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I was having a discussion today that turning off the engine while still moving along at about 20mph, while the engine is still revving will do no damage to the car, apart from the obvious safety issues (no brakes, speedo etc).

I think this is because the oil pump is still turning and the engine is being lubricated so it'll be fine to do this.

Am I wrong?

Will removing the ignition key while the engine is still turning damage my car?

What will happen if this did happen?

I obviously do not want to try it in case I destroy something and ruin my car or crash as I have no brakes in an emergency!

UPDATE: I have a manual transmission, fuel injected. I mean just turning the key to ignition, not actually removing it

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    I fully disagree that the engine will just stop, on an automatic transmission. How many cars from the 80's-90's used to die all the time and they were never damaged. My car died while on the freeway 60+mph, due to the ignition switch in the steering wheel column falling apart (Yay for Ford!). My car did not stop, nor screech to a halt. Yes power steering and brakes were worse than manual, but I just put the shifter into neutral and started it up, then back to drive mode.
    – Aaron
    Mar 31, 2016 at 21:06
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    @Aaron the fact that you had to put it in neutral and start the engine proves that the engine was stopped. It didn't hurt anything, but the engine was not turning.
    – JPhi1618
    Apr 1, 2016 at 4:43

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What will happen is dependent on the configuration of your vehicle.

Automatic transmission - Due to the torque converter, the engine will just stop. This is bad for the transmission, because the parts are moving, but the pump is not as the pump is run by the input shaft. This is similar to towing, where they typically recommend no more than 5 miles at no more than 25mph with the drive wheels on the ground.

Fuel injection with manual - If left in gear, engine will continue spin, but the injectors and spark plugs will not fire. Oil pump will keep everything lubed. As soon as you turn the key to ON again, it will run like normal.

Carb with manual - Similar to FI, except fuel will still be pushed through the engine. This is not good for wear, but will not cause immediate damage.

Also to consider, most vehicles lock the steering when the key is removed. The brakes usually have 1-2 presses left after the engine is shut off.

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    Thorough answer - I'd only add that you will loose power steering (if the engine is off, but key stays in) as well as power brakes. Both will continue to work, but will become much more difficult, possibly to the point of loosing control or not being able to stop in a reasonable distance.
    – JPhi1618
    Mar 31, 2016 at 18:15
  • @JPhi1618 If the power steering pump is belt driven and the engine is still spinning it should still be working I believe. Maybe not 100% due to losing constant RPM but long enough to get to the side of the road.
    – Ben
    Mar 31, 2016 at 20:48
  • I was assuming an automatic transmission when I wrote that. With a manual transmission and not pushing in the clutch you probably would have engine speed and power steering.
    – JPhi1618
    Mar 31, 2016 at 20:55
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    @alephzero Sounds like how it is here in the US. We typically have Off, Unlock, Accessory, On, Start. Unlock is not found on all vehicles. It is used to unlock the steering, but not have the ignition on. Engine will turn off in the Acc, unlock, or off positions
    – rpmerf
    Apr 1, 2016 at 11:03
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    @MathuSumMut Engine braking should be about the same until you are at about idle speed. I believe there is still a tiny bit of fuel injected while engine braking. If you want to test it, drive down the road, be in a gear where you are at around 3-4k RPM. Start engine braking. Turn off the throttle for a second - to the ACC position, not all the way off where it locks the steering. If the exhaust get quieter when you shut the engine off, the engine was injecting some fuel while the ignition was on.
    – rpmerf
    Jan 29, 2018 at 14:08

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