Mechanical fuel pumps deliver fuel to carbureted engines and are found on older vehicles. They are mounted on the side of the engine, run on engine momentum, and provide a constant supply of fuel to the carburetor. They are easy to replace, but can’t handle high pressure fuel systems and have been replaced by electronic fuel pumps.
A mechanical fuel pump is a device that delivers fuel to the engine. Due to the fact that mechanical fuel pumps generally work better on carbureted engines, they are generally found only on older vehicles.
A carburetor squats over the engine in many older cars and delivers fuel to the combustion chambers through the intake ports. Some older cars had fuel lines under the gas tank so that gravity would do the work of transporting gasoline from the tank to the carburetor. However, many older models needed a little extra help getting fuel from the tank to the carburetor. Therefore, the mechanical fuel pump, which was usually located on the side of the engine, was designed to run on engine momentum and provide a constant supply of fuel to the carburetor.
A mechanical fuel pump is mounted on the side of the engine. A lever or pushrod on the pump passes through an opening in the side of the engine and aligns with a special lobe on the camshaft. As the camshaft rotates, the lobe moves the lever up and down, raising and lowering a flexible diaphragm inside the pump. With the movement of the diaphragm, gasoline travels down the fuel lines and into the pump. From there, the gasoline is pushed into the carburetor, which uses the engine’s vacuum to draw fuel into the combustion chambers.
Because diaphragm fuel pumps run directly off of the engine, they will only run when the engine is running or starting. This prevents dangerous fuel buildups in the lines. Mechanical fuel pumps also do not pressurize the system very much: most carbureted fuel systems run as low as 4 pounds per square inch (psi) and rarely exceed 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 a mechanical fuel pump, it will leak fuel. The fuel line is below the gas tank, so gravity will continually pull fuel down the line if you don’t cap it. You’ll also need to make sure to properly align the lever or pushrod up with the engine’s camshaft when you install the new fuel pump, so you don’t accidentally damage the pump by cranking the engine.
Eventually, electronic fuel injection systems, which were favored for greater precision and control, replaced carburetors. When this happened, cars also began to require fuel systems that ran at high pressure, usually between 40 and 60 psi. Because a mechanical fuel pump can’t handle a system under that much pressure, they were eventually dropped in favor of the higher performance electronic fuel pump.
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