PFI systems — why the electric fuel pump primes for about two seconds at key-on In a programmed fuel injection (PFI) system, the in-tank or in-line electric fuel pump typically runs for about two seconds at key-on (before cranking). What is the purpose of this brief operation?

Introduction / Context:
Electronic fuel-injected engines need stable rail pressure for accurate metering at start. The ECU briefly energizes the fuel pump at key-on to prepare the system.

Given Data / Assumptions:

• PFI system with electric pump, fuel rail, pressure regulator or returnless control.
• ECU controls the pump relay/driver.
• Engine is not yet cranking.

Concept / Approach:
The short prime builds pressure in the fuel line and rail, purges vapour pockets, and ensures that injectors deliver the correct mass of fuel at first crank. Without priming, extended cranking could be required as pressure slowly builds.

Step-by-Step Solution:

Verification / Alternative check:
Fuel pressure gauge connected to the rail shows rapid rise to specification at key-on before cranking. Many OEM service manuals describe this prime routine.

Why Other Options Are Wrong:

• Diagnostics: not the primary reason; fault checks occur simultaneously by ECU.
• Pump warm-up: unnecessary; pumps are cooled and lubricated by fuel during normal operation.
• Choke effect: obsolete carburetor concept; EFI meters fuel precisely.
• Immediate canister purge: occurs when warm and running, not at key-on.

Common Pitfalls:
Failed pump relay or immobilizer logic prevents priming; leaking injectors or bad check valves cause pressure bleed-down leading to long cranks.

Final Answer: