Weak Fuel Pump Symptoms: 5 To Look Out For

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When a fuel pump nears its end, it starts showing us that total failure is imminent, so what are the weak fuel pump symptoms to look for?

  • Engine Runs Rough
  • Fuel Pump Noise
  • Bad Throttle Response
  • Engine Dies Randomly
  • Engine Struggles To Start

With the symptoms shown, let’s go more in-depth on why these symptoms appear and how to diagnose them.

a bosch 044 fuel pump cut open to show the innards
A cut-open Bosch 044 high-pressure fuel pump by © Raimond Spekking, https://commons.wikimedia.org/w/index.php?curid=72071143

5 Weak Fuel Pump Symptoms

1. Engine Runs Rough

The engine of the vehicle needs three ingredients to run, air, spark, and fuel, and since the fuel pump’s job is to supply the fuel rail or carburetor of the vehicle with the fuel, it can result in the car running rough.

Since there isn’t enough fuel brought to the fuel rail or carburetor, the engine RPM will be inconsistent and cause the engine to shake and sputter.

2. Fuel Pump Noise

Inside a fuel pump are an electrical motor, bearings, and the pump assembly with the blades and one-way valve.

All these can fail over time, but if the bearings fail, the pump will start whining or humming when the vehicle is turned on.

It might not be audible from inside the cabin, depending on how good insulation your vehicle has, but with a door or window open, you can hear it, especially as the fuel tank empties.

3. Bad Throttle Response

You can imagine the engine running rough already on idle; it will be worse once you start driving.

As the demand for fuel rises, the fuel pump can’t keep up, and this causes the car the lurch and chokes out when you start accelerating.

It feels like someone else has control over the throttle pedal and is deliberately going on and off it in quick succession.

4. Engine Dies Randomly

While I haven’t experienced this on fuel-injected cars, the vehicles with carburetors were prone to letting the engine die haphazardly while driving.

I didn’t include fuel-injected vehicles because once the fuel pump can’t supply fuel to the fuel rail at the required pressure, the car won’t start anymore.

While carbureted vehicles have little reservoirs inside the carburetor called the fuel bowls, and they need little to no pressure to fill when there is enough demand, the carburetor will empty the bowl. 

Still, the fuel pump won’t have enough flow to keep it topped up, resulting in the engine dying.

5. Engine Struggles To Start

It happens at the worst time and places when the car doesn’t want to start on fuel-injected vehicles. 

The pump might have enough pressure to let the vehicle start, but it won’t keep running as the demand for fuel increases.

On carbureted cars, the startup sequence uses a lot of fuel compared to fuel injection and can just be enough for the fuel pump not to keep up, so the car will stumble and shake and might start but eventually die again.

a mechanical fuel pump and inline fuel filter for a carburetor engine
An example of a mechanical fuel pump used on carburetor engines and an inline fuel filter.

How To Diagnose A Bad Fuel Pump

Once you have experienced the symptoms, you must consider why the fuel pump is bad.

There can be multiple things that can cause the fuel pump to appear broken or not functioning.

1. Blown Fuse

The fuel pump runs on the car’s 12-volt system, and to ensure that there are no fire hazards, every part of the 12-volt system needs a fuse.

A fuse is the weakest part of the electrical circuit, so if the resistance or amps needed surpass what the circuit is designed for, the fuse would blow.

So look in your vehicle’s owner’s manual, look up where the fuel pump fuse is located, and see if it is blown.

2. Broken Relay

Relays are used to switch on high-amperage electronics in the vehicle’s 12-volt system. The fuel pump is one of those electronics that use a relay.

Find the fuel pump relay’s location in your vehicle owner’s manual or online; in this instance, it will help to have a friend.

One of you has to be close to the relay, and the other needs to switch the vehicle to the run position on the ignition barrel.

Once the key is turned to the run position, you will hear the relay click as it powers the electrical circuit; if it does not click, the relay needs to be replaced.

3. Power At The Pump

With both of the above steps checked, you can now find the fuel pump wiring plug. 

It will either be underneath the vehicle, close to the fuel tank or through an inspection hole in the car’s interior underneath the rear seats.

Once you have found it, you can separate the plug and test the individual pins inside when the car is in the run position if there is power at the plug.

If there is no power, there is a fault somewhere on the circuit between the relay, fuse, and fuel pump plug; it can be a bent or semi-removed pin or the next thing.

4. Inertia Switch

The inertia switch is a very acute electrical switch that disables the circuit of electricity to the fuel pump when the vehicle is in a crash.

This minimizes the chances when the fuel line is ruptured during a crash, and the fuel pump keeps pumping out fuel, thus increasing the chances of a fire.

The switch is usually located in the trunk of a vehicle near one of the taillights; the inertia switch has a small button on the top, which you can depress and reset the inertia switch.

If you drive over a pothole or something in the boot that hits the inertia switch, it can disable the electric circuit.

Chris Fix made a very easy-to-follow video on how to go through all these diagnoses:

How to Diagnose and Replace a Fuel Pump

Final Thoughts On Weak Fuel Pump Symptoms

Electronics, especially those with motors inside like a fuel pump, will always die at some point, so it is in your best interest to know the symptoms of a fuel pump that is failing.

Hopefully, you found this article helpful, and it will help you keep your car on the road and experience many trouble-free miles.


How Does A Car Act When The Fuel Pump Is Going Out?

The car will struggle to start, once it starts running, it will idle very roughly, and on the move, it will stutter and hesitate under acceleration.

Can You Drive With A Weak Fuel Pump?

Yes, but I don’t recommend doing it; it is advised to take the vehicle as fast as possible to a mechanic or fix it yourself.

Can A Fuel Pump Be Weak But Still Work?

To some degree, yes, it might have enough oompf left in it to let the car start and idle but not enough for it to accelerate or move.