Symptoms of a Failing Fuel Pump


Normal car engines have four different strokes, hence being referred to as four-stroke engines. The strokes are intake, compression, combustion, and exhaust. The combustion stroke requires fueling and a spark which is provided by the spark plugs. Without fuel however, combustion can’t take place. Therefore, fuel is one of the most important components for an engine to run properly.

Fuel starts in the gas tank and has to travel all the way to the cylinders for combustion to happen. There are a number of things that the gas has to pass through first such as the fuel pump, fuel filter, fuel lines, injectors, and so on. A failure in any one of these components can prevent gas from getting to the cylinder, therefore preventing combustion and proper engine operations.

One of the most common fuel system failures is the fuel pump. This guide is going to discuss what the fuel pump is, differences in fuel pumps, symptoms of bad fuel pumps, and why fuel pumps fail.

What is a Fuel Pump and How Does it Work?

A fuel pump is responsible for sending gasoline from the gas tank to the engine so that the cylinders can fire. The fuel usually goes from the fuel tank, into the fuel lines, through the fuel filter and then to the injectors which then delivers the gas to the cylinders. The fuel pump is responsible for pulling the gas from the tank to the lines.

There are two types of fuel pumps: mechanical and electronic. Mechanical pumps tend to be used on older carbureted vehicles. Mechanical fuel pumps are driven by the camshaft and a diaphragm that essentially siphons fuel from the tank. Electronic fuel pumps on the other hand are most common in modern vehicles. These are also usually located within the gas tank whereas mechanical pumps sit in the engine bay since they are driven by the crank.

Electronic pumps utilize a small motor inside of them which is powered by the battery. When the ignition is turned, the motor starts spinning to build pressure within the fuel pump. Once the pump is pressurized it can draw fuel into the pump and then through the lines and filter which then feed into a fuel rail.

Low-Pressure vs High-Pressure Fuel Pumps

There are two forms of common fuel injection: port injection and direct injection. Port injected vehicles tend to have one fuel pump whereas direct injected engines will have two. Port injection refers to when fuel is delivered or sprayed into the intake ports, instead of directly into the engines cylinders. Since fuel is delivered into the intake ports, the fuel doesn’t need to be highly pressurized, therefore requiring these engines to only need one low-pressure pump.

Direct injection vehicles utilize a fuel injector for each cylinder. These fuel injectors operate at thousands of psi, and therefore are very high-pressure. Because of the pressure the injectors need, these engines need a high-pressure fuel pump. They utilize a low-pressure pump which sits in the fuel tank. The low-pressure pump pulls gas from the tank and sends it through the lines to the high-pressure pump. The high-pressure pump then pressurizes the fuel for the injectors.

Since high-pressure pumps operate at extreme pressure, they are more prone to failing than any other fuel pump.

Symptoms of a Bad Fuel Pump

A bad fuel pump will have a significant impact on drivability and engine performance. With that being said, fuel pumps can also fail over time rather than instantaneously, making a bad fuel pump harder to diagnose. These symptoms are the most common things you’ll notice when your fuel pump goes bad.

1. Engine shuts off while running or accelerating

Fuel pumps usually deteriorate over time, which is why they tend to slowly fail over time. When this happens they slowly lose their ability to draw fuel from the gas tank. Eventually, the pump will be so weak that it can’t deliver enough fuel for the engine to even start.

But before this happens you will likely run into issues with the engine shutting off while it is idling or while you are accelerating. At idle, the pump might be strong enough to supply adequate fuel for the engine to run. However, once you accelerate the engine requires more fuel. The fuel pump can be weak enough to the point it cannot deliver enough fuel for the engine to do more than idle.

2. Rough idling and misfires

A second common symptom is rough idling caused by misfires. Misfires occur when the engines cylinder either doesn’t fire or doesn’t fire at the proper time. It usually feels like the engine is hiccupping.

When the fuel pump is bad, it doesn’t deliver enough gas to the cylinder which will then prevent the cylinder from firing, causing a misfire. Misfires are usually easy notice since they tend to be accompanied by P0300 engine fault codes.

The downside of misfires as a symptom is that they can be caused by a multitude of problems such as injectors, fuel filter, spark plugs, ignition coils, and so on.

3. Fuel pump whining noise

On modern electronic fuel injected vehicles, the fuel pump “primes” when you turn the ignition over. You can hear the ignition turning over and starting to spin up and pressurize. It does this so that it can deliver enough fuel to start the engine. A bad fuel pump usually has a very whiny noise to it during startup and while the engine is running or idling. You will either hear this noise from around the back seat on the gas tank side of the car, or from the engine bay depending on which pump is failing.

Port/peripheral injected vehicles will mostly hear the sound from the back seat or gas tank area since they don’t have high pressure pumps.

4. Long crank and slow starting

An engine taking a long time to start or crank can be the sign of a bad fuel pump. Fuel pumps have to send fuel to the engine before it can start. Therefore, a bad pump can can cause a slow start or even a no start if it isn’t powerful enough to deliver enough fuel to the engine.

Long crank and no starts can also be caused by battery problems. If it isn’t a battery issue then it is probably a starter or a fuel pump issue. You will probably notice a period of long crank and slow starting before it stops starting completely.

Why Fuel Pumps Fail

Fuel pumps usually fail because of heat, stress, or contamination. These can be caused by a number of issues:

  • Bad fuel
  • Running out of gas
  • Constantly driving with a very low tank
  • A bad fuel filter overworking the pump

Poor fuel can cause sediment and particles to be sent through the fuel pump which can damage the motor, causing the pump to fail. This is most common on diesel engines since diesel fuel is less refined and dirtier than gasoline.

Running out of gas and frequently driving with very low fuel levels is terrible for the fuel pump. The less gas there is in the tank the harder the fuel pump has to work to draw the fuel. This causes overheating in the pump which then results in the internal motor failing.

Lastly, a bad or clogged fuel filter will cause less fuel to be delivered to the engine. The ECU will then realize not enough fuel is being delivered and signal for the fuel pump to work harder and pump more fuel, causing the pump to get overworked and fail.


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