* Prime and Pressure Check: The primary reason is the key-on-engine-off (KOEO) self-test. When you turn the ignition key to the "on" position (but don't start the engine), the fuel pump runs briefly to build pressure in the fuel lines. This ensures that the fuel system is primed and ready to deliver fuel immediately when the engine starts. Once it reaches the correct pressure, it shuts off. If it didn't do this, you'd have a slight delay before the engine started each time.
* Preventing Engine Flooding: A short prime also minimizes the risk of flooding the engine with fuel, especially if the car has been sitting for a while.
* Fuel Pump Protection: This short cycle helps to prevent overheating or premature wear on the fuel pump motor itself by avoiding continuous running when not needed.
However, if this is a *new* behavior for your vehicle, or if it's accompanied by other problems (like difficulty starting, rough running, or lack of power), it could indicate a problem. Possible causes of a fuel pump malfunctioning in a way that's *different* from the normal short prime include:
* Faulty Fuel Pump Relay: The relay might be failing and not allowing the pump to run for its normal duration.
* Fuel Pump Pressure Sensor Issues: The sensor might be malfunctioning, incorrectly signaling the pump to stop prematurely.
* Low Fuel Pressure: A problem elsewhere in the fuel system (clogged filter, leaking injector, etc.) might be preventing the system from reaching its normal pressure, causing the pump to shut off early.
* Faulty PCM (Powertrain Control Module): The computer could be faulty and sending the incorrect signals to the fuel pump.
If the 3-second run is a recent change and you're experiencing starting or running problems, it's best to have a mechanic diagnose the issue. They can test fuel pressure and check the fuel pump, relay, and other related components.
