How to test fuel pump (inside gas tank)

When a fuel pump starts to fail, your car can have a variety of symptoms – from not starting at all, to stalling unexpectedly, or even just feeling sluggish and weak, especially when you try to accelerate. Figuring out if the pump is the problem is a key step in fuel system diagnostics.

Here’s how you can check your fuel pump:

1. Listen for the Prime: Most electric fuel pumps run for a few seconds when you first turn the ignition key to the ‘ON’ position (before you crank or start the engine). This is the system’s “priming” or building pressure.

• Turn the key to ‘ON’ and listen carefully near the fuel tank. In many cars, the pump is inside the tank, often under the back seat or in the trunk area.
  
• What to listen for: A low, steady whirring, buzzing, or humming sound for a couple of seconds.
  
• What if you hear nothing? This is a strong sign the pump isn’t running. The problem could be the pump itself, the fuel pump relay, the fuse, the wiring, or the command signal from the computer.

2. Check Fuel Pressure (Your Best Friend!): As we discussed before, checking the actual fuel pressure with a gauge is the most definitive way to test the pump’s performance.

• Connect your fuel pressure gauge and perform the Key On/Engine Off and Engine Idling tests.
  
• What the readings tell you: If pressure is low or zero during the prime or while idling (and you’ve checked the filter isn’t totally blocked), it’s highly likely the pump isn’t moving enough fuel or creating enough pressure. A rapid pressure drop during the leakdown test also points strongly to a faulty check valve within the pump.

3. Check for Power and Ground: If you don’t hear the pump running and pressure isn’t building, the next step is to see if the pump is even receiving the electrical signal to turn on.
   

• You’ll need to locate the electrical connector for the fuel pump. This might be a plug on top of the fuel tank (requiring access often from under the car or under the rear seat) or sometimes an inline connector further up the fuel line. Finding a wiring diagram for your car is super helpful here to identify the correct wires.
  
• Using your multimeter or a test light, check for battery voltage (around 12V) on the power wire and a good ground connection on the ground wire at the connector while someone turns the key to the ‘ON’ position (to catch the prime) or while cranking/engine running (depending on your car’s system design).
  
• What this tells you: If you have both power and ground at the connector when the pump should be running, but the pump isn’t making noise or building pressure, the fuel pump itself is almost certainly faulty. If you don’t have power or ground, the problem is upstream – in the fuse, relay, wiring, control module, or perhaps an inertia safety switch that’s tripped.

By listening for the pump, confirming its performance with a pressure test, and checking for electrical power, you can confidently diagnose whether a fuel delivery problem is caused by the pump itself or an issue in the circuit powering it.

Figuring out if your fuel pump is working correctly gets you closer to solving many frustrating car problems!

P.S. I have a full DIY Mechanic video course (if you are a more video-visual learner), you can join HERE.