How To Check A Mechanical Fuel Pump

How To Check A Mechanical Fuel Pump?

Knowing how to check a mechanical fuel pump can be your saving grace if you’ve got an old car. Just like knowing crucial skills like changing spark plugs, plugging a tire, or testing a flasher unit, knowing how to check a mechanical fuel pump will not just help you save money but also ensure your car runs faster and outlives its shell life ( Check out How to make your car faster).

Worried about how you’re going to educate yourself on this skill from the confines of your home? We’ve got you covered, so don’t panic! This article will serve you with all the details you need to know. This article will serve as a perfect guide for you, from enlightening you regarding performing pressure tests using a mere plastic bottle to teaching you how to check a mechanical fuel pump without any external help.

What is a mechanical fuel pump?

A device that transports fuel to a car’s engine is a mechanical fuel pump. These pumps work most effectively on carbureted engines. This is why you’ll find them mostly in older models of vehicles.

The mechanical fuel pump is placed on the engine’s side. It was designed to give a constant fuel supply to the carburetor. This pump has a lever that goes into an opening placed on the engine’s side. Then, it meets a lobe placed on the camshaft. When the camshaft moves, the lobe shifts the lever. Placed in the pump is a flexible diaphragm. The lobe’s movement moves the flexible diaphragm up and down. Then, the gasoline is transferred to the carburetor. The carburetor uses the engine’s vacuum to push fuel inside the combustion chambers.

Common problems in a mechanical pump

We know that a mechanical pump performs the pivotal task of delivering fuel to the engine. However, it can also be met with several faults that can hinder its performance. Thus, first, understand the problem to perform a proper diagnosis of the pump. Therefore, the following is a list of some of the most common problems that occur in a mechanical pump:

1.      Pressure Issues

A mechanical fuel pump should pump an average of more than 60 PSI in a modern-day vehicle. Vintage cars can have lower pressure, i.e., around 4-6 PSI. Check your car’s pressure. Perform tests if it is lesser than the required PSI amount.

You can perform two tests. An output pressure test is the first test. Connect the metal output and the test gauge via a clamp and a fuel hose. Ensure the connection is properly sealed. Then, keep on cranking the engine for 20 seconds. This will provide you with an accurate pressure reading.

The second method is to test the fuel volume. Collect some fuel samples using an empty bottle while cranking the engine for 30 seconds. A well-functioning mechanical pump will give out 4-6 ounces of gas.

2.      Leaky Fuel system

A leaky fuel system is a common problem in older car models. Fuel can leak from the metal tube, internal diaphragm, and metal hose.

3.      Leaky Engine oil

The gasket is meant to act as a seal. However, due to the constant vibration of the engine, the bolts tend to loosen up. This causes oil to leak out from the spaces. Leaky engine oil should be treated and replaced immediately as the presence of detergents tends to cause more damage.

How to check a mechanical fuel pump

If your car stops running while still having petrol, your fuel system is faulty. In such instances, it’s important to know how to check a mechanical fuel pump since the problem highly likely lies there.  

The first step is to disconnect the carburetor’s fuel line. Now, while disconnecting, you need to catch the remaining fuel inside. Do so by placing a plastic bag at the end of the fuel line. Use a rubber band to secure the bag once you’ve collected the fuel. Lastly, use a starter to turn over the engine.

If the fuel doesn’t spurt out, the fuel pipe either has a leak or blockage. It may also mean that the fuel pump isn’t functional. Check If your fuel pump has screws. If it doesn’t, then it means it is sealed, and it is beyond repair. It’s unusual for a mechanical pump drive to fail. However, you may observe some wear and tear on the linkages, pins, and operating lever.

  • Diaphragms

Diaphragms tend to leak. A leaky diaphragm affects the engine when it is at high speed. Be wary if the petrol leaks to the pump from the camshaft as it can cause an explosion.

  • Valves

A small cage holds the seating and disc in a valve. To put the valve on its seating, put pressure in a single direction. However, pressure on the other side will open the valve by lifting the disc.

Dirt in the valves will hinder proper seating. To clean the dirt, you need to overhaul the mechanical pump.

  • Gaskets

Loose fuel pipes and gaskets can also leak along with the pump body and the diaphragm seal.

  • Loose mountings

Another potential problem is loose mountings. This fault often goes unnoticed as the pump remains out of sight. Loose mountings can cause extreme oil leaks. Hence, fix them quickly.

Pump: Removal and Replacement

  • There is a high chance that the fuel lines will gush out petrol once disconnected. Thus, to prevent this, keep old pencils and tapered plugs beside.
  • Wire clips are used to connect some lines. Cut the wire clips using side cutters. Then, use new screw clips as a replacement. As a temporary relief, you can also use twisted wires.
  • Ensure that the camshaft is beneath the lever when you fit the pump. Otherwise, it will damage the operating cam.
  • Lastly, replace the gaskets and screw in the bolts. Make sure to not overtighten them.

Pump Operation: Inspection

  • The most common way of testing a pump is by putting a pressure gauge and vacuum gauge on each side.
  • However, do not test it by pumping in compressed air; otherwise, the diaphragm swill burst.
  • Finding the fault of a sealed pump is as good as nothing. A faulty sealed pump will always need replacement.


So, the next time you experience a leaky fuel, low pressure, or engine oil situation, know that it’s time you learn how to check a mechanical fuel pump before it’s too late.