Turning off your car at 55 mph

Me and a friend were arguing over this issue. If you were driving down the road at say 55 mph or at any speed and just turned the car off what if anything would happen to the engine and or car?

Steering column LOCKED !
NO power steering.
NO power brakes !


…you forgot the CRASH!! tinkle tinkle.

It depends partly on your car. You could:

Loose or reduce braking and steering ability. It is not a good idea.

Many cars today are designed to shut off the fuel to the engine even under normal conditions. My car will shut down the fuel totally anytime you let up on the accelerator and the engine is being turned faster than idle speed.

If you removed the key, you could also lock the steering wheel.

I have heard, but don’t really know that some cars can damage their engines doing that trick.

I don't recommend the idea.

Be sure you have arranged for a chase vehicle with a camera crew to film your experiment! Some of the cable channels pay around $55,000 for a segment like this, especially if it results in a crash…

Normally, your vehicle will just coast to a stop…They won’t pay anything for that…

For one, this would be bad for an automatic transmission, which would be turning without proper lubrication.

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In gear?
Manual or automatic?

If it’s an automatic and you leave it in gear your wife will divorce you when she sees the bill for the new tranny. Assuming you don’t crash.

If it’s a manual and you leave it in gear and you can avoid crashing, and it’s a typical modest-compression 4-banger, you’ll slowly come to a stop. Less slowly of you downshoft to assist the process.

Oh, and as others have pointed out, there’s the little problem with the loss of brake assist and power steering.

Yeah Mtn Bike makes yet another good point. If you had a manual trans and went into neutral…didnt take the key out or lock up the steering column… Not much would take place. YOu would be rolling down the road…with a few power assisted stops left in your brake booster… SO if you hit the brakes more than say 3x…the next time you do it will be unassisted…so it would be harder to stop than you are used to. No pwer steering either…but you can still use the brakes and steer. Other than that…not much.

Now in an automatic…I dont think the tranny would like that much unless you went into neutral…all else would be the same though.

Any why are we doing this again?

I’m surprised no one has asked this: Why would you do that?

Sounds like someone made a really weird wager to me.

No, indy is a regular. He’s just trying to start up another 100 post thread, thats his job.


I do this occasionally, but at generally at a lower speed. My car has this quirk in which the ABS light sometimes comes on for no apparent reason. Restarting the car clears the light. So when driving and the light comes on, I push in the clutch, kill the ignition for a second or two, then turn the key to ON (not start). Releasing the clutch pedal restarts the engine and all is well. The ABS light stays off.

If my car had automatic transmission instead I would have to shift into neutral first and then restart using the starter. If done on a clear road, I see no particular problems, either mechanical or safety related. But it pays to know your car.

Oh, and as others have pointed out, there’s the little problem with the loss of brake assist and power steering.

You likely wouldn’t lose either. Not at first, anyway.

Let’s assume (since the OP did say “at 55”) that you either have a MT or an AT with the torque converter locked. The spinning wheels would be (in effect) “reverse driving” the engine, which would still be spinning. Your closed throttle (no point in stepping on the gas, is there?) would produce plenty of vacuum for braking, and the PS pump would still be driven by the spinning engine.

(As for electric PS, dunno.)

Obviously, turning the key to “off” and removing would activate the locking mechanism on the steering wheel, which would be very bad…but simply killing the ignition (by turning the key to “accessory on”) would have little immediate negative effect* (other than coasting down.)

  • W/R/T a fuel-injected car…things could get (ahem) interesting with a carb’d car on the “restart.”

The reason the question came up was I have a 2002 Kia Sedona mini van which needs a timming belt change but is on hold till I can raise the LARGE sum of money it takes to get done. My friend said that if I hear it break I can just turn the car off thus limiting the damage to the engine since I only drive on city streets and such and never get over 55 mph, I said that would do damage also and you can cause an accident.

Turning off will be of no help if the timing belt breaks.

First, when the cam stops spinning in sync with the crank all power is immediately lost anyway.

Second, the crank will free spin at least a couple of revolutions, much more if in gear and driven by the transmission.
It only takes one turn of the crank to drive a piston up into an open valve.
Even if you could react quickly enough to take it out of gear at least one valve would get struck, requiring head removal.

I can’t believe more of you have not lost power at highway speeds. You simply lose power steering, power brakes after depleting the resevoir, and you coast to the side of the road. Normally you would shift to neutral and try a re-start, but generally nothing else happens. Something wrong if you can’t control your car with manual systems to avoid a crash. Its not easy to steer or brake but then you should be allowing space around you anyway.

“My friend said that if I hear it break I can just turn the car off thus limiting the damage to the engine”

Your friend is wrong.

Once the belt breaks on an interference engine, the damage is essentially instantaneous. When he refers to “hearing the belt break”, I assume that he is referring to the sound of your engine itself suffering catastrophic damage as pistons and valves collide.

As ok4450 has stated, your engine will run very normally–right up to the milisecond after the belt snaps.
Unless you are able to somehow shift to neutral and simultaneously shut off the ignition in that milisecond, the damage has been done.

Your friend is wrong.

Even if he is magic and can turn the ignition off, even if he does it 100 msec before the belt breaks, the crankshaft is still going to keep turning more than long enough to do serious damage.

How about staying in gear with the clutch out? That way, the crank is still turning due to the motion of the car. Power steering(assuming it’s old school hydraulic) and power brake keep working. When you turn it back on, ignition resumes immediately, and there’ll be no cranking the car.

Actually, if you leave it in gear, you won’t lose power steering until you are going fairly slow. This is because the transmission (even an automatic) will keep turning the engine.

100? Please the goal now is 150…:wink: