Modern vehicles use electric fuel pumps, located inside the fuel tank. Due to the complexity of the pump and difficulty of access, replacement can be very expensive. With care, failure of the fuel pump is often preventable.
Why are pumps mounted in the tank?
Carbureted vehicles do not need much fuel pressure. The pump on these models produces between two and six pounds per square inch of pressure. These are often diaphragm pumps, mounted on the engine. The diaphragm pump has good suction, but not too much pressure. When we operate the pump, we draw fuel from the tank and pump gasoline into the carburetor bowl.
Injected vehicles require a much higher and precisely regulated fuel pressure. They meet these needs with a rotary displacement pump. This style of a pump produces high pressure that is very precise. The design also lacks suction to draw fuel from a long distance. What the rotary pump lacks in suction it more than makes up for in pressure. Submerging this type pump under fuel, provides head pressure for the pump.
Placing the fuel pump in the tank also cuts assembly costs. Fuel tanks arrive at the manufacturer, complete with the pump. This saves installation of a fuel tank and a pump.
Most vehicles today employ an electric fuel pump module
Early fuel pumps were replaceable, separately from the level sensor and pickup tube. More modern designs employ a fuel pump module. The fuel pump module consists of a fuel pump, internal wiring, anti drain-back valve and a strainer. A sender unit sends a signal to the fuel level gauge to show the fuel level in the tank. They may also include a fuel filter, as part of the module. These fuel pump modules are most often available only as an assembly. A few have fuel senders and fuel filters that are separately replaceable.
The pump is the heart of the fuel pump module and does the work of moving fuel from the tank to the injection system. A small permanent magnet motor drives an integral pump. Fuel enters the pump through the strainer, flows through the motor and exits under pressure.
Fuel pressure is critical and the pump maintains it within a narrow range. Older systems used a fuel pressure regulator in the front of the vehicle. The fuel pressure regulator returned excess pressure from the engine fuel rail to the fuel tank. This offered an advantage of allowing fuel to cool.
More modern systems regulate pressure by varying pump voltage or with an internal pressure relief valve. Pumps with relief valves simply dump excess pressure back to the fuel tank.
Fuel flowing through the fuel pump cools and lubricates the internal components
To cool and lubricate the internal components, fuel must flow continuously through the fuel pump. Insufficient flow limits cooling and lubrication of the pump. A leading cause of fuel pump failure is running the fuel tank low. This is particularly critical on late model vehicles without a fuel pressure return system. Running such a vehicle out of fuel once can permanently damage the fuel pump.
When the fuel level is low, the pump has to work much harder to produce the same pressure. This is because the reduced fuel weight no longer pushes fuel into the pump. Instead the pump must draw the fuel in. A low fuel level also means less fuel to dissipate heat and lubricate the pump. The combination of an overworked pump, reduced cooling and lubrication will likely damage the fuel pump.
Another major cause of fuel pump failure is dirty fuel
Contaminants that enter the fuel tank are drawn through the fuel pump. The strainer on the pump will try to remove larger (more than 70 microns) particles. Unfortunately, much smaller (30-40 microns) particles pass right through and do most of the damage. Low quality fuel damages the fuel pump and can cost much more than any savings from a lower price.
Debris that enters the pump wears the commutator and brushes on the motor. The debris can also stick the anti drain-back valve. As the brushes and commutator wears, the motor pulls more amperage to produce the same pressure.
Additional amperage, from a worn pump, often burns the connectors inside the fuel pump. Burned connectors and harnesses on failed pumps are extremely common. Always check connections carefully, before replacing the fuel pump. Failure to replace a burned connector will cause the replacement pump to fail very quickly.
Fuel pumps don’t always fail completely
A no start condition is often a symptom of a failed fuel pump. Fuel pumps can also fail in other ways. Worn brushes often produce a whine noise from the fuel tank. A fuel pump with problems will often produce a loud whine.
In the outlet of the fuel pump is an anti drain-back valve. This valve closes when we turn off the pump and prevents fuel from running back into the fuel tank. Extended engine cranking-time is often a symptom of a bad anti drain-back valve in the fuel pump.
Worn brushes, commutators and broken/loose windings in the fuel pump motor or harness can also cause the vehicle to die. This often happens after driving a while. Often the vehicle starts again after the pump cools down. Such problems are intermittent and can be very difficult to diagnose.
Most late-model vehicles no longer have separately replaceable fuel filters. Instead the fuel filter is part of the fuel pump and not separately available. This is unfortunate as fuel quality is a real problem and a clogged filter may mean a new fuel pump. More than ever, purchasing only high quality fuel is important.
Needlessly replacing fuel pumps
A lack of fuel pressure will cause a vehicle not to start but a no-start does NOT necessarily mean the fuel pump is bad. Dozens of things, other than a bad fuel pump, can prevent an engine from starting. A lack of fuel pressure does NOT necessarily show a bad fuel pump.
Modern vehicles have several things that can disable the fuel pump. Security systems often cut power to the fuel pump if they think someone is stealing the vehicle. Even oil pressure switches can disable the fuel pump on some vehicles. Experts estimate as much as 50% of all fuel pumps replaced are mis-diagnosed. Fuel pump replacement does not solve the problem as the pump was not the cause.
Preventing fuel pump problems
Preventing fuel pump failure is far less expensive than replacing the pump. Using quality, name-brand fuel normally prevents many problems. Buying fuel only from high-volume stations also helps.
Ethanol in fuel can add other problems. Fuel with ethanol will go through phase separation if it remains too long in the tank. The alcohol and fuel separate and fuel becomes unusable. We should never allow gasoline containing ethanol to remain in the tank more than three months. Regularly driving the vehicle is necessary to prevent problems.
Do not purchase fuel while a tanker-truck is filling the station tanks. This can stir debris that has settled in the tanks. Another factor is not allowing the tank to fall below one-quarter. Keeping the tank above this level can greatly extend the life of the fuel pump.