

It can be hard to find out info here in the UK as GM didn't release the suburban/tahoe/yukon/yukon xl in the UK, although recently they started selling the escalade and H2.
There's probaly less than 100 of them in the country and garages want to charge a premium for their know how. But I guess its the price I pay for having an American car in England, your cars are so much better than ours.

[BenScoobert]

Coasted it today, to observe any spike in RPM, there was none.

[some_other_dave]

The only issues I know of are that some auto transmissions suffer the aforementioned lubrication problems if you coast with the engine off. With it on, there shouldn't be any problems.

-soD