3 Common Mechanical Fuel Pump Problems

Find Out How to Handle Issues and Keep Your Car Running

Small Block Chevy Fuel Pump. Photo by Mark Gittelman

The standard mechanical fuel pump found on classic cars is very reliable. With that said, nothing automotive lasts forever. In the case of an external fuel pump, situations will arise that require testing and replacing this component. Here we'll talk about some of the common classic car fuel pump problems facing automobile enthusiasts and car collectors. Learn about testing volume, pressure and finding bolt torque specifications.

1. Pressure Problems With the Fuel Pump

On modern automobiles, the average fuel pump pressure is over 60 PSI. On classic cars with mechanical style fuel pumps, pressure is between four and six PSI. When a lack of pressure or output is suspected, there are two clear-cut tests that can answer the question, "Is my fuel pump bad?" The first test is a simple pressure output test. Many inexpensive old school vacuum testers can read mechanical fuel pump pressure as well as vacuum.

It's recommended to connect the test gauge to the metal output line using a spare piece of rubber fuel hose and clamp. With tight connections verified, crank the engine over for 20 seconds. This will provide a full pressure reading. Obtaining a reading after the fuel filter will also test the condition of the filter. A second test before the fuel filter should yield the same numbers if it’s in good shape.

The second procedure is to perform a fuel volume test.

This is necessary, because it's possible for the unit to produce pressure, but not the correct volume. An effective shade tree mechanic trick is to use an empty 12-ounce clear soda bottle to collect a sample. With a partner cranking the engine for 30 seconds, the mechanical fuel pump should push four to six ounces of gas into the bottle.

2. Fuel System Leaks

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 found on classic cars between 30 and 60 years old. The internal rubber diaphragm is capable of lasting a long time because gas is a petroleum product that helps extend the life of the rubber diaphragm through lubrication.

Another common place for a fuel leak to develop is the rubber hose and metal tube that leads from the tank to the fuel pump. Since the metal tube is exposed to the elements, it's common to see these rusted through to the point where the fuel is leaking out. In the same respect, 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 you can get your hands on. Use the specialized and reinforced rubber fuel hose in this situation.

3. Engine Oil Leak

On many automobiles, the fuel pump actuator arm passes through the timing case cover. This arrangement allows the constant rotating motion of the camshaft or crankshaft to operate the arm.

This is accomplished through a push rod and eccentric lobe that resembles a single lobe of a camshaft. In the example of a small block Chevy V-8, for every engine revolution, the fuel pump actuator is pushed and released one time.

Where the fuel pump mounts to the timing case 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.

Tips for Replacing Mechanical Fuel Pumps

There are several best practices to follow when replacing the fuel pump or the sealing gasket. On most internal combustion engines utilizing an exterior mounted mechanical fuel pump, the gasket is used alone without silicone or sealers from the factory.

In the situation where the timing cover is made out of aluminum, clean the sealing surface by hand without using abrasive materials. Cleaning pads can remove the soft aluminum material, creating an uneven surface with low spots.

The integrity or straightness of the aluminum surface is checked with a small straight edge and a set of feeler gauges. If the area has low spots more than half the thickness of the replacement fuel pump gasket, room temperature vulcanization (RTV) silicone can be used to fill this gap. Although this is a last resort before replacing the timing cover, it's often successful with proper curing time before restarting the engine.

When the mechanical fuel pump leaks oil from a silicone or composite style gasket the cause can often be traced back to improperly tightened pump mounting bolts. The fuel pump bolt torque specification is usually around 25 to 35 foot-pounds, but can vary on different models. Regardless of the exact specification, the best way to assure the bolt is tightened properly is to use a torque wrench. In order to make sure it holds in this position, apply a small amount of thread locking compound before reassembly.