Common Issues with Low-Pressure Mechanical Fuel Pumps

Older model cars with carburetor engines use low pressure mechanical fuel pumps to pump fuel from the fuel tank to engine. They are mounted outside the fuel tank, mostly on the side of the engine. With the advent of fuel injection systems, most mechanical fuel pumps are being replaced with electrical fuel pumps, albeit, few old car owners are still retaining these mechanical fuel pumps.

Low pressure mechanical fuel pumps are less functional compared to electric fuel pumps and incur frequent problems that lead to driveability issues. However, on newer model cars with GDI equipped engines, mechanical fuel pumps are making a comeback. There is an electric fuel pump in the tank delivering fuel to a high pressure engine mounted mechanical fuel pump.

In this article, we will discuss only about the common issues with low pressure mechanical fuel pumps used in carburetor engines along with the ways to identify and resolve them.

Common issues with mechanical fuel pumps: Common issues faced by engines with low-pressured mechanical fuel pumps are:

Vapor lock: Vapor lock happens when the liquid fuel in the fuel delivery system changes to gaseous form and blocks the passage of the fuel. As the mechanical fuel pump is located in the engine compartment, the high engine heat on the pressurized side of the pump boils the fuel in the fuel lines leading to vaporization of fuel.

Due to vapor lock, the operation of the fuel pump gets disturbed. It causes loss of feed pressure, which results in loss of power transmission to the engine or complete stalling of vehicle. Common signs of vapor lock are no or low fuel pressure, dry carburetor air horn and no accelerator pump discharge, stalling, hard starting and low power.

Fuel foaming: Fuel foaming occurs when cold fuel enters a hot carburetor. Fuel foaming leads to a series of short jerks on acceleration and finally results in dead engine. The common signs of fuel foaming are black smoke emitted from the exhaust pipe, wet carburetor air horn and starting of engine after a long wide open throttle crank.

Alcohol mixtures: Gasoline additives such as alcohol mixtures and octane boosters affect the volatility of the gasoline and result in performance and driveability issues. These additives also cause fuel system corrosion, dislodge rust and foreign particles into the fuel tank leading to fuel filter clogging and finally affecting the functionality of a fuel pump.

Causes of fuel pump failure: A worn-out or a leaky diaphragm inside a mechanical fuel pump leads to fuel pump failure. A leaky diaphragm either results in fuel leak or loss of fuel pressure. This eventually increases the pressure on the pump making it fail. Even leaky inlet or outlet valves and broken spring also result in the same.

Recognizing a bad fuel pump: Before your fuel pump completely ceases to work, it displays some signs of failure. Identify these signs and rectify the problem as early as possible. Though there are professional troubleshooting techniques to check the functionality of the fuel pump, there are few other simple ways which help you identify the bad fuel pump even if you are an amateur.

In the initial stages of a bad fuel pump, you may experience lack of power, starting trouble, sputtering, etc.
Look at the fuel pump. Dripping of fuel from the pump indicates failed diaphragm, hence, replace the fuel pump.
Inspect the throat of the carburetor by removing the air cleaner, then pump the throttle linkage and check if any fuel is squirting into the carburetor. No squirting of fuel can indicate failed fuel pump.
Disconnect the fuel line at the carburetor and place it in a container. Start the engine and see whether the fuel pump is pumping the fuel. Steady spurts of fuel indicate proper functioning of fuel pump, while weak stream or no fuel can indicate bad fuel pump.
Pull out the dipstick to check the oil level. Oil level above the full mark can indicate leaking diaphragm, as it lets the fuel leak into the crank case therefore thinning the oil and raising the oil levels.

Timely repair/replacement avoids engine problems: Engine performance gets affected negatively if the fuel pump issues are not addressed in a timely manner. If the fuel pump is unable to deliver the fuel with required pressure, it starves the engine and finally ceases its operation. Hence, timely identification and fixing of the problem is quite essential to avoid engine problems.

Make sure that you are using reliable after-market products while replacing engine components. It is important for better performance of the fuel delivery system.