Mechanical Fuel Pumps – Basic Information And Troubleshooting

Mechanical Fuel Pumps - Basic Information And Troubleshooting
Mechanical Fuel Pumps - Basic Information And Troubleshooting

The standard mechanical fuel pumps found on classic cars, are very reliable.

With that said, nothing automotive lasts forever. So, at some point, mechanical fuel pumps will fail.

Firstly, most mechanical fuel pumps, are used on older engines, that have carburetors.

How Mechanical Fuel Pumps Work

Mechanical fuel pumps, siphon fuel from the gas tank. It then pushes it to the carburetor, when the engine is cranking or running. All mechanical fuel pumps, used on inline six cylinder and V8 engines, are diaphragm type. As a result, no adjustments or repairs are possible.

Fuel Pump Arm
Fuel Pump Arm

The pump is operated by, an eccentric lobe on the camshaft or the camshaft gear. On six cylinder engines, the eccentric rides, directly on the fuel pump arm. On V8 engines, a push rod between the camshaft eccentric and the fuel pump, operates the, mechanical fuel pumps arm.

Is The Problem, The Fuel Or The Fuel Pump:

  • If the engine lags during acceleration, after having idled for a long time; then you may be experiencing vapor lock. Also, other symptoms include, lack of fuel pressure, no accelerator pump discharge; or dry carburetor air horn. You can also experience this, on a hot day while driving.
  • If, after filling the tank, the engine lags or jerks repeatedly; fuel foaming could be the cause. Fuel foaming happens when, cold gas comes in contact, with a hot carburetor fuel bowl.
  • Gasolines that contain octane boosters or alcohol, may alter volatility; which can cause drivability problems or hard starting. Consequently, alcohols can corrode components, throughout the fuel system. This will result in, particles that clog the fuel filter and alter, the air/fuel mixture.
  • An engine that misfires, runs lean, hesitates, or stalls; could have a leaking diaphragm or valve, within the fuel pump. Both of these problems, cause the fuel pressure to drop; starving the carburetor of gasoline and creating the above symptoms. Finally, if the pump dies, the car will not start or run.

Checking The Fuel Pump:

  • Remove the air cleaner. While looking in the throat of the carburetor, pump the throttle linkage. A working pump, will squirt fuel into the carburetor. However, if no fuel appears; verify the tank has gas and check the fuel line and fuel filter for blockages.
  • Visually inspect the pump. Inspect the fuel pump visually. If fuel is dripping, the diaphragm is faulty and you need a new pump.
  • Another way to check the pump is to; disconnect the fuel line at the carburetor and place it in a container. Crank the engine to see if, the pump is pushing any fuel through the line. Strong steady spurts of fuel, mean the pump is working. No fuel or a weak stream means, a bad pump, a plugged fuel filter, fuel line blockage.
  • You should also check fuel pump pressure. Connect a fuel pressure gauge to the pump outlet; or tee a gauge into the fuel line at the carburetor. Crank the engine and note the pressure reading on the gauge. If there is no pressure, or if pressure is less than specifications, replace the pump.

Pumps – (Leaking Fuel):

  • Most mechanical fuel pumps, have a weep hole, on the bottom side of the unit. When the internal diaphragm leaks, fuel escapes through the weep hole; to notify the vehicle owner of a malfunction. This is one of the more common fuel pump problems. Usually found on classic cars, between 30 and 60 years old. The internal rubber diaphragm, is capable of lasting a long time. So, you could actually have, fuel leaking into your oil. Gas is a petroleum product, that helps extend the life of the rubber diaphragm through lubrication.
  • Another common place for a fuel leak, is the hoses that lead from the tank to the fuel pump. Since the metal tube is exposed to the elements; it’s common to see these rusted and leaking. The rubber hose that connects the metal tube to the fuel pump, can also dry rot and leak. A common mistake is, to replace this small section of rubber hose with any scrap piece. Use the specialized and reinforced rubber fuel hose in this situation.

Pumps – (Leaking Oil):

  • On many automobiles, the fuel pump actuator arm, passes through the timing cover. This arrangement allows the constant rotating motion, of the camshaft or crankshaft to operate the arm. That also adds another place for oil to leak.
  • Where the fuel pump mounts to the timing cover, a gasket provides a tight seal. Although capable of long term reliability, often engine vibration, will cause the bolts in this area to loosen up. When this happens, it’s possible for oil to seep out, around the fuel pump to timing cover gasket. If the leak continues long enough, replace the seal. Because, the detergents in the engine oil will eventually damage it.

Noises From, Common GM Pumps:

  • A knocking or ticking noises can come from, a broken spring on fuel pump arm.
Common GM Fuel Pump
Common GM Fuel Pump

How To Replace Mechanical Fuel Pumps:


So, there are two holes in the passenger side front of the engine. These holes were for engines that used front engine mounts. The top bolt hole, aligns with the fuel pump push rod and will have a 3/8″ x 3/4″ bolt. This bolt needs to be removed and a longer 3/8″ bolt installed in its place. Some block assemblies are supplied from the factory with a 3/8″ by 7/8″ bolt with (2) thick washers.

This will allow the (OEM) bolt to be re-inserted without the washers to secure the rod in place. Once the (2) fuel pump mounting bolts are tightened, the bolt securing the push rod can be removed. They can be re-inserted with the washers under the head thereby preventing contact with the rod, but sealing the hole. The longer bolt, is used to gently hold the fuel pump push rod, in the retracted position.


The camshaft has a lobe on it, that causes the fuel pump push rod, to move in and out. This is what makes the fuel pump actually pump fuel. Total travel of the pump rod, is about 0.394′ (10 mm). This lobe needs to be positioned; in such a way that the rod is furthest back, away from the pump. While this isn’t absolutely necessary, it can make installing the pump a little easier.

So, with the longer bolt in the upper hole, carefully tighten the bolt, until it holds the push rod. It should stop it from sliding back out; remember just finger tight or you’ll bend or nick the push rod. Remove the fuel lines, plug them to prevent dirt from entering. Remove the mechanical fuel pumps mounting bolts, then the pump. Clean all the way around the fuel pump hole in the spacer.


Sometimes the hole in the front of the block, can not be accessed. An alternative is to; remove the spacer plate and push rod and put some heavy grease on the push rod. This grease will (for a while) hold the pushrod to the cam, in the retracted position. Reinstall the spacer plate, use a new gasket. With the gasket on the fuel pump, insert the fuel pump arm into the hole in the spacer plate. Once the pump arm is in position, you’ll have to push against the pump’s return spring pressure. This will get the pump’s mounting holes, close enough to the spacer, to start the bolts.


Now you will need to loosen the bolt, holding the push rod in position; so the push rod can move. Also, you may need to rotate the engine a bit, to let the fuel pump cam, retract the push rod. Tighten the bolts evenly, until the pump meets the spacer. DO NOT forget, to remove the longer bolt, holding the push rod. Replace the original bolt into the same hole, with a little sealer on the threads. Finally, reattach the fuel lines and do not use teflon tape. Now you know more about, mechanical fuel pumps.

Related Info And Specs:

GM Applications – Fuel Pump Push Rod Info


Product Fits 1955 to 1990 – Diameter: 0.5 In.  – Length (In): 5.75 In.  – OEM #3704817

Fuel Pump Block Off Plate

Block Off Plate
Block Off Plate

Only necessary if, switching to an electric fuel pump, to plug the hole and stop leaks.


So, I would say the most common problem, is a clogged fuel filter. As a result, make sure you follow your manufacturers recommendations, as to when you should change the fuel filter. Consequently, this information, should be in your owner’s manual. Now you know more about, mechanical fuel pumps.

Thank You !