Scary Near Miss: Has anyone else experienced this?

[Madcoder]

Originally Posted by Yahooligan

You sound a bit confused, so I'll just explain it in a way you might understand better.

When coasting downhill in gear with a modern EFI engine the engine is being "driven" by the road, which is turning the tires, which turns the driveshaft, transmission, and engine. The fuel demand is zero, no fuel is delivered, and the engine is putting out no power. The engine is actually "absorbing" power in the form of engine braking, which will slowly slow you down. In this state you are using no fuel because none is required. The engine doesn't die because it's not disconnected from the drivetrain or the road, all of the other systems are still functioning as well, there's just no fuel so the engine is acting like a big air pump.

When freewheeling downhill in neutral, the engine is disconnected from the road so in order to stay running in this state requires fuel, just as if you were sitting parked in your driveway. Take away the fuel here and the engine dies, you lose power steering, eventually you lose power brake assist, etc.

Maybe thinking about a bicycle will help?

Coasting downhill in gear is sort of like coasting downhill on a bike with your feet still in the pedals. Let's assume it's an older bike that can't freewheel, if the bike is moving then the pedals are moving. In this scenario your feet never leave the pedals, your legs are moving but you're not doing any work. Have you disconnected anything from the chain or tires by coasting? Nope. Are you able to transition to pedaling without changing anything else? Yup. Are you what's moving your legs? Nope, the movement of the wheels on the ground is moving them for you.

Now let's do the same scenario, only this time you take your feet off the pedals. If you want to move your legs now, what supplies the power/energy? Your body. Is it easy to transition back to pedaling now that your feet are removed from the pedals which continue to spin at a pretty fast rate? Nope.

Not the best example, but maybe it helps get the point across. Kinetic energy turns the engine and keeps it "running" when your foot is off the gas but it's still in gear. Kinetic energy requires no gasoline.

This does make sense. Thanks.

So the computer somehow recognize that it doesn't need to spray any fuel into the cylinders and that some other force is powering the car i.e. downhill? What sensors, parameters does it need to determine this?

[occar]

Just that there is 0 throttle input and the engine is maintaining X RPMs.

My GT500 has an AFR gauge built in, and you can tell when it shuts off the fuel because it pegs the gauge (though it won't display over 20). It will do it in 3rd-6th gear, when the RPMs are at least 1250 or 1500 (I've not isolated the exact value) and there is 0 throttle input. It will maintain that state, even if RPMs drop below the threshold once it is engaged. Any throttle or clutch input will start fueling again.

[Madcoder]

Originally Posted by occar

Just that there is 0 throttle input and the engine is maintaining X RPMs.

My GT500 has an AFR gauge built in, and you can tell when it shuts off the fuel because it pegs the gauge (though it won't display over 20). It will do it in 3rd-6th gear, when the RPMs are at least 1250 or 1500 (I've not isolated the exact value) and there is 0 throttle input. It will maintain that state, even if RPMs drop below the threshold once it is engaged. Any throttle or clutch input will start fueling again.

Very cool. I'll have to pay more attention to my AFR now

But as for saving fuel. If the car is in drive it slows down much quicker then if coasting in neutral.
So I would imagine, if not going downhill that you would still save more fuel by being in neutral.

[gooney0]

On these modern cars using neutral will often cost you more fuel. When you let off the accelerator in gear it will actually shut off the gas and use the transmission to avoid stalling. If you take the car out of gear you have to use gas to idle.

In older cars this worked as they would simply waste the extra fuel during engine braking.

[Yahooligan]

Originally Posted by Madcoder

Very cool. I'll have to pay more attention to my AFR now

But as for saving fuel. If the car is in drive it slows down much quicker then if coasting in neutral.
So I would imagine, if not going downhill that you would still save more fuel by being in neutral.

If you don't want to slow down as quickly then shift into a higher gear. Coasting in neutral for any measurable distance isn't particularly safe, either. Should you have to make an emergency maneuver or speed up to get out of the way of another vehicle or object then you're screwed, the time it takes to get back into gear and accelerate could be the difference in avoiding an accident and being involved in one.

I have never been in a situation where coasting in neutral would be beneficial for any reason. Just don't do it, especially with an automatic. If you need to get out of the way of something and the car happens to go into limp mode when you shift back into drive then you just screwed yourself. When the car slows enough that the wheels and transmission gear can no longer turn the engine due to low RPMs then the PCM will disengage the torque converter clutch and it will be just like you're in neutral only in this case you'll be able to get out of the way without a problem.

If your automatic car is slowing too quickly then you must be using AutoStick and manually downshifting. Just leave it in the high gear and let the transmission do what it's programmed to do when you're off the throttle and slowing down. There's no need to manually downshift an auto, even in AutoStick mode.

[laurag146]

I almost pooped my pants reading that...glad you're safe. You were given correct advice on how to propery use auto-stick.

[10ChallSRT8]

When my cam was first put in, the tuning was not quite right and the car used to stall at freeway speeds. I'd romp it at 60, get up to 75 and engine would shut off. Much like OP scenario, here I was coasting at freeway speeds. I'd pop into nuetral and try to restart but it would not work. I'd have to coast to shoulder of road, stop, then I could start. I'm thinking thats just some safety feature, limp mode, whatever. Mine has since been retuned and runs great without stalling.

[Madcoder]

Originally Posted by Yahooligan

If you don't want to slow down as quickly then shift into a higher gear.

5th is as high as it goes

Originally Posted by Yahooligan

There's no need to manually downshift an auto, even in AutoStick mode.

I completely agree with everything you've said up until this one. Well partially.

I don't NEED to down-shift but I do because it's fun. Just like none of us NEED a 400+hp car.

Plus in autostick I can drop only one gear as compared to just going into passing gear could jump 2 gears, which maybe too much.

Thanks for taking time to explain all of the BTW.

[adrake23]

You never have to put the transmission in neutral when driving. My salesperson told me to do this as well. All you need to do is hold the shifter or paddle shifter to the right and it will go back into drive.


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[Redliner]

Yahooligan.......thanks for the explanation. I'm an engineer and great at physics and totally understand the "air-brake" scenario as what trucks do to slow down. I just didn't realize that the engine (in a driven mode) would require or better yet, receive "zero" fuel. That was what convinced me now. One other brought up the coasting further in neutral thing. He is correct. I coast considerably further (I'll say 50% further) in neutral than in drive but that is due to the engine braking going on, simple as that). I NEVER do this coasting with anyone even close to me behind me or with anyone just ahead of me. and most importantly, NEVER in the Challenger because you simply can't ! It won't let you. Again, thanks for the fuel system explanation which is modern technology that I'm not up to speed on.