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In every fuel injected car that i drove that had electric fuel pump, i always hear the fuel pump buzzing sound after turning the ignition on however after a couple of seconds the buzzing sound goes away. Why is that? Is it because after a couple of seconds the pump fully pressurizes the fuel and this causes the buzzing sound of the pump to be "eaten away" so we dont hear the sound anymore?

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Most fuel systems have a pressure-switch. So when the power is first applied the fuel system is unpressurized. The fuel pump is then turned on to build up pressure so the engine can start.

Once the fuel system reaches the designed pressure the pump is turned off.

If you listen closely you will hear the pump reactivate from time to time as the engine is running to keep the pressure up.

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  • oh i see thanks, i used to think that pump always ran at constant speed to keep the pressure in the fuel rail and bleed off excess pressure through valving and stuff
    – Timothy Scherer
    Feb 15 at 10:15
  • @jwh20 - Im pretty sure most systems are just timed. When the system is first activated, the the pump is turned on for a period of time, then is shut off until the engine is started. Once started, it runs all of the time. Not saying all systems are this way, but I believe most are.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Feb 15 at 11:10
  • Some may be but I have drained a fuel tank by disconnecting the fuel line, switching on the ignition, and having it pump into a gas can. It was clearly pressure sensitive for that model, a 2000 Chevy Tahoe. I am sure there are both types but I think the pressure switch setups are more common today with fuel rails and direct injection.
    – jwh20
    Feb 15 at 16:30
  • Older early systems used a signal from the oil pressure switch. The pump would run for several seconds and if no oil pressure was detected it cycled off. This also acted a safety circuit in an accident. If the engine stalled the pump would shut off. Some also had an inertia switch, they caused issues with off roaders and service trucks with push bumpers and were often bypassed.
    – mikes
    Feb 15 at 19:54
  • @Paulster2 I think you are correct for all the vehicles I've ever worked on. If you turn the key from off->on->off->on you'll hear the pump kick on twice, which wouldn't happen if it was pressure-controlled. The fuel pump just turns on as part of the start cycle so the fuel gets to the engine quickly and the engine starts quickly. After the start cycle the fuel pressure sensor takes over while the vehicle is operating.
    – David
    Feb 15 at 21:03

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