[D101h]

Is it possible to realy turn off cars engines(like when you fall in the water), and not freeze the driver? If similar post exists post the link.

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You cannot "turn off the engine" but you can TogglePlayerControllable which will freeze them. This will however prevent then hitting enter to exit the car, you'll need to use RemovePlayerFromVehicle to then remove them from it. (Perhaps type /leavecar, to leave a frozen car)

Also, you cannot natively detect if the player has hit water.

[D101h]

Would be possible then to "soak" the engine somehow?

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The engine cannot be stopped at all, you have to freeze the player to give the effect it's not going to move anywhere.