Automotive and Mechanics Fuel Systems & Combustion 2 — Questions and Answers
Question 1: What is the function of the evaporative emission control (EVAP) system?
- Cool intake air
- Prevent fuel vapors from escaping to the atmosphere (Correct answer)
- Recirculate exhaust gases
- Filter fuel before injection
Correct answer: Prevent fuel vapors from escaping to the atmosphere
The EVAP system captures fuel vapors from the tank and routes them to the intake manifold to be burned, reducing hydrocarbon emissions.
Question 2: Which sensor provides the ECU with feedback to adjust the air-fuel ratio in closed-loop operation?
- MAP sensor
- Throttle position sensor
- Oxygen sensor (Correct answer)
- Coolant temperature sensor
Correct answer: Oxygen sensor
The oxygen (O2) sensor measures exhaust oxygen content and feeds back to the ECU to fine-tune the air-fuel mixture in closed-loop mode.
Question 3: What does a high long-term fuel trim (LTFT) positive value indicate?
- Engine is running rich
- ECU is adding fuel to compensate for a lean condition (Correct answer)
- Fuel pressure is too high
- Injectors are over-fueling
Correct answer: ECU is adding fuel to compensate for a lean condition
A positive LTFT value means the ECU is adding fuel, indicating the engine is running lean, often due to a vacuum leak or weak fuel pressure.
Question 4: What is the purpose of a fuel pressure dampener?
- Regulate maximum fuel pressure
- Reduce pressure pulsations from the fuel pump (Correct answer)
- Filter fuel contaminants
- Prevent fuel siphoning
Correct answer: Reduce pressure pulsations from the fuel pump
A fuel pressure dampener absorbs pulsations created by the high-pressure fuel pump, ensuring smooth, consistent fuel delivery.
Question 5: Which component in a carbureted engine controls the amount of fuel mixed with air at idle?
- Accelerator pump
- Power valve
- Idle mixture screw (Correct answer)
- Choke plate
Correct answer: Idle mixture screw
The idle mixture screw adjusts the fuel-air ratio at idle by controlling how much fuel bleeds into the idle circuit.
Question 6: What causes fuel injector 'dead time' (offset)?
- Fuel pressure variance
- The delay between the ECU signal and when the injector actually opens (Correct answer)
- Injector filter restriction
- Low battery voltage
Correct answer: The delay between the ECU signal and when the injector actually opens
Injector dead time is the lag between the ECU's open command and actual needle lift, which the ECU compensates for by adding offset time to pulse width.
Automotive and Mechanics Fuel Systems & Combustion 2
What is the function of the evaporative emission control (EVAP) system?