3 Common Mechanical Fuel Pump Problems

How to Handle Them and Keep Your Car Running

Fuel pump
Small Block Chevy Fuel Pump. Photo by Mark Gittelman

The standard mechanical fuel pump found in classic cars is very reliable. Nevertheless, nothing automotive lasts forever. If your car has an external fuel pump, you'll probably run into situations that will require you to test and possibly replace this component. Learning how to diagnose these problems is essential for keeping your car in shape.

Pressure Problems

On modern automobiles, the average fuel pump pressure is over 60 PSI (pounds per square inch). On classic cars with mechanical style fuel pumps, however, the pressure is much lower—between four and six PSI. If you suspect that your fuel pump is not producing enough pressure, there are two tests you can perform. The first is a simple pressure output test. Many inexpensive old-school vacuum testers can read mechanical fuel pump pressure just as well as vacuum pressure.

To perform this test, you should connect the test gauge to the metal output line using a spare piece of rubber fuel hose and a clamp. Once the connection is sealed, crank the engine over for 20 seconds. This will provide a full pressure reading, letting you know whether or not the pump is still in working order.

The second way to evaluate the pump is to perform a fuel volume test. This should be done even if you've already done the first pressure test, as pumps can sometimes produce pressure without producing it at the right volume. An effective mechanic trick is to use an empty 12-ounce soda bottle to collect a fuel sample while a partner cranks the engine for 30 seconds. If the mechanical pump is working correctly, it will push four to six ounces of gas into the bottle.

Fuel System Leaks

Most mechanical fuel pumps have a weep hole on the bottom 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 in classic cars that are between 30 and 60 years old.

Another common place for a fuel leak to develop is in the rubber hose and metal tube that lead from the tank to the fuel pump. Since the metal tube is exposed to the elements, it's not uncommon for it to become rusted through to the point that the fuel is leaking out. Likewise, the rubber hose can also become worn out over time, leading to fuel leaks. If you notice that this component is damaged, replace it with a reinforced rubber fuel hose.

Engine Oil Leaks

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 drive the arm. 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 this gasket is usually reliable, engine vibration can cause the bolts to come loose. When this happens, oil can seep out around the fuel pump. If you notice a leak in this area, replace the seal as soon as possible, as detergents in the engine oil can cause further damage.

Tips for Replacing Mechanical Fuel Pumps

There are several best practices to follow when replacing the fuel pump or the sealing gasket. If the timing cover is made out of aluminum, for example, you should clean the sealing surface by hand without using abrasive materials. Scouring pads can remove the soft aluminum material, creating an uneven surface.

The integrity or straightness of the aluminum surface can be 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, RTV (Room-Temperature-Vulcanizing) silicone can be used to fill the gap. Although this is a last resort before replacing the timing cover, it's often successful given proper curing time before restarting the engine.

When the mechanical fuel pump leaks oil from a silicone 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. Regardless of the model of the fuel pump, the best way to ensure the bolt is tightened properly is to use a torque wrench. To make sure the bolt stays in place, apply a small amount of thread-locking compound before reassembly.