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I was thinking about a recent question here as well as that 2009 Toyota incident. I'm not particularly paranoid of this happening, as it's highly unlikely, but I do like to mentally run through sticky situations so I'm prepared if they do happen (this has helped me out in the past a few times, e.g. a popped brake line in tight high speed traffic).

So consider a passenger car accelerating out of control. Let's assume:

  • Worst case: For whatever reason, turning the ignition switch off has no effect. Also turning off ignition has risk of power steering and power brake loss, as well as risk of engaging steering lock, so it isn't plan A.
  • Everything else in the vehicle is functioning correctly, in particular the transmission has no issues.
  • The engine can overpower the brakes (e.g. due to vacuum loss at WOT affecting brake assist / power brakes).
  • It doesn't matter why this is happening, it just is.
  • The priority is to stop the vehicle without putting the driver, passengers, or other people in danger. Doing permanent damage to the vehicle is fine.

In a manual, which my car is, I figure if something ever causes uncontrolled acceleration plan A would be to pop the car into neutral and handle it from there.

But I don't have an automatic to play with, and I'm not really familiar with them. So my questions are:

  1. Will I always be able to throw an automatic into neutral at any speed? Or can the shifter get stuck (sort of like in a manual how it's hard to pop it out of gear with the clutch closed).
  2. What about in a fully drive-by-wire vehicle? Is there some regulation that says switching to neutral must still be a direct mechanical/hydraulic action, or otherwise states that a request to shift to neutral must not fail, or is it possible that a reasonable ECU may e.g. refuse to go into neutral because of some condition that may result from a stuck throttle?

And a subquestion: Would there be a better way? I'm just stuck on the neutral thought train.

This isn't a question about why this would happen or how unlikely it is to happen (very unlikely), it's a question about being prepared for a really weird situation and having a strategy to handle it safely.

I'd be able to answer this on my own if I had another car to play with, but unfortunately, I don't.

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    I'm voting to close this question as off-topic because it is not about car maintenance. – HandyHowie Jul 31 '16 at 11:03
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    @HandyHowie: You'll have plenty of maintenance to perform after executing this maneuver :) – Lightness Races with Monica Jul 31 '16 at 13:40
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    I'd like to see another answer to this question. The existing ones didn't really address the points made in the question. – Lightness Races with Monica Jul 31 '16 at 13:41
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    @Lightness Yeah me too. I flip flopped with the check mark a few times but I'm going to leave it open for now. Also fwiw if the community wants to close this question I respect that, I wouldn't agree but I can understand it being seen as borderline. – Jason C Jul 31 '16 at 13:47
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    On every vehicle i've owned, there is a place in the ignition between ON and LOCK called UNLOCK where the engine is off, but the steering wheel is unlocked. However, I could see someone missing that unlock position in a panic situation. The booster should have enough vacuum for 1-3 good presses left with the engine off. Power steering isn't going to matter much above 10mph. – rpmerf Aug 1 '16 at 10:33
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I would think that you'd want to impose the largest load that you can on the engine, probably in the highest gear you can get into, pull over and then shut the engine down. In real life, in traffic, I'd be leery of trying to shut the ignition off, for fear of engaging the steering lock.

So, I think I would:

  1. Apply the brakes hard (maybe add parking brake).
  2. Shift from drive to the top gear (make the engine work hard to accelerate, don't allow downshifting).
  3. Steer to edge of road.
  4. Switch off ignition once safely off of road.
  5. Put transmission in neutral or park.
  6. If the engine has not shut down pull fuel pump fuse, or discharge a fire extinguisher into the air intake, or block the air intake, or pull the distributor primary lead – whatever seems like an expeditious way to force the engine to shutdown. That's one of those things that would be worth checking out and thinking about specifically in advance.

Just curious, on a modern passenger car, can the engine overpower the brakes? I could see one of the design rules being "brake capacity adequate to stop car even if engine is stuck at WOT."

How dangerous is a runaway engine? Would it be better to just move a safe distance from the car rather than trying heroics to stop the engine if the ignition switch isn't working?

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    "Just curious, on a modern passenger car, can the engine overpower the brakes?" .... Money shot right here; this is the answer. This isn't a forum for contrived hypotheticals. Why can't you shut off the ignition? How will the engine overpower the brakes? What kind of paranoid scenario are we prepping for?? Honestly, I'm all about safety, but I'm not pulling fuses or remapping my ECU on the fly. I'm stomping the living bejuices out of my brakes, as if my life depended on it, because it clearly does. It's not a Bugati Veyron. If your brakes work, they will work. – SteveRacer Jul 31 '16 at 4:51
  • I've had this happen a couple of times in an old car with stick shift and an old-style carburettor (no electronic engine management). The cause was icing in the carb while the engine was still warming up, and ice freezing the throttle butterfly open. The engine could definitely overpower the brakes. Switching off the ignition would lock the steering, which isn't a very smart idea driving in traffic. My solution was put the car out of gear, which killed the engine when it over-revved. Then wait till the ice in the carb melted and the gas pedal started working again, and carry on driving! – alephzero Jul 31 '16 at 5:00
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    "The engine could definitely overpower the brakes." - well, maybe not in 5th gear, but it certainly could in 3rd, which was the actual situation both times it happened. – alephzero Jul 31 '16 at 5:04
  • "discharge a fire extinguisher into the air intake" sounds like you'd be risking hydrolock (which may do permanent damage to the engine even if over-revving didn't) – Random832 Jul 31 '16 at 5:12
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    The brakes did not work in those accidents because at WOT there is no vacuum (well, very little vacuum), so the power brake vacuum assist stops working. – Jimmy Fix-it Jul 31 '16 at 14:49
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The safest option for the vehicle is to turn the ignition switch to off, protect the engine, then turn the key back to "on" without turning the engine back on, to prevent steering lock.

Since you ruled that out, shifting to neutral would prevent the uncontrolled acceleration, and you would hope the rev limiter will prevent engine damage. This works for an automatic, you don't even need the "button" at the top of the shifter, you can always slide the shifter from drive to neutral (I do this all the time when approaching a stop or traffic light).

If you want to go nuts and if your fuse box is accessible from the cabin while the vehicle is in motion (this rules out those vehicles where the driver or passenger door needs to be open to access the fuse box), learn how to pull your fuel pump fuse by touch. This only works for those vehicles where said fuse is in the cabin fuse box and not under the hood. Pulling the fuse for the fuel pump will cause fuel starvation and the engine will safely shutoff in a few seconds.

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    "The safest option for the vehicle is to turn the ignition switch to off, protect the engine." For the vehicle perhaps, but not for yourself or the people around you. Absolutely do not do this. Can't stress that enough. – Lightness Races with Monica Jul 31 '16 at 13:40
  • And yeah the thing I'd worry about the most turning off the vehicle is power steering and brake loss, and risking the steering lock engaging. – Jason C Jul 31 '16 at 13:44
  • Oops! Forgot about steering lock. I'll edit my answer. – tlhIngan Jul 31 '16 at 16:51
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The technologies are going forward and any decent car should be able to stop a roaring engine with a brake pedal. However, if it doesn't work, you have to switch the ignition key off, and leave the engine turning over to pump your brake booster and power steering (if it's hydraulic). If it still doesn't stop, get it to neutral, stop the car, jump out, open the bonnet, and put the flat hand in front of intake duct (choke the engine), or if you have any available rug near by, faster would be to block the exhaust.

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Here's my two cents. This happened to me last week so the event is fresh in my mind.

  • don't panic

  • put it on neutral thus separating the connection between the engine and the tranny

  • shut off ignition.

  • coast the car to the side of the road .

2

Even in the Toyota cases, it was well documented that the only thing a driver needed to do was shift into neutral, and the car would stop accelerating.

The situation you mention is quite the extreme hypothetical. It reminds me of the kind of thinking pilots are trained for ("What happens if the fuel tank adjustment slider comes off in your hand when you're switching tanks? Do you know where you can land?").

I would run down a list, like Allan mentioned. Start with the realistic cases first. 99.999% of times, I'd expect you to be able to either put the car into neutral or turn the ignition off. It'd be a pretty spectacular failure that causes both to fail at the same time. Even in the drive-by-wire cases, this is safety critical stuff, so they put redundancy into it. I'd also be using the brake to slow me down. If it's not working, I'd try the emergency brake.

Okay, so now we're in a special sort of case. We have a runaway engine, jammed transmission, brakes don't work, ebrake doesn't work, and the ignition isn't stopping the car.

“What to do if you find yourself stuck in a crack in the ground underneath a giant boulder you can't move, with no hope of rescue. Consider how lucky you are that life has been good to you so far. Alternatively, if life hasn't been good to you so far, which given your current circumstances seems more likely, consider how lucky you are that it won't be troubling you much longer.” - Douglas Adams, Hitchiker's Guide to the Galaxy

At this point, you need to change your focus. Throw any plans you had out the window. Don't focus on what you think you should have in this scenario. Instead, start with what you do have. Do you still have tires? Is the steering wheel still in control of them? What's outside. Is there a gravel median you can use to slow yourself down, or is it a sheer cliff? Is there a runaway-truck ramp within reach? Can you get in a controlled collision with a nearby car so that you don't get in an uncontrolled collision with one later with more disastrous results? Think outside the box. It's hard to prepare for this, because if so many systems all failed simultaneously, there's a really good chance that a bunch of other stuff failed too.

The closest to a real answer you might have comes from the diesel truck maintenance world. This is hearsay, but it seems believable. A diesel engine can run away from you unless you can stop the fuel flow. In a case where this occurs, a mechanic would keep a phone book nearby. You'd simply toss the phonebook on the air intake and let the engine suck page after page until it is air-starved enough to cease its runaway actions. The idea was that the pages of the phonebook were thin enough that they wouldn't cause damage. Again, hearsay, so don't go stopping your truck with a phone book on my account, but it does point out the sort of out of the box thinking called for with a runaway engine.

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Yes, you can change the gear to neutral at any speed. The neutral is a safety feature to disconnect the engine from the wheels. It is similar to the clutch on manual transmission cars.

On a modern computer-controlled engine, over-revving is impossible even with a stuck throttle.

Turning off the ignition may activate steering lock (unless you are careful to turn the key to the exact position where steering lock is not on) and disable power steering. Also, if you turn off the ignition and pump the brakes, vacuum in the booster is lost (although with a throttle stuck completely open pumping the brakes will anyway lose the vacuum).

Based on this, I would change the gear to neutral, brake without pumping the pedal, and then turn off the ignition after the car is stopped.

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Install a guillotine throttle on the engine, in addition to the primary throttle. They are a safety device used in high performance vehicles, usually diesel engines. The throttle is normally completely open, but a mechanical action causes it to snap shut instantly. Here is a great video.

https://www.youtube.com/watch?v=OgP3r1CMv6M

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