Will this kill my transmission?

[MizBlackCrow]

Hey guys. Forum newb here. But I came across an idea that seems brilliant and, because it's me, is probably not. Here's my thought process:

Background: I drive a '99 Corolla VE with a 3AT (sadness!). I drive mostly in the city with a few highway jaunts. I'm averaging 26-28 mpg, which isn't terrible. But it could be more. Oh, and the awful thing is, I have no tachometer (so none of my measurements can really be scientific).

So I'm considering starting to put it into neutral when I'm idling at a light and when I'm coasting, then putting it back into drive when I need to accelerate or start from a stop. (This is quite easy for me, since I don't have to push the button on my shifter to switch from N to D). I tried it a couple times on my drive home, and here are my findings:

1) I can coast a LOT farther! I think this is because with my foot off the gas with the car in drive, I guess there's some engine braking going on? Whereas with the car in neutral, there's no resistance and pure momentum...
2) The revs are lower when coasting. Obviously, since the engine isn't trying to do any work.
3) I have no way to know this empirically, but I'm guessing that the engine revs lower at idle in neutral? I know that to avoid a slight lurch I have to rev it juuuuust slightly to put it in drive at a standstill.

The real question, though, is whether or not I'm going to destroy my transmission with this. I've already gathered that I need to rev-match to put it into gear when driving or even at a rest. But I'm concerned about the life of my (already 12-years / 115k mile) transmission. Any idea if it'll be worth it? It's way too soon to tell if I'll get any better FE by doing this (but if it's not crazy dangerous for my car, I'd be willing to give it a shot).

Oh, one last thing... a friend of mine seems to think that I could do damage to my transmission/engine by braking hard while coasting with the car in neutral. I can't see any way for this to happen, but I thought I'd throw it out here... is my friend crazy?

Thanks in advance.

~MBC

[Tony Silva]

Brilliant idea if it actually worked. Your car can get more than just a damaged transmission. There are lots of sensors that manage the engine emissions system. Engine operating temp vs engine air intake speed vs the vehicles actual driving speed. The computer takes all these (and more) into its consideration on how to make adjustments to whats going on under the hood. Plus, what happens if you suddenly have to speed away to avoid a collision? What are you gonna do then? Put the car back in gear and gently bump the throttle up to match the engine speed with the road speed ao as not to damage the transmission? Or are you gonna yell, "Oh! Shiiiiit!!" and immediately floor the gas pedal in a panic put the car in gear at the same time? Gas pedal to the floor...engine speeds up and the transmission finally slams into gear.


And just so you'll know, coasting your car to a stop is rediculously hard on the brake pads and brake rotors. Yes there are alot of manual transmissions in small cars AND those brake rotors and pads are actually bigger than those that are on a car with an automatic transmission because there is some vehical coasting going on with a manual.

One more thing...the air that your motor sucks in operates the brake booster. At 3,000 rpm's the motor is sucking alot of air which is good for slamming on the brakes repeatedly, however, lets say you are doing 80 on the freeway...bout to get off...you throw the tranny into neutral and now your engine is spinning at only 600 rpm's but you're still doin 80mph and theres a problem at the end of the offramp ....you're screwed!!!! Slamming on your brakes 4 or 5 times and the engine might sputter enough that it stalls, now you have no power brakes and no power steering.

[vgames33]

Many modern vehicles cut or reduce fuel while decelerating in gear down to a certain RPM. Going into neutral requires fuel to keep the engine at idle, but none (or very little) is required to keep the engine at 3000 while decelerating because the momentum of the vehicle is turning the engine.

Just leave it in drive.

Braking in neutral won't hurt anything.

[procaddytech]

No, it won't kill your transmission. It will make the car harder to stop because you won't have the advantage of engine braking to slow you down and your brakes may overheat and fade. The control module will see a neutral input so the emissions sensors will be fine. You will have plenty of vacuum for the brakes because an engine pulls the most vacuum at idle, with no load. BUT, it is not a good idea because the downshifts help the car to stop, and as mentioned earlier if you needed to make a sudden change in driving you would end up revving the engine and then banging the trans back into gear or putting it into gear at a low rpm and then the engine braking would rev the engine as it tries to catch back up. Put it in neutral at a stoplight if you want, this will reduce transmission fluid temperature and engine load. Just don't forget you are in neutral when the light turns green or a panic rev and bang into gear will happen. As most say, just leave it in Drive, it is an automatic.

[Moppie]

Originally Posted by Tony Silva

Brilliant idea if it actually worked. Your car can get more than just a damaged transmission. There are lots of sensors that manage the engine emissions system. Engine operating temp vs engine air intake speed vs the vehicles actual driving speed. The computer takes all these (and more) into its consideration on how to make adjustments to whats going on under the hood. Plus, what happens if you suddenly have to speed away to avoid a collision? What are you gonna do then? Put the car back in gear and gently bump the throttle up to match the engine speed with the road speed ao as not to damage the transmission? Or are you gonna yell, "Oh! Shiiiiit!!" and immediately floor the gas pedal in a panic put the car in gear at the same time? Gas pedal to the floor...engine speeds up and the transmission finally slams into gear.


And just so you'll know, coasting your car to a stop is rediculously hard on the brake pads and brake rotors. Yes there are alot of manual transmissions in small cars AND those brake rotors and pads are actually bigger than those that are on a car with an automatic transmission because there is some vehical coasting going on with a manual.

One more thing...the air that your motor sucks in operates the brake booster. At 3,000 rpm's the motor is sucking alot of air which is good for slamming on the brakes repeatedly, however, lets say you are doing 80 on the freeway...bout to get off...you throw the tranny into neutral and now your engine is spinning at only 600 rpm's but you're still doin 80mph and theres a problem at the end of the offramp ....you're screwed!!!! Slamming on your brakes 4 or 5 times and the engine might sputter enough that it stalls, now you have no power brakes and no power steering.

Good points about safety and not being able to drive around something when the car is in neutral, but your wrong about brakes boost and engine speed.

An engine generates the most vacuum at idle, so provides the most boost at idle.
It's one of the many good reasons to lift off the gas while braking

[Moppie]

Originally Posted by MizBlackCrow

Hey guys. Forum newb here. But I came across an idea that seems brilliant and, because it's me, is probably not. Here's my thought process:

Welcome to AF!


Is the car an Auto or Manual?

If it's an auto, then putting in neutral while sitting in at the lights, or in traffic could save you some fuel.
While in gear the engine has to pump fluid through the gear box which does add a small amount of load to it. You won't notice it's happening because the engine management system compensates by adjusting the cars idle speed, but it does so by adding more fuel and air.

You won't save fuel by coasting.
Modern fuel injected cars with a locked connection between the transmission and engine actually stop injecting fuel when the car is coasting. There is no need to run fuel into the engine as the cars own momentum keeps it turning over.
On cars with non lock up torque converters then fuel delivery drops to the same level as it does at idle.

Any saving you might get by being able to coast further is more than out weighed by the increased wear on your brakes.


They are some good ideas, and it's good that you've thought of them and wanted to put them to the test though, even if they don't actually work.