some adjustments and new parameters on CarVehicle3D

autoload/persistence/settings/input_controls.gd

@@ -38,6 +38,7 @@ const VehicleReverseAccelerate: StringName = &"vehicle_reverse_accelerate"
 const VehicleSteerRight: StringName = &"vehicle_steer_right"
 const VehicleSteerLeft: StringName = &"vehicle_steer_left"
 const VehicleHandBrake: StringName = &"vehicle_hand_brake"
+const StartVehicleEngine: StringName = &"start_vehicle_engine"
 
 const PerformanceMetrics: StringName = &"performance_metrics"
 const PauseGame: StringName = &"pause"

components/motion/3D/vehicle/3D/car/car_vehicle_3d.gd

@@ -40,6 +40,7 @@ var vehicle_steer_left_action: StringName = InputControls.VehicleSteerLeft:
 @export var engine_reverse_acceleration: float = 100.0
 ## The maximum value the wheels can be turned at
 @export var max_rpm: float = 500.0
+@export var start_engine_action: StringName = InputControls.StartVehicleEngine
 @export_category("Steering")
 ## Decides how much a wheel can be turned. Higher values means that it can turn more easily
 @export var kb_steering_ramp_up_factor: float = 30.0
@@ -68,13 +69,20 @@ var engine_on: bool = false:
 				stopped_engine.emit()
 
 
+func _unhandled_input(_event: InputEvent) -> void:
+	if Input.is_action_just_pressed(start_engine_action):
+		engine_on = !engine_on
+
+
 func _enter_tree() -> void:
 	add_to_group(GroupName)
 
 
 func _ready() -> void:
 	_update_input_actions()
 
+	process_mode = PROCESS_MODE_PAUSABLE
+
 
 func _physics_process(delta: float) -> void:
 	motion_input.update()

project.godot

@@ -264,6 +264,11 @@ vehicle_steer_left={
 , Object(InputEventJoypadMotion,"resource_local_to_scene":false,"resource_name":"","device":-1,"axis":0,"axis_value":-1.0,"script":null)
 ]
 }
+start_vehicle_engine={
+"deadzone": 0.5,
+"events": [Object(InputEventKey,"resource_local_to_scene":false,"resource_name":"","device":-1,"window_id":0,"alt_pressed":false,"shift_pressed":false,"ctrl_pressed":false,"meta_pressed":false,"pressed":false,"keycode":0,"physical_keycode":4194309,"key_label":0,"unicode":0,"location":0,"echo":false,"script":null)
+]
+}
 
 [internationalization]