A mechanical fuel pump is a device that delivers fuel to the engine. Due to the fact that mechanical fuel pumps generally work best on carbureted engines, they are typically found only on older vehicles.
A carburetor squats over the engine on many older cars and delivers fuel into the combustion chambers via intake ports. Some older cars ran the fuel lines below the gas tank so that gravity does the work of carrying gasoline from the tank to the carburetor. However, many older models needed a little additional help to get the fuel from the tank to the carburetor. The mechanical fuel pump, which was usually located on the side of the engine, was therefore designed to run off of the engine’s momentum and provide the carburetor with a steady supply of fuel.
A mechanical fuel pump mounts to the side of the engine. A lever or push rod on the pump passes through an opening in the side of the engine and lines up with a special lobe on the camshaft. As the camshaft turns, the lobe moves the lever up and down, raising and lowering a flexible diaphragm inside the pump. With the motion of the diaphragm, gasoline is drawn down the fuel lines and into the pump. From there, the gasoline is pushed into the carburetor, which uses the vacuum of the engine to pull fuel into the combustion chambers.
Because diaphragm fuel pumps run directly off of the engine, they will only work when the engine is running or being started. This prevents dangerous buildups of fuel from occurring in the lines. Mechanical fuel pumps also do not pressurize the system very highly: Most carbureted fuel systems run as low as 4 pounds per square inch (psi), and rarely ever more than 15 psi.
Since diaphragm fuel pumps are located right on the side of the engine, replacing this part is relatively easy and quick. However, despite the low pressure levels of carbureted fuel systems, if you disconnect the line that feeds into a mechanical fuel pump, it will drip fuel. The fuel line is below the gas tank, so gravity will continuously pull fuel down the line if you do not plug it. You will also need to be sure that you correctly line the lever or push rod up with the engine’s camshaft when installing the new fuel pump, so that you do not inadvertently damage the pump when you turn the engine over.
Eventually electronic fuel injection systems, which were favored for greater accuracy and control, replaced carburetors. When this happened, cars also began to require fuel systems that ran under high pressure, generally between 40 and 60 psi. Because a mechanical fuel pump cannot support a system under that much pressure, they were eventually cast aside in favor of the higher performing electronic fuel pump.