An inline fuel pump can take on different forms. One type found in most modern fuel-injected vehicles can be found inside of the vehicle's fuel tank. The fuel pump actually sits in the fuel tank and connects to the fuel line inside of the tank. Another type can be found mounted along a vehicle's frame or chassis. This type of fuel line is often found on carbureted vehicles as well as high-performance vehicles.
The difference between an inline fuel pump and the traditional fuel pump is that a traditional fuel pump attaches to the engine block. This type of pump is typically driven off of the camshaft via an eccentric wheel. The wheel is attached to the camshaft in a matter which allows it to operate in an oblong circle. As the fuel pump arm rides on the wheel, it makes a pumping motion as the wheel wobbles up and down. An inline fuel pump is typically battery-operated and is wired into the vehicle's electrical system.
One advantage the inline fuel pump has over the traditional block-mounted pump is the lack of heat-related vapor lock. As fuel is heated, it can tend to vaporize. This causes a condition known as vapor lock, where the pump can no longer pump and move the fuel to the engine. By locating the fuel pump away from the engine and mounting it in a location away from any heat source, this condition is rarely a factor in the pump's performance. Most inline fuel pump systems use a return line to bleed off excess fuel to the fuel tank, thereby relieving pressure and heat away from the fuel pump.
By nature, most engine-mounted fuel pumps are pulling the fuel from the rear of the vehicle forward to the engine bay. The inline fuel pump pushes the fuel from the rear-mounted fuel tank forward to the engine compartment. As in most mechanical operations, physics makes it much easier to push than to pull an object. Thus, the rear-mounted fuel pump is able to work using less effort to transfer the fuel from the tank to the engine than an engine-mounted system.
With the inline model of pump being electronically-operated, it maintains the pressure and flow of fuel much more consistently than an engine-driven pump that is subject to variations in engine speed. The inline fuel pump also allows for the fuel system to be purged as the electric system is switched on. The engine-driven pump is not able to pump fuel until the engine has been cranked and started.