Coasting with the car on neutral
 

Is there any damage that can be done (Long OR Short term) from frequently putting any car with Automatic Transmission into neutral to coast to a stop (and somehow get better MPG)?

I've done this once on a freeway that went downhill, but now I'm getting worried that I could be damaging the transmission at the expense of MPG.

You won't really get any better mpg by doing that. If you're coasting downhill and the engine is at 2000rpm, it will not be using any more fuel than if it was at 700rpm....the fuel rack in the IP dictates the fuel used, and if you're not on the throttle, then the engine isn't using any more than an idle-amount of fuel. So essentially its pointless to do that....

It will actually use more fuel.

If the engine is idling it is burning fuel.

If instead, the engine is trying to idle, but the RPM is above idle, the rack will close to the minimum fuel position which is less than the idle flow.

However, it will do no damage to coast in neutral with the engine idling.

Huh, I didn't know the rack could close even more than idle. You learn something new every day. :D

but his question was.....
 

So, aside from that, how about the answer to his question??

His second question- will the transmission be damaged by engaging it at highway speeds while the engine is idling?

What does the oracle say about that?

No.

Its better to keep it in gear. In neutral everything is spinning at different speeds and wearing. In gear its locked into one ratio so only one gear set is moving.

Good one! I was actually looking for that answer!

The OP mentioned coasting to a stop and putting it in gear, fine. Putting it back in gear at highway speeds should IMO be avoided.

Coasting in nuteral
 

This is illigal in some states as it tends to damage cat converters due to consant cooling and heating cycles.

I know; the temp difference is very slight, but over the long run every bit of wear adds up.

Plus the price of Mercedes cats is not cheap.

Pooka

oops!
 

Sorry, forgot I was on the Diesel forum. Old age, you know.

Pooka

Still illigal in some states. classified as "control of vehicle" or something like that...

I have done it before, on some of the stoplights I have are a 60mph to stop. That is a lot of coasting at idle. I did see a slight MPG gain over 2 tanks. But not worth it to me.

I would be concerned about how well the secondary fluid pump on the tranny would be circulating the tranny fluid, keeping pressure and cooling at an optimum level.

If the engine is idling, it'll be fine. I've towed cars with the engine idling in neutral for long distances.