How to detect a bad fuel pump in your car

Car engine
Car engine

A fuel pump’s primary task is to deliver fuel from the storage tank to the engine, and works with multiple other auto parts to accomplish that duty. That can make it difficult to know whether or not it’s failing, unless you know the symptoms of a bad fuel pump.

The most important key to diagnosing a car is listening and understanding how it normally works, runs, and acts on a daily basis. What sounds does it make? What rattles? What are its normal RPMs? With this knowledge base, and an attentive mindset, you’ll notice when the vehicle starts running irregularly.

Realizing the vehicle starts hard, struggles to maintain idle, or flexes less power is the first step in accepting there’s an issue and setting out to fix it. Because so many car parts are interconnected and interdependent, diagnosis is not always a simple exercise. It requires trial and error, and pinpointing the exact source often requires the process of elimination.

That’s what we’re here to discuss, in regards to fuel pumps. Follow The Drive’s thorough guide to bad fuel pump symptoms and figure out what the heck is going on with your ride.
What Is a Fuel Pump?

A fuel pump is a mechanically or electrically controlled mechanism that sends fuel from the gas tank through the fuel filter to the fuel rail. From there, fuel is distributed to the injectors and sprayed into each engine cylinder combustion chamber. On older vehicles, the pump sends fuel to a carburetor.

Types of Fuel Pumps

Automobiles have primarily transitioned away from mechanical pumps on the engine block in old cars to electric pumps inside the gas tank on modern vehicles. Below, we break down the various types of pumps and how they work.

Mechanical fuel pump: A mechanical fuel pump uses positive displacement with a plunger or a diaphragm. In a cyclical action, the pump draws in fuel, traps it, then moves that fuel toward the carb and engine, and repeats the process.

Electric fuel pump: An electronic fuel injection system on modern vehicles requires higher pressure, which is accomplished, in part, by using an electric fuel pump rather than a mechanical module. Here are a few examples of electric pumps:

  • In-tank: The majority of new vehicles sold today use fuel pumps that are located inside the fuel tank. This helps protect the module, and the gas helps keep it cool.
  • Inline: Much easier to replace than an in-tank pump, an inline pump is located between the gas tank and the engine and is installed within the fuel system track. These are often mounted under the car or in the engine bay.
  • Rotary vane: Like mechanical pumps, rotary vane pumps use positive displacement to move the fuel along. Inside a rotary vane pump, a rotor with “paddles” operates off-center within a cavity. Because it is off-center, it creates a crescent-shaped pocket of space that lets the fuel in. The vanes close the small amount of fuel as the rotor moves, and release the fuel once it crosses the outlet valve. Different types of vane pumps include rolling vane, sliding vane, flexible vane, and swinging vane.
  • Gerotor: A gerotor pump also uses positive displacement but uses different parts to achieve that method. Within a gerotor pump, there are two interlocking gears, a toothed inner gear and an outer ring gear with smooth curved lobes. The inner gear is offset from the outer gear. Thus, when they rotate, small pockets of fuel can be trapped, pressurized, and released.

How Long Do Fuel Pumps Last?

Fuel pumps are not part of a regular maintenance schedule and only need to be replaced when they fail. Most fuel pumps should last well past 100,000 miles.

Symptoms of a Bad Fuel Pump

Though these symptoms could be sourced to a faulty fuel pump, that is not always the case.

Car Won’t Start

There are a host of issues that could prevent a car from starting, but a lack of fuel is one of the first things to check. If the fuel pump can’t send fuel to the engine, it won’t start.

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