Electric Fuel Pumps – How Do They Work – How Can They Fail

Electric Fuel Pumps
Electric Fuel Pumps

Electric fuel pumps are fairly simple but rely on many other systems to function properly.

The Electric fuel pump is responsible for delivering pressurized fuel from the fuel tank to the engine.

When the ignition is activated, the (PCM) energizes a relay that supplies voltage to all electric fuel pumps.

The motor inside fuel pump starts to spin and runs for a few seconds to build pressure.

Generally you will find that electrical fuel pumps are located in the fuel tank in order to use the fuel in the tank to cool the pump and ensure a steady supply of fuel.

Electric fuel pumps can be heard as a quiet whine or hum in most cars.

Most fuel pumps are built to last a long time.

However, as the vehicle reaches high mileage, it is not uncommon for fuel pumps to require replacement.

So, What really goes on when you turn the key or press the start button.

Electric Fuel Pump – How Do They Work

Electric Fuel Pump
Electric Fuel Pump
  1. Fuel is drawn into the pump through an inlet tube and filter system.
  2. The fuel then exits the pump through a one-way check valve (which maintains residual pressure in the system when the pump is not running), and is pushed toward the engine through the fuel line and filter.
  3. The fuel filter traps any rust, dirt or other solid contaminants that may have passed through the pump.
  4. Fuel then flows to the fuel supply rail feeding individual fuel injectors.
  5. A fuel pressure regulator on the fuel rail maintains fuel pressure, and routes excess fuel back to the tank.
  6. On newer vehicles with returnless (EFI) systems, the fuel pressure regulator is located in the fuel tank.
  7. There is no fuel return line from the engine back to the tank.
  8. Fuel pumps run continuously once the engine starts, and continues to run as long as the engine is running.
  9. The pump may run at a constant speed, or variable speed depending on engine load and speed.
  10. If the engine stalls, the (PCM) will detect the loss of the (RPM) signal and turn the pump off.

Fuel Pump – How Can They Fail

Electric Fuel Pump
Electric Fuel Pump

Fuel pumps should last the life of the vehicle, but can fail because of:

  • Contamination inside the fuel tank (dirt or rust)
  • Fuel starvation (running out of fuel)
  • Overheating (always driving with a low fuel level)
  • Low voltage (wiring problem)
  • Overwork (trying to overcome a restriction caused by a plugged fuel filter)

The harder a pump works, the hotter it runs and the more amps it pulls through its power circuit.

So, When a fuel pump fails, it often just quits without any warning. You are driving along fine one minute, then suddenly your engine stalls. Or, you come out to start your car in the morning only to discover it cranks but won’t start.

Fuel Pump – Failure Signs

Relay
Relay

So, How can you tell if a bad fuel pump is causing your no-start problem? One way is to listen for pump noise after turning the ignition key on. No pump noise would tell you the pump is not running. The fault may be a bad pump, or it could be a bad fuel pump relay, fuse or wiring connection.

On most vehicles, a failed fuel pump will not set any diagnostic trouble codes or check engine light. The engine will crank normally, and it will have spark, but it will not start because of no fuel.

One way to tell if a no-start is due to no fuel is to spray some aerosol starting fluid into the throttle. If the engine starts, runs a few seconds, then dies, it has spark and compression but is not getting any fuel from the fuel pump.

Electric Fuel Pump – Failure Symptoms

  • Loud Whining Noise From the Fuel Tank
  • Difficulty Starting
  • Engine Sputtering
  • Stalling At High Temperatures
  • Loss of Power Under Load
  • Engine Surging
  • Poor Fuel Mileage
  • Engine Will Not Start
Fuel Pressure Regulators
Fuel Pressure Regulators

NOTE:

Overheating is among the top factors in causing a fuel pump to wear out. Mounting the fuel pump on the bottom of the tank helps. This keeps the pump cool, but only if there’s enough fuel to keep the pump covered. Additionally, If the fuel level gets too low, the pump can begin sucking in air with the fuel.

This creates further problems by throwing off the correct mix of fuel and air in the engine. For these and other, non-pump-related reasons, many experts recommend never letting your fuel level get down below a quarter-tank full.

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Conclusion

Most times fuel pump problems are hard to diagnose. Specially because some symptoms are similar to those produced by problems in other systems.

By understanding the basic process of the fuel pump, you’ll have a better understanding of what happens when a fuel pump goes out — and how to recognize when it’s happening.

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