NAPA Fuel Systems & Fuel Delivery 2

Question 1

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

Accepted Answer

By varying fuel pump speed based on engine demand

Answer

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?

Accepted Answer

14.7:1

Answer

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?

Accepted Answer

Manifold Absolute Pressure (MAP) sensor

Answer

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?

Accepted Answer

The ECM is adding fuel to compensate for a lean condition

Answer

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?

Accepted Answer

To return excess pressurized fuel and heat back to the tank

Answer

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

NAPA - National Automotive Parts Association Certified Mechanic Practice Test

NAPA - National Automotive Parts Association Certified Mechanic Practice Test

NAPA Fuel Systems & Fuel Delivery 2