Fuel Systems & Fuel Delivery Flashcards

In a returnless fuel system, how is fuel pressure regulated?

Answer: By varying fuel pump speed based on engine demand

Returnless fuel systems use a variable-speed electric pump controlled by the PCM to match pressure to demand, eliminating the need for a return line.

What is the stoichiometric air-to-fuel ratio for a gasoline engine?

Answer: 14.7:1

A 14.7:1 air-to-fuel ratio by mass is stoichiometric for gasoline, providing theoretically complete combustion and optimal catalytic converter efficiency.

Which sensor primarily calculates air mass in a speed-density fuel injection system?

Answer: Manifold Absolute Pressure (MAP) sensor

Speed-density systems use the MAP sensor combined with engine RPM and temperature data to calculate air density and required fuel delivery.

What does a large positive long-term fuel trim (LTFT) value indicate?

Answer: The ECM is adding fuel to compensate for a lean condition

A positive LTFT means the ECM is commanding extra fuel delivery because it detects a lean condition that requires correction.

What is the function of the fuel return line in a conventional fuel injection system?

Answer: To return excess pressurized fuel and heat back to the tank

The return line carries unused fuel and heat away from the engine back to the tank, cooling the pump and stabilizing fuel rail pressure.

What is 'vapor lock' in a fuel system?

Answer: Fuel vaporizing inside the lines before reaching the engine, causing fuel starvation

Vapor lock occurs when fuel converts to vapor in the supply line, preventing liquid fuel flow to the engine and causing stalling or hard starts.

What is the purpose of the inertia switch found in some vehicles' fuel systems?

Answer: To shut off the fuel pump automatically in the event of a collision

The inertia switch is a safety device that cuts power to the fuel pump when a sudden impact is sensed, reducing fire risk after a collision.