Symptoms of a Bad or Failing Fuel Pump

The basic components of a fuel pump

What is a fuel pump?

The fuel pump does exactly what its name entails - pump fuel! It is responsible for moving fuel from the gas tank through fuel lines and into the fuel rail, from where it is injected into the combustion chamber. It is critical in supplying a consistent stream of fuel to the engine to ensure smooth operation and expected acceleration when you press the gas pedal. Although it works in conjunction with the rest of the fuel system, the fuel pump is a common failure point in a variety of car models.

Symptoms of a Bad or Failing Fuel Pump

A faulty fuel pump can lead to a wide range of symptoms and engine performance problems. Unfortunately, bad fuel system components in modern vehicles usually do not exhibit symptoms over time and may only be noticeable when the component fails, especially when the component is electronically controlled like the fuel pump.

A bad fuel pump has symptoms similar with other bad fuel system and ignition components including fuel injectors, the fuel pressure regulator, fuel tank pressure sensor, and even a bad gas cap.

Here is a list of possible symptoms caused by a bad or failing fuel pump:

• Engine won't start

• Lack of Power

• Engine misfires

• Sputtering or surging under heavy loads

Let's take a look at each of these symptoms and what they mean below.

1. Engine won't start

A gasoline engine needs three basic things to run – spark, fuel and compression. A faulty fuel pump can prevent fuel from reaching the engine, resulting in a vehicle that won’t start.

2. Lack of power

An engine that is starved for fuel due to a weak fuel pump may suffer from lack of power and sluggish acceleration.

3. An engine misfire

Complete combustion requires the proper amount of fuel and air. A bad or failing fuel pump can result in an air/fuel mixture that is lean (too much air, not enough fuel). This can result incomplete combustion, otherwise known as a misfire.

4. Sputtering or surging under heavy loads

The engine requires more fuel under heavy loads. A weak fuel pump can’t meet this demand. As a result, surging may occur.

Again, many of these symptoms are shared amongst other fuel system and ignition components. Thankfully, modern OBD-II diagnostic symptoms and even basic testing by an experienced mechanic make bad fuel pumps relatively easy to diagnose.

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How does a fuel pump work?

The fuel pump is a direct current (DC) motor mounted in the fuel tank. It does just what its name implies – it pumps fuel from the tank to the fuel injectors. The injectors then spray fuel into the engine.

There are two types of fuel systems in use today: continuous and return-less. How the fuel pump operates is a little different in these two systems.

Continuous Return Fuel System: Older vehicles have what’s referred to as a continuous return fuel system. With this design, fuel is delivered from the tank to the fuel rail by the fuel pump. The fuel injectors are located in the fuel rail, as is a pressure regulator. Since the fuel pump delivers more fuel than the engine needs, the regulator is used to maintain proper fuel pressure. The regulator accomplishes this by routing excess fuel back to the tank, via a return line.

Return-less Fuel System: Most modern vehicles use a return-less fuel system. As the name indicates, this type of system has no return line to the fuel tank and no externally mounted regulator. Instead, fuel pressure is controlled in tank. The fuel pressure regulator is often built into the fuel pump assembly. This design allows for more precise fuel control and reduced emissions.

Some return-less fuel systems regulate fuel pressure by varying the fuel pump’s duty cycle (on time). The fuel pump is usually either controlled by the powertrain control module (PCM) or a dedicated fuel pump control module (FPCM). A fuel pressure sensor is used to monitor system operation.

Regardless of the fuel system design, most modern fuel pumps come as an integrated assembly (sometimes referred to as a module). The assembly also includes the fuel pump sending unit, which is the sensor used to read fuel level. In some cases, the fuel pump assembly may also contain the FPCM.

How to Troubleshoot a Fuel Pump Problem

Before condemning the fuel pump, it should be tested. This is typically done by measuring fuel pressure with a mechanical gauge. On return-less fuel systems, it may also be possible to check fuel pressure using a diagnostic scan tool.

Fuel pump location

In addition to measuring fuel pressure, fuel volume may also be checked. This is done by removing a fuel line and inserting it into a container. Then, the pump is operated for a specific period of time. The amount of fuel gathered in the container is compared to manufacturer’s specifications.

If the fuel pump is found to be faulty, it should be replaced. On most modern vehicles, the pump is integrated into a fuel pump assembly. The entire fuel pump assembly must be replaced as a unit.

Fuel Pump Replacement Cost?

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