Automatics and Neutral?

[Gone4]

I've read people arguing both sides of this coin so I wanted to know what everyone here thought or knew. Is it safe to transmission pretty quickly. I've also seen the same arguments for overdrive.

I ask because switching into neutral on the mountains is very very useful sometimes...

[roflwaffle]

I've done it for around 10k miles so far. I've read others have done it longer w/ no ill effects. That being said there will always be naysayers who whine about the possibility of working on their own car, so if you're in that camp, don't bother. Otoh, if you find the idea of a swap to a manual transmission if/when the auto fails appealing, git 'er done!

[tputus]

Some manufacturers warn against, some dont. On a Volvo v70 diesel, the revs go up to about 1300 rpm until you press the brake pedal enough for the brake light to switch on. Shifts smoothly to a suitable gear at any speed anyway.
I can not think of any way to damage the 'box if the engine is running. That way you must have normal oil pressure to lube the bearings? Disclaimer: I'm not a gearbox expert.

[elhigh]

Rule no. 1: the engine must run if the wheels are turning. Seriously. I don't know how many miles it takes to ruin an automatic, but the input shaft is integral to the lube getting around inside there while it's turning, so there's that.

Here's another thing: shifting into neutral in a rental Impala, I noticed my instant MPG would skyrocket, but usually not bury the gauge at its upper limit of 99. Coasting with my foot off the gas, but the tranny still in gear, the instant MPG would hit the upper limit and stay there until I got back on the gas. The reason:

The ECU was completely shutting off the fuel to the engine. The tranny was dragging the engine along at or above the cruise control setspeed, no more fuel was needed, and the engine was doing above idle speed, so no fuel needed there, either. The fuel injectors simply stopped injecting.

Your experience may vary, especially if you're driving a carbureted vehicle. This was my experience, and that was in a late-model large sedan that was, frankly, optimized to do its utmost with every gallon at cruising speed. For such a large car, 27.8mpg at 75mph is really very very good.

[Widetracker]

Hot Rod discusses this in a tech question this month for sticks. I'll see if I can dig up the page number. Basically the same conclusion though. In a carburated car it makes sense but with EFI it actually uses more to idle than engine braking since the injectors are not squirting fuel.

[roflwaffle]

Originally Posted by Widetracker

In a carburated car it makes sense but with EFI it actually uses more to idle than engine braking since the injectors are not squirting fuel.

The difference is that the car in gear won't travel as far as the car in N does, even if the car in N is using fuel. In certain situations, such as steep downhills, N would result in the car going too fast, so coasting in gear is a win-win since there's N will result in excessive gas use and excessive brake use, or excessive gas use and the dangers associated with breaking the speed limit. It depends on average fuel consumption of some distance at some speed/load compared to peak load as well as idle fuel consumption. This should help out with what's happening.

[DifferentPointofView]

What About MFI? I don't know about the neutral thing. My vehicle coasts about twice as far in gear than in neutral. Neutral doesn't get the slight pushing force of the tranny as is in gear, and my vehicle's electronic system shifts seamlessly into neutral at high coast speeds anyway, and goes into gear when it's supposed to be out of O/D because the rpm's aren't way high like in neutral would be.

[roflwaffle]

If the engine speed under some amount, usually ~1500rpm when warm, the ECU will start injecting fuel and the auto will pretty much be idling in gear, even if your foot is off the gas pedal.

[DifferentPointofView]

What exactly does throwing it in neutral do? shut off fuel and the drive train from moving? All I know is, I can coast way farther in gear than out of gear, and that it will be in neutral coast when I'm first coasting.

[roflwaffle]

N disconnects the engine from part of the transmission, so instead of moving the whole engine/transmission, the wheels only move part of the trans. When you're coasting in gear the above some speed the engine is operating just about as inefficiently as possible.