I got the offroad vehicle and was driving across the map, when I got into gear 3 and increased up to around 80 km/ph and stopped the car the engine noise went silent but I was still able to drive.

When I got to my destination I realised I couldn't turn the engine off the option was not there, so I removed the car battery, radiator and spark plugs but the car was still running so I logged out and logged back in to see the car engine is no longer running but when I attempted to drive it again there was still no option to start the engine.

I have recently got another car and managed to recreate the same situation and it seems to occur when your engine sound disappears.