A more sophisticated approach to adding more miles to each gallon of gas comes from the air bleed and vapor bleed devices. An air bleed device sends additional air into the carburetor. They are typically installed on the positive crankcase ventilation (PCV) line or replace the idle-mixture screws.
The EPA has tested more than 20 air bleed devices and has found only one that slightly increased fuel mileage, but at the cost of increasing exhaust gases coming from the engine. Although one of these products showed a small increase in gas mileage, before you add an air bleed system into your car, consider the fact that the federal government considers some of these systems to be illegal tampering when installed.
A vapor bleed system, sometimes referred to as a mixture enhancer, works in a similar way but instead of just adding more air, it vaporizes the fuel going into the PCV manifold. Some vapor bleed systems work by taking liquid fuel on its way into the engine, and mixing it with air from a pressure line. The pressurized air and fuel mix together in a chamber until the fuel becomes vaporized and is then released into a line that feeds into PCV manifold.
The idea behind vaporized fuel is that the engine will burn the fuel more completely, not wasting any of the fuel and so increasing the mileage of each gallon. As opposed to the additives we talked about earlier these systems need to be installed in the engine compartment and certain modifications and additions added to the engine. Out of all the vapor bleed and mixture enhancer products that the EPA tested, not one of them showed any improvements to gas mileage.
But if you're still looking for that silver bullet for your car's gas tank, you can try your luck on the next page.