NAPA Fuel Systems & Fuel Delivery 2 — Questions and Answers
Question 1: In a returnless fuel system, how is fuel pressure regulated?
- By a mechanical pressure regulator at the fuel rail
- By varying fuel pump speed based on engine demand (Correct answer)
- By a pressure relief valve inside the fuel tank
- By adjusting the fuel injector duty cycle
Correct 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.
Question 2: What is the stoichiometric air-to-fuel ratio for a gasoline engine?
- 12.5:1
- 9.0:1
- 14.7:1 (Correct answer)
- 16.5:1
Correct 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.
Question 3: Which sensor primarily calculates air mass in a speed-density fuel injection system?
- Mass Airflow (MAF) sensor
- Throttle Position Sensor (TPS)
- Crankshaft Position Sensor (CKP)
- Manifold Absolute Pressure (MAP) sensor (Correct answer)
Correct 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.
Question 4: What does a large positive long-term fuel trim (LTFT) value indicate?
- The engine is running rich
- Fuel pressure is too high
- The ECM is adding fuel to compensate for a lean condition (Correct answer)
- The fuel injectors are oversized for the engine
Correct 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.
Question 5: What is the function of the fuel return line in a conventional fuel injection system?
- To deliver extra fuel during wide-open throttle
- To recirculate fuel continuously through the filter
- To relieve vapor lock by venting fuel vapors
- To return excess pressurized fuel and heat back to the tank (Correct answer)
Correct 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.
Question 6: What is 'vapor lock' in a fuel system?
- A solenoid valve that secures the fuel filler cap
- Fuel vaporizing inside the lines before reaching the engine, causing fuel starvation (Correct answer)
- Ice formation inside the fuel line restricting flow
- A blocked vent tube creating negative pressure in the tank
Correct 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.
Question 7: What is the purpose of the inertia switch found in some vehicles' fuel systems?
- To shut off the fuel pump automatically in the event of a collision (Correct answer)
- To modulate fuel pump speed based on engine RPM
- To prevent the fuel pump from overheating during extended idling
- To reduce in-tank fuel pump operating noise
Correct 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.
NAPA Fuel Systems & Fuel Delivery 2
In a returnless fuel system, how is fuel pressure regulated?