These devices, which often attach to spark plug wires or the car's distributor, supposedly improve fuel burn by creating an "ionic corona" around the engine. Supposedly, this performs the same function as fuel ionizers, just closer to the point of ignition.
As mentioned before, the supposed molecular breakdown caused by ionic forces can't really happen. The molecular separation the ionizer claims to produce is what happens during combustion, and gasoline in its unburned form is a poor conductor. It stands to reason that, if gasoline were volatile enough to break down when exposed to a small electrical field carried by a bolt-on ionizer, the much larger field generated by the spark plugs, alternator and distributor would cause the same breakdown well before the gasoline reached the engine [source: Herning].
This is possibly one of the worst hoaxes thrown at drivers. In tests, ionizing devices revealed themselves to be little more than bundles of cleverly packaged wire, which could cause short circuits or fire hazards if attached to the wrong parts of a car's engine [source: Allen]. This is definitely one hoax to avoid at all costs.