Coasting downhill in my car in neutral: how bad?

Factual Questions
1

To amuse myself, sometimes I slip my car’s automatic transmission into neutral when I’m driving down mountains, letting gravity do its thing.

Admitting that this isn’t exactly “helping” my car, how much is it hurting it? My two concerns center on: (a) does doing this on a hot day, when the radiator needs to really circulate those fluids, yet the car is basically idling and (b) does it “hurt” (whatever that means) the transmission when I pop it back into drive at about, say, 50 mph? :rolleyes:

Thanks.

2

I can only speak for the heating question. Idling the engine creates considerably less heat than running it up at speed. The reason temps tend to skyrocket while idling is generally because you’re stuck in traffic with virtually no airflow through the radiator. On the other hand if you’re rolling downhill at 40 or 50 you’re cool.

3

On a manual car (and I assume auto as well) don’t switch off the engine to save fuel when doing this, as the breaks will not work with the engine off (on a Ford Fiesta any way this is true). Luckily I experimented on a very shallow hill, and restarted the engine easily enough.

4

It shouldn’t hurt anything besides wearing out the brakes faster and heating up and possibly warping the drums and rotors. The radiator will circulate the coolant the same as with the trans in gear. Popping it back in gear at 50mph also shouldn’t hurt anything as long as everything is working as it should. In some states it’e illegal to coast in neutral because you might need to accelerate quickly to avoid an accident.

5

Your brakes will work but you wont have any power boost from the vacuum created by the engine which may make you think they don’t work at first.

Either way, it is extremely unwise to kill your car when it is moving, period. Especially at highway speeds.

6

Carl said it is a matter of control. You’ve given away your control.

7

My friend Carl would say it is a matter of control. You give away your control.

8

Drat.

9

Or rather, My friend Carl said Drat. :wink:

10

Imagine what would happen if you turned the key one click too far and, all of a sudden, your steering wheel locked :eek:

Please do keep this possibility in mind if you ever need to turn off the ignition while driving (e.g. stuck accelerator).

11

In my car at least, it’s impossible to turn the key to the “lock” position if the shifter is anywhere but “park”.

12

I imagine I’d turn the key one click in the other direction, thereby fixing the problem.

13

If you turn the engine off with the key the steering wheel doesn’t lock until you pull the key out. But then it is such a habit to pull the key out after switching the engine off you might do this without thinking.

14

I have heard that in an auto tranny you are running the gears and not the pump so you might be wearing out the gears and burning the oil.
Also some places have laws against coasting in neutral, California maybe. Google shows that Maine had one but it was repealed.

15

The answer to the actual question is:

You are probably not helping, but not hurting.

Modern automatic transmissions generally allow the drive shaft to freewheel faster than the engine speed. You can experiment to see if you really travel faster and further with the transmission in neutral instead of in gear. If you have a tachometer, see if the engine speed drops to idle speed when you release the gas. If it does, shifting to neutral has no effect.

If you have an auto that does not allow freewheeling, and choose to drive in neutral, brakes are made for stopping cars. Any additional wear on brakes saves tremendous wear on the engine. Brakes are easy to replace.

If the engine is on, your tranny is getting oil.

In many states, it is illegal to turn off the engine to coast.

Hope this helps.

16

You are doing a disservice to your transmission. If you are coasting at 40 mph, the internal parts of your transmission are turning at a 40 mph clip too. But with the engine idling, the transmission is not get the hydraulic pressure to keep the clutch packs in contact. This will cause premature wear inside the transmission. It is not as severe as towing a car on the drive wheels with the engine off but close to it. You could get away with coasting in GM cars with 2 speed automatics produced from the early 50’s to the mid 60’s. The hydraulic pump was driven off the output shaft of the transmission, not the input shaft like virtually all other automatic transmissions. Cars with these transmissions suffered during periods of idling in place. The transmissions would overheat and quit working. The nice thing about these transmissions is that they cars that had them could be push started.

17

It has alread been said, but I think it’s worth repeating: DON’T SWITCH OFF THE ENGINE WHEN YOUR CAR IS MOVING!

It is very dangerous, as your steering may lock. Also, you lose power assistance to the brakes, meaning you have to pump the pedal much hard than you would normally.

18

I didn’t say a thing about turning off the engine.

So far, we have some Dopers who says idling has no ill effect, while others say it does.

Any others?

19

So is racer right or is ronbo?

20

My guess is that little to no damage would be done to the transmission. IF any damage IS done, it would be almost theoretical.

What you do lose is the engine braking effect. If the hill in question is long, or particularly steep, you will increase the wear on your brakes and in severe cases you might end up with brake fade due to heat. Plus, if the car is out of gear, you lose the ability to quickly accelerate, should that be required.

Coasting down a hill is never a good idea unless the engine dies and stopping would be dangerous. Switching the engine off while coasting is an even worse idea, unless the accelerator is stuck.