Right now, the only way to turn the engine on in a vehicle is to accelerate.
It would probably be more appropriate to select an "Engine on" action instead.
Part of the reasoning behind this suggestion is helicopters during autorotation: if you turn the engine off, it's probably because you don't want it to be on, so when you raise collective to cushion the landing, the engine stays off instead of turning back on - especially when you have a damaged tail rotor.
Now of course, autorotations are still impossible in the current build, but that doesn't mean vehicles should magically turn their engine on when you push a pedal or raise a lever.
To sum up: make turning the engine on the same as the current "Engine off" action.